The study does not show that the structure preserved all marsh functions or that nitrogen removal was unchanged at every site; it documents selective functional convergence within a single artificialized salt-marsh landscape. The investigation used a space-for-time comparison at five locations in the Rivière-du-Loup embayment on the south shore of the Upper St. Lawrence Estuary.

Two unvegetated sites sheltered behind the breakwater were classified as impacted, two sites beyond the structure served as references—one vegetated by cordgrass, Sporobolus alterniflorus, and one unvegetated—and a site immediately seaward of the breakwater was treated as intermediate. The seaward site resembled the references across the measured sediment, oxygen and benthic-flux variables, suggesting relatively little direct breakwater influence there, although the investigators cautioned that it might also have captured offshore effects. By comparing locations rather than sampling the same sites before and after construction, the design associated spatial differences with long-term exposure to the structure without directly reconstructing the construction history at each site.

The breakwater was installed in 2009 as a 300 m-wide barrier of boulders to reduce erosion threatening a major highway. A storm coupled with high tides in 2010 transported substantial sediment that became trapped landward of the structure. The main sampling campaign took place in June 2021, near the beginning of the salt-marsh growing season, when plant roots were active but the aboveground canopy remained short enough to incubate intact vegetated cores.

A supplementary campaign in June 2022 filled specific gaps at the intermediate site and repeated selected sediment measurements; the spatial patterns were consistent between the two years. Seawater temperature and salinity were similar during sampling, at 9 °C and 19.6 in 2021 and 10 °C and 18.7 in 2022. Sediments across the study area were silty clay, but the two landward sites differed clearly from the references.

Their median grain size was smaller, while organic carbon and total nitrogen tended to be higher. This pattern was consistent with the low-energy depositional environment created behind the high-crested structure, where wave energy is reduced and fine, organic-rich particles accumulate. Carbon-isotope signatures remained similar among sites, indicating that the observed difference was primarily in the quantity of deposited material rather than the apparent source of the carbon.

The intermediate seaward location again resembled the reference sediments. Such differences in grain size, organic-carbon content and nutrient content are expected when engineered structures reorganize sediment delivery, but their ecological significance depends on how the altered habitat affects organisms and microbial processes. Those physical changes altered the internal chemistry of the sediment.

At the impacted locations, diffusive oxygen uptake was as much as 2.3 times higher than at reference sites, and oxygen penetrated less deeply, consistent with greater oxygen consumption during the breakdown of accumulated organic matter. Nitrate declined rapidly with depth and fell below the detection limit at roughly 4 cm in impacted sediment, compared with about 6 cm in reference material. Ammonium and phosphate concentrations increased more sharply with depth behind the breakwater, consistent with enhanced organic-matter mineralization and reductive processes.

Reactive iron extracted with ascorbate was concentrated near the surface at the impacted sites, implying that aerobic reactions were confined to a thin surface layer. By comparison, the vegetated reference had a deeper oxidized zone that the researchers associated with marsh-root oxygenation. These profiles show that similar rates measured later at the sediment-water interface emerged from markedly different sediment structures.

The biological community changed as clearly as the sediment. Sampling recovered 2,657 macrofaunal individuals representing 14 taxa, and hierarchical clustering separated the impacted sites, the vegetated reference and the unvegetated reference into three distinct assemblages. Behind the breakwater, high densities of small oligochaetes and the polychaete Hediste diversicolor characterized the fauna.

These organisms are associated with fine, organic-rich sediments, and both can act as substantial bioturbators. The vegetated reference supported small grazers including amphipods and gastropods, while the unvegetated reference had relatively high densities of the bivalve Mya species and greater total biomass. Functional richness was higher at the two references than at the impacted sites, although functional evenness did not differ significantly among them.

Thus, habitat alteration did more than change which species were present: it reduced the breadth of biological functions represented within the communities, even where selected process rates converged. Intact-core incubations showed no simple separation of all affected and reference areas in dark total benthic oxygen uptake, nitrate exchange or ammonium release. These fluxes did differ significantly among individual sites, so the result was not an absence of site-level change.

Instead, their broader pattern was more complex than a uniform breakwater effect: several values were higher at the southern impacted site and the vegetated reference than at the northern impacted, intermediate and unvegetated sites. Total denitrification also differed significantly among sites. It ranged from 6 to 75 µmol m−2 h−1, within the range previously reported for temperate intertidal mudflats and salt marshes, and was highest at the vegetated reference and southern impacted site.

More than half of denitrification at both impacted sites and the vegetated reference was supported by nitrate produced inside the sediment through nitrification. At the unvegetated reference, less than 10% was attributed to that coupled pathway, indicating greater reliance on nitrate supplied from the water column. Stable-isotope tracing helps explain how the apparently similar outcome was achieved.

The southern impacted site had the densest subsurface bioturbation among the affected locations. Burrowing and irrigation by H. diversicolor and oligochaetes increase oxygen transfer, organic-matter mineralization and nitrification in the upper sediment, thereby supplying nitrate that can be reduced immediately nearby. The vegetated reference reached a broadly similar result through a different mechanism.

Cordgrass roots release oxygen into the rhizosphere, creating a subsurface oxidized layer where nitrification can occur; the deeper reactive-iron maximum was consistent with this process. The resulting nitrate was then consumed by denitrification in adjacent anoxic sediment. Root oxygenation at the marsh and animal-mediated sediment irrigation behind the breakwater thus supported comparable nitrogen-removal functions through distinct biogeochemical routes, with locally produced nitrate serving an important role at both.

The two affected sites were not functionally identical. The northern impacted site contained less macrofauna and maintained more reducing conditions than the southern site, apparently limiting internal nitrate production and therefore denitrification. The southern site’s greater bioturbator density partly compensated for its lack of marsh roots and preserved more of the processes measured in vegetated sediment.

By contrast, the unvegetated reference had low organic-carbon availability, restricting ammonium regeneration and nitrification, while its denitrification depended mainly on external nitrate. Its larger burrowers may also have exerted effects beyond the scale of the intact cores used for flux measurements. Comparisons with an earlier bulkhead study further indicate that impacts from shoreline hardening are structure-specific: the present breakwater accumulated organic-rich sediment and supported bioturbators, whereas vertical structures can cause coastal squeeze and reduce total marsh area even when nitrogen removal per unit area remains similar.

Several limits qualify the findings. The incubations were conducted in darkness to standardize comparisons, excluding photosynthetic oxygen production by microphytobenthos and marsh plants, although root oxygen release can persist without light. Oxygen microprofiles and porewater nutrients were measured on one core per location, creating potential pseudo-replication and requiring cautious interpretation of statistical results.

The terminal inventory of isotope-labelled nitrogen gas in sediment porewater was not quantified, so some nitrogen produced during incubations may not have reached the overlying water. Measurements covered selected benthic processes—respiration, nutrient exchange and denitrification—rather than the full suite of services supplied by a natural marsh. Altered communities may also be more vulnerable to additional pressures even where individual process rates remain similar.

At the seascape scale, compensation appears incomplete. The southern impacted area covers about 18,800 m², roughly three times the approximately 6,300 m² of marsh directly lost through breakwater construction, but less than one-third of the approximately 62,700 m² lost through the combined effects of erosion and construction. A first-order estimate therefore suggests that the measured fluxes in the southern area might offset some of the functional capacity lost from the marsh directly removed by the structure, provided the rates adequately represent spatial heterogeneity.

The results demonstrate selective functional convergence, not equivalence between engineered and natural coastlines. Assessing coastal development consequently requires more than a comparison of average process rates: habitat area, sediment conditions, community composition, the origin of nitrate and vulnerability to additional stressors must all be considered when evaluating the consequences of shoreline hardening.

Subject of Research: Breakwater effects on salt-marsh sediment ecology, macrobenthic communities and benthic nitrogen cycling

Article Title: Different paths, same destination: similar benthic nitrogen cycling sustained by distinct biogeochemical mechanisms in artificialized coastal systems

Article References: Pascal, L., Dansereau-Macias, É., Guay, C.-A., Didier, D., Nozais, C., Sirois, L., Zouaghi, F., & Chaillou, G. (2026). Different paths, same destination: similar benthic nitrogen cycling sustained by distinct biogeochemical mechanisms in artificialized coastal systems. Biogeosciences, 23(20), 7327-7343. https://doi.org/10.5194/bg-23-7327-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-7327-2026

Keywords: coastal breakwaters, salt marshes, benthic nitrogen cycling, denitrification, sediment biogeochemistry, macrobenthic communities, bioturbation, rhizosphere oxygenation, nitrification, St. Lawrence Estuary, Different, paths

The research, conducted by Rafael Hernández Maqueda and Fernando Del Moral at the University of Almería, compared two adjacent greenhouses with similar soils and crop histories at the Palmerillas Experimental Station of the Cajamar Foundation. One greenhouse was managed conventionally, receiving inorganic fertilizers dissolved in the irrigation water—a practice known as fertigation that dominates Mediterranean greenhouse horticulture. The other followed an ecological intensification strategy: each year, the residues of the previous eggplant crop (Solanum melongena) were incorporated back into the soil together with organic amendments in the form of manure, while inorganic fertilizer use was deliberately restricted. In total, only about 40 percent of the nitrogen applied in the ecological system arrived in soluble mineral form, compared with 100 percent in the conventional greenhouse.

After six years of this contrasting treatment, the researchers sampled soil from the top 15 centimeters along randomly selected drip lines in each greenhouse, collecting nine samples per greenhouse in May 2021 at the end of the growing season. The samples were split between classical physicochemical analysis and molecular biology: aliquots frozen at minus 80 degrees Celsius yielded DNA for quantitative real-time PCR, which the team used to count copies of bacterial 16S rRNA genes, fungal ITS regions, and four genes central to denitrification—the microbial pathway that converts nitrate ultimately to nitrogen gas, and which can leak the potent greenhouse gas nitrous oxide (N2O) into the atmosphere if the process stalls midway.

The physical and chemical results were striking. Total organic carbon in the ecologically intensified soil reached 31.6 grams per kilogram, nearly 2.6 times the 12.2 grams per kilogram measured under conventional management—a proportional increase the authors note exceeds values typically reported in the literature, even though the levels still fall short of the thresholds recommended for optimal greenhouse horticulture. Water-soluble organic carbon, a rapidly cycling fraction that responds to management changes far faster than total carbon, more than tripled, rising from 0.23 to 0.68 grams per kilogram. Total nitrogen increased by roughly half, and the carbon-to-nitrogen ratio climbed from 5.98 to 9.1, a shift with important consequences for the microbial community structure.

Soil structure improved in parallel. The percentage of water-stable, resistant aggregates—the glues and scaffolds that hold soil particles together against erosion and compaction—rose from 21.3 to 33.2 percent, an increase of about 1.5-fold. Because aggregate stability is one of the most robust indicators of a soil’s capacity to resist degradation, this single number encapsulates much of what ecological intensification promises: a soil that behaves less like an inert growing medium and more like a living, self-organizing system. Soil pH, meanwhile, dropped by about 0.7 units, from 8.6 to 7.9, moving the alkaline greenhouse soil closer to the range in which most nutrients are most available to plants.

The biological data told an equally compelling story. Bacterial abundance, measured as 16S gene copies, remained statistically similar between the two systems. Fungal abundance, however, exploded: ITS copy numbers rose from 0.27 to 2.11 log10 copies per gram of dry soil, and the fungal-to-bacterial ratio increased roughly threefold, approaching but not quite reaching statistical significance. The researchers attribute this fungal boom to the lignified eggplant residues and mature compost incorporated under ecological intensification—substrates rich in complex carbon compounds that fungi, with their enzymatic apparatus for degrading recalcitrant material, exploit far more effectively than bacteria. Intensive mineral fertilization, by contrast, is well known to suppress fungal relative abundance and push communities toward bacterial dominance.

The denitrification genes provided perhaps the most nuanced findings. Both clades of the nosZ gene, which encodes nitrous oxide reductase—the only known enzyme capable of converting N2O back into inert atmospheric nitrogen—were significantly more abundant under ecological intensification. The nirK and nirS genes, which encode the nitrite reductases that initiate the nitrogen-producing steps of denitrification, also trended upward, though their increases did not reach significance. Critically, the ratio of N2O-consuming nos genes to N2O-producing nir genes exceeded 2 in both treatments and remained statistically similar between them, suggesting that neither system had an inherent advantage in its potential to suppress nitrous oxide emissions. The authors caution that this genetic proxy is imperfect: denitrification is only one of several microbial pathways that generate N2O, and gene abundance does not always translate into enzyme activity in the field.

Correlation analysis revealed the web of interactions underlying these patterns. Total organic carbon correlated positively with fungal ITS abundance (Rho = 0.700), and the carbon-to-nitrogen ratio likewise tracked fungal abundance. Soil pH showed consistent negative correlations with fungal ITS, nirK, and nosZ1 copy numbers, reinforcing recent findings that pH and organic carbon are among the dominant drivers of microbial community structure. Intriguingly, the four denitrification genes correlated positively with one another, hinting at tightly coupled denitrifier populations, while water-soluble organic carbon—despite its reputation as a microbial energy source—showed only weak links to the biodiversity indicators, suggesting it may not reliably reflect shifts in microbial abundance in these soils.

Then comes the caveat that gives the study its edge. Electrical conductivity, the standard proxy for salinization risk, was 7.4 times higher in the ecologically intensified greenhouse—4.52 versus 0.61 decisiemens per meter. Sodium quadrupled, potassium tripled, and magnesium rose 1.7-fold, while calcium actually declined by about a third, possibly because organic acids released from the decomposing residues promoted calcium leaching. The eggplant residues and compost derived from greenhouse crops are inherently rich in mineral salts, and under the low-rainfall, closed-canopy conditions of a Mediterranean greenhouse, those salts accumulate rather than wash away. The authors recommend rethinking amendment choices—avoiding high-salt biomass and exploring combinations of fresh and mature organic materials—to preserve the structural and biological gains without pushing salinity into dangerous territory, and to reduce reliance on salt leaching, which itself threatens groundwater quality.

The study’s design is deliberately exploratory—a single case study with two greenhouses rather than a replicated field trial—so its conclusions must be generalized with care. Yet its message resonates far beyond Almería. As European agriculture confronts the European Union’s Farm to Fork strategy and the global imperative to cut agrochemical inputs, ecological intensification offers a credible path toward soils that store more carbon, hold together better, and host richer microbial communities. What this six-year experiment makes clear is that the transition is not a simple substitution of organic for inorganic inputs but a systems problem: the composition of the amendments, the salt loads they carry, and the denitrifier communities they cultivate all interact in ways that only long-term, carefully monitored experiments can untangle. The next step, the authors argue, is to pair high-throughput sequencing with direct measurements of nitrous oxide fluxes and leachates, so that the genetic signals observed here can be translated into the climate and water outcomes that ultimately matter.

Subject of Research: Effects of ecological intensification on soil quality indicators and denitrification gene abundance in Mediterranean greenhouse soils

Article Title: Enhancing Greenhouse Soil Quality Through Ecological Intensification (EI). A Case Study

Article References: Hernández Maqueda, R., & Del Moral, F. (2026). Enhancing Greenhouse Soil Quality Through Ecological Intensification (EI). A Case Study. Spanish Journal of Soil Science, 15, Article 15184. https://doi.org/10.3389/sjss.2025.15184

Image Credits: AI Generated

DOI: 10.3389/sjss.2025.15184

Keywords: ecological intensification, soil quality, greenhouse horticulture, denitrification, nitrous oxide, soil organic carbon, salinization, fungal-to-bacterial ratio, qPCR, organic amendments, Almería, nitrogen cycling

The study combined dynamic chamber measurements, laboratory soil analysis, DNA sequencing and a parameterized upscaling model. Twenty-five soil samples were collected in April 2021 from Linzhi in the plateau’s southeast, spanning grasslands, croplands, forests, wetlands and bare land across an altitude range of 150 to 7,782 meters. Each composite topsoil sample was sieved, characterized for pH, moisture, carbon and inorganic nitrogen content, and then placed in a multi-gas dynamic measurement system that flushed purified, contaminant-free air over the soil while it dried from maximum water-holding capacity. Concentrations of HONO, NO, NO2 and NH3 in the chamber headspace were tracked continuously with cavity-enhanced absorption spectroscopy, chemiluminescence and laser-based analyzers, allowing fluxes to be calculated with detection limits below a few nanograms of nitrogen per square meter per second.

The results show that every soil type emitted all four gases during wetting and drying cycles. Optimum fluxes averaged 21.6 ± 8.4 ng N m−2 s−1 for HONO, 43.7 ± 14.7 for nitric oxide, 15.8 ± 1.3 for nitrogen dioxide and 190.0 ± 116.3 for ammonia. Cropland soils were consistently the strongest emitters per unit area, with HONO fluxes reaching 42.1 ± 28.1 ng N m−2 s−1, a pattern the authors attribute to fertilization, faster nitrogen turnover and abundant nitrogen-cycling microbes. Integrated over a single wet-dry cycle, HONO release ranged from 0.02 to 0.30 mg N m−2 and ammonia from 0.47 to 3.07 mg N m−2, while NO2 showed net consumption rather than emission in most soils.

What drives these emissions is a combination of soil chemistry and microbial ecology. Spearman correlation analysis linked HONO and NO fluxes to soil pH, nitrogen content and the abundance of key functional genes such as amoA, narG, nirK and nosZ, which encode the enzymes of nitrification and denitrification. Sequencing of more than 3.8 million high-quality bacterial, archaeal and fungal gene sequences revealed that cropland soils were dominated by Crenarchaeota, ammonia-oxidizing archaea that can explain the low ammonium but high nitrification rates observed there, while Actinobacteriota, hardy Gram-positive bacteria that thrive at low temperatures, correlated strongly with NO emissions from Tibetan croplands. Grassland and cropland soils carried the highest loads of nitrogen-cycling genes, whereas bare soils, despite their vast extent, had the lowest gene abundance.

The team also ran a fumigation experiment to ask how the plateau’s own polluted air might feed back on its soils. Forest, grassland and cropland soils were exposed for five days to either pure NO at 10 or 50 ppb, or to mixtures of NOx and ozone at 10 ppb NOx plus 30 ppb O3 and 50 ppb NOx plus 100 ppb O3, concentrations that bracket the 25 to 100 ppb surface ozone levels previously reported on the plateau. The responses diverged sharply. Forest soils responded to fumigation with HONO and NO emissions peaking two to eight times above pre-exposure levels, accompanied by a rise in soil pH of roughly 0.53 units and significant increases in the denitrification genes nxrB, nirK and nirS. Cropland and grassland soils moved in the opposite direction, with HONO and NO peaks falling by about 31 percent in croplands and by 68 and 84 percent respectively in grasslands, as nitrogen accumulated as nitrate rather than escaping as gas.

The authors interpret this asymmetry through the lens of deposition and soil buffering. Forests are efficient at capturing atmospheric nitrogen, so added NOx and ozone appear to accelerate internal nitrogen transformation and push more gas out, while fertilized croplands and grasslands shift toward mineralization and nitrate retention under oxidative stress. Soil pH, which rose under fumigation and correlated positively with HONO and NO emissions, likely contributed to the forest response. The team is careful to note that these short-term chemical feedbacks were discussed qualitatively but not folded into the annual emission model, which instead rests on long-term in-situ soil properties.

Scaling the chamber results to the entire plateau using a parameterized model that accounts for soil temperature, precipitation and fertilizer application yielded annual emissions of 7.0 ± 3.4 Gg N yr−1 for HONO, 11.6 ± 7.8 Gg N yr−1 for NO and 20.3 ± 7.0 Gg N yr−1 for NOx in 2021. Counterintuitively, bare soils, which cover roughly 43 percent of the plateau, emerged as the largest regional contributor, accounting for 43 percent of HONO, 46 percent of NO and 67 percent of NOx emissions, consistent with earlier findings that desert soils and biological crusts are potent nitrogen emitters. Croplands, despite their high per-area fluxes, contributed only 8 percent of HONO because they occupy just 0.3 percent of the land area, a striking reversal of the global pattern in which farmland dominates soil nitrogen emissions.

To translate these fluxes into atmospheric consequences, the researchers combined their land-use-weighted HONO fluxes with observations from the Nam Co Multisphere Observation and Research Station at 4,730 meters elevation. Soil emissions were found to produce a volume mixing ratio of about 10.5 pptv per hour, equivalent to roughly 10.5 percent of the external, NOx-independent daytime HONO source previously identified in the plateau’s background atmosphere. Combined with an estimated 35 pptv per hour from surface nitrate photolysis, these two ground-level pathways jointly explain about 45.5 percent of the missing HONO that models have struggled to account for in this remote environment.

The NOx story is equally consequential. Soil emissions of 20.3 ± 7.0 Gg N yr−1 dwarf the plateau’s weak anthropogenic NOx sources of less than 5 Gg N yr−1, establishing soils as the dominant natural NOx source in the region. The HONO-to-NOx emission ratio for bare soil, 0.49, is consistent with the elevated daytime HONO/NOx concentration ratio of 0.31 ± 0.06 observed at Nam Co, which far exceeds the 0.02 to 0.05 expected from photo-stationary internal cycling alone. That chemical disequilibrium, the authors argue, is direct evidence that soil emissions are actively reshaping the plateau’s atmospheric nitrogen balance rather than merely adding to it.

Under the plateau’s intense sunlight, where photolysis rates for excited oxygen atoms reach twice their sea-level values, soil-emitted HONO rapidly splits to produce hydroxyl radicals, the atmosphere’s principal oxidizing agent, while soil NOx feeds tropospheric ozone formation. As the plateau continues to warm and wet, the authors warn that soil reactive nitrogen emissions are likely to increase, creating a positive feedback that strengthens atmospheric oxidation capacity, potentially exacerbating ozone pollution and altering the lifetime of greenhouse gases. Capturing this climate-chemistry feedback accurately in Earth system models, they conclude, will be essential for projecting air quality, ecosystem stability and nitrogen cycling across the Third Pole in the decades ahead.

Subject of Research: Soil emissions of reactive nitrogen gases and their atmospheric chemistry impacts on the Tibetan Plateau

Article Title: Measurement report: Soil reactive nitrogen gas emissions in the Tibetan Plateau

Article References: Deng, L., Chen, Y., Ye, C., Wang, R., Wang, R., Fu, Z., Zhao, H., & Wu, D. (2026). Measurement report: Soil reactive nitrogen gas emissions in the Tibetan Plateau. Atmospheric Chemistry and Physics, 26(20), 14617-14636. https://doi.org/10.5194/acp-26-14617-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14617-2026

Keywords: Tibetan Plateau, reactive nitrogen, nitrous acid, HONO, nitrogen oxides, soil emissions, atmospheric chemistry, ozone, microbial nitrification, denitrification, nitrogen cycle, air quality

The team measured nitrous oxide, carbon dioxide, and methane fluxes continuously above a 1.55-hectare cropland in Oensingen, canton of Solothurn, using the eddy covariance technique, a micrometeorological method that samples air at high frequency and integrates gas exchange over the entire field. The laser spectrometer and infrared gas analyzer mounted at the site captured half-hourly fluxes from September 2022 to October 2023, spanning the final month of a temporary grass-clover ley, the full winter wheat growing season, and a subsequent summer cover crop. This is, to the authors’ knowledge, the first study to report eddy covariance nitrous oxide fluxes over a complete winter wheat season, and the numbers it produced were sobering: cumulative emissions reached 5.5 kilograms of nitrous oxide nitrogen per hectare over the 282-day season, with yield-scaled emissions of 1.1 kilograms per tonne of grain, both figures well above values typically reported for wheat.

Raw flux data alone cannot explain why emissions spike when they do, so the researchers turned to machine learning. They trained an XGBoost model, a gradient-boosted tree ensemble, to predict nitrous oxide fluxes from a set of engineered predictors including soil temperature, water-filled pore space, precipitation, cumulative fertilizer nitrogen applied over the preceding 30 days, time since fertilization and soil preparation, and gross primary productivity, the ecosystem’s photosynthetic carbon uptake, which served as a proxy for crop activity and nitrogen demand. The model achieved an R-squared of 0.36 on an independent hold-out period, reproducing the timing of the main emission episodes even if it underestimated peak magnitudes. The real power came from interpreting the model with SHAP, or Shapley Additive exPlanations, a framework borrowed from game theory that decomposes every prediction into additive contributions from individual drivers.

The SHAP analysis did something no previous study had managed: it tracked, day by day, how the relative importance of each driver shifted across the cropping season. Over the full year, nitrogen fertilization was the dominant driver pushing emissions upward, accounting for 43 percent of all positive contributions, followed by soil moisture at 19 percent and gross primary productivity at 17 percent. But the temporal breakdown told a far richer story. During crop establishment in October 2022, when herbicide killed the grass-clover ley and winter wheat was direct-seeded, emissions were driven mainly by soil disturbance and low crop activity. Throughout winter, cold soils suppressed emissions, while low photosynthesis kept a persistent positive pressure on fluxes. In early spring, fertilization took over as the dominant force, with soil moisture amplifying the peaks.

Those spring peaks were dramatic. After the first fertilization events on 27 and 28 February, which supplied 111 kilograms of nitrogen per hectare as a mix of mineral ammonium nitrate and dairy slurry, fluxes surged in three distinct waves through March, peaking at a daily mean of 13.6 nanomoles per square meter per second on 12 March, with topsoil water-filled pore space above 80 percent. The critical insight is what was missing from the field at that moment: plants. Leaf area index was still below 1 square meter per square meter, and photosynthetic activity was minimal, meaning the crop could not yet absorb the nitrogen being poured into the soil. Soil mineral nitrogen climbed to 74 kilograms per hectare ten days after fertilization, an enormous microbial feast that wet, cool soil converted into nitrous oxide. By contrast, the smaller final application of 27 kilograms in early April, made when the canopy was already developed and nitrogen demand was high, produced only a modest emission pulse.

The study’s most elegant finding concerns what happened when the crop was growing hard. From mid-April to late May, when leaf area index rose from 2.5 to 4 and photosynthesis peaked, daily nitrous oxide fluxes fell to near zero despite warm soil and moisture levels above 60 percent throughout the profile. The SHAP dependence plot for gross primary productivity showed a clear shift from positive to negative contributions as productivity increased, revealing that active vegetation acts as a competing nitrogen sink, starving soil microbes of the substrate they need to produce the gas. Because soil moisture was already in the model, this effect cannot be explained away by transpiration drying the soil; it points to genuine substrate limitation, a plant-mediated brake on emissions that the authors interpret through the nitrogen synchrony framework, in which losses track the mismatch between nitrogen supply and crop demand.

To identify which microbes were actually making the gas, the team deployed static chambers and analyzed the stable isotope composition of the emitted nitrous oxide, focusing on site preference, the intramolecular distribution of nitrogen-15 within the molecule, which is largely independent of substrate composition and therefore a direct fingerprint of the production pathway. Combined with oxygen isotope ratios and a Monte Carlo mixing model called FRAME, the measurements showed that heterotrophic bacterial denitrification, possibly with nitrifier denitrification, dominated production, contributing 74 to 86 percent during the three weeks after fertilization. This made sense given the poorly drained silty clay soil, which kept water-filled pore space high and oxygen scarce. Intriguingly, the fraction of nitrous oxide further reduced to inert nitrogen gas rose from roughly 40 to 50 percent in the first week to about 78 percent by 17 March, suggesting that as conditions shifted, the microbial community became more thorough at finishing the job.

The greenhouse gas accounting added a further twist. Nitrous oxide emissions offset roughly 70 percent of the season’s net carbon dioxide uptake by the wheat, a far larger offset than the 10 to 12 percent reported for Swiss maize and pea in earlier eddy covariance work. When lateral carbon fluxes were included, the field emerged as a substantial net greenhouse gas source of 1349 grams of carbon dioxide equivalents per square meter, driven by the carbon exported in grain and straw. Even the fertilizer-free cover crop period emitted nitrous oxide at nearly the same daily rate as the wheat season, because post-harvest tillage and warm temperatures stimulated nitrogen mineralization while the slowly establishing crop could barely absorb it. Notably, the cover crop still halved post-harvest emissions compared with a year when the field was left bare, supporting Swiss policy that promotes continuous soil cover.

The practical implications are direct. The two most vulnerable windows, crop establishment and early spring, are both characterized by surplus soil mineral nitrogen meeting limited plant uptake under favorable moisture and temperature. Mitigation, the authors argue, should therefore focus on synchronizing nitrogen supply with demand: adjusted timing and split applications, site-specific and precision fertilization, enhanced-efficiency fertilizers, rapid soil cover after harvest, and avoiding the near-simultaneous application of mineral fertilizer and slurry, a combination that supplies both nitrate and easily degradable carbon to denitrifiers. Improving drainage on wet soils may also help by shortening the anaerobic episodes that favor denitrification. The XGBoost-SHAP framework itself is portable, requiring no site-specific parameterization, and the authors suggest coupling it with process-based biogeochemical models that could supply continuous simulated soil nitrogen as predictors. For a gas that has long evaded targeted control, the message is refreshingly concrete: it is not just how much nitrogen you apply, but when the crop is ready to use it.

Subject of Research: Temporal dynamics of nitrous oxide emission drivers and microbial sources in a fertilized winter wheat field

Article Title: What matters when? Temporal development of drivers and sources of nitrous oxide emissions in winter wheat

Article References: Turco, F., Feigenwinter, I., Allemann, L., Hörtnagl, L., Liebisch, F., & Buchmann, N. (2026). What matters when? Temporal development of drivers and sources of nitrous oxide emissions in winter wheat. Biogeosciences, 23(19), 7151-7182. https://doi.org/10.5194/bg-23-7151-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-7151-2026

Keywords: nitrous oxide, winter wheat, eddy covariance, machine learning, SHAP, denitrification, stable isotopes, greenhouse gases, nitrogen fertilization, soil moisture, cropland, climate mitigation

The research, led by Noémy Chénier of Empa, the Swiss Federal Laboratory for Science and Technology, together with colleagues at the University of Basel, focused on two closely related Pseudomonas species that are workhorses of denitrification research. Pseudomonas chlororaphis subsp. aureofaciens carries the copper-containing nitrite reductase NirK, while Pseudomonas chlororaphis subsp. chlororaphis carries the structurally distinct cytochrome cd1-type enzyme NirS. Both strains lack the enzyme NosZ, which normally consumes nitrous oxide, so the gas simply accumulates and can be measured cleanly. The team grew both organisms under strictly anoxic conditions and tracked the isotopic composition of the emitted nitrous oxide in real time using quantum cascade laser absorption spectroscopy, complemented by conventional isotope ratio mass spectrometry of the accumulated product.

Two isotopic quantities sit at the heart of the study. The first is site preference, the difference in nitrogen-15 abundance between the central and terminal nitrogen atoms within the linear nitrous oxide molecule. Because bacterial denitrification is widely assumed to yield site preference values near zero per mil through the canonical nitric oxide reductase NorB, deviations from this value are routinely interpreted as evidence for other production pathways such as fungal denitrification, hydroxylamine oxidation, or abiotic nitrite reactions. The second quantity is the oxygen-18 signature, which reflects how much of the oxygen in nitrous oxide came from the original nitrate versus from exchange with surrounding water before nitrite was reduced.

The most striking result concerns site preference. When the researchers measured the accumulated gas at the end of closed batch incubations, both species produced values clustered near zero per mil, exactly as the textbook picture predicts. But the continuous laser measurements told a different story. During the earliest phase of nitrous oxide production, site preference transiently climbed to values between roughly plus eight and plus twenty per mil before converging back toward zero as the reaction progressed. These excursions were reproducible, most clearly in the NirK-bearing strain, yet they would be completely invisible in any experiment that only measures the pooled product at the end.

The authors propose a plausible enzymatic explanation. Early in denitrification, rapid nitrate and nitrite reduction can transiently accumulate nitric oxide faster than NorB can handle it, while the expression of NorB itself may still be ramping up. Under such high nitric oxide stress, alternative detoxification enzymes, particularly flavohemoglobins, can step in and reduce nitric oxide to nitrous oxide with a distinctly elevated site preference, an effect demonstrated directly in recent work on engineered bacteria. Other candidate systems, including the hybrid cluster protein pathway and flavodiiron proteins, may also contribute. The implication is unsettling for isotope-based source apportionment: a site preference of five to fifteen per mil, often read as evidence of mixed or non-denitrification sources, could in principle arise entirely within a denitrifying cell simply because different nitric oxide reductases took turns.

The oxygen isotope results challenge an even more entrenched assumption. In the widely used denitrifier method, which converts nitrate to nitrous oxide for isotope analysis, the NirS-bearing species is known to exchange a large fraction of its oxygen atoms with water, while the NirK-bearing species was long thought to exchange almost none. That contrast has been extrapolated into a general rule that oxygen exchange behavior is dictated by nitrite reductase identity. The new experiments dismantled this rule. Under active growth, the NirK-bearing Pseudomonas aureofaciens exhibited oxygen exchange ranging from about 38 percent in gas-flushed bioreactor incubations to essentially complete exchange in closed batch cultures, vastly exceeding the values below nine percent reported under denitrifier-method conditions.

The NirS-bearing strain told a complementary story. Pseudomonas chlororaphis maintained consistently high but less variable exchange, around 66 percent, during active growth. Crucially, when the researchers harvested actively growing cells and resuspended them in a defined nitrate medium, mimicking the stationary-phase, washed-cell conditions of the denitrifier method, both strains reproduced exactly the canonical low- and high-exchange behavior reported in the classical literature. This demonstrates that the famous NirK-versus-NirS exchange dichotomy is not an intrinsic property of the enzymes at all, but an artifact of the specific physiological regime in which the denitrifier method operates.

What governs the exchange, the authors argue, is the lifetime of the nitrite intermediate pool. Oxygen exchange between nitrite and water requires nitrite to persist long enough to approach isotopic equilibrium with its surroundings. During active growth, when biomass synthesis and enzyme expression proceed simultaneously, the coupling between nitrate reduction and nitrite reduction loosens, nitrite lingers, and exchange increases. In resuspension assays, fully induced cells reduce nitrate rapidly and tightly, leaving nitrite little time to equilibrate, so nitrate-derived oxygen is retained. The oxygen-18 signature of nitrous oxide therefore behaves as a composite signal shaped by nitrate source, exchange dynamics, and branching isotope effects, all of which fluctuate with metabolic state rather than with enzyme identity.

The consequences ripple outward across biogeochemistry. Models that assign fixed isotopic endmember values to denitrification, nitrification, and abiotic pathways may misattribute sources in soils, oceans, groundwater, and wastewater treatment plants whenever the physiology of the microbial community shifts. The authors do not suggest discarding existing reference ranges, but they insist that physiological state, intermediate dynamics, and environmental context must be built explicitly into any interpretation of nitrous oxide isotopocules. Time-resolved measurements, they show, are essential, because pooled endpoint values can average away the very signals that reveal which enzymes were actually at work.

For a field that has relied on static isotopic fingerprints for more than two decades, the message is both humbling and energizing. Nitrous oxide isotope signatures are not fixed constants stamped by enzyme identity; they are conditional expressions of living metabolism. Pairing laser-based isotopocule monitoring with transcriptomics, proteomics, and targeted inhibitor experiments could finally connect the isotopic record to the specific enzymes operating at each moment, opening the door to genuinely mechanistic source attribution for one of the planet’s most important greenhouse gases.

Subject of Research: Isotopic variability of microbially produced nitrous oxide during bacterial denitrification

Article Title: High nitrous oxide isotopic variability during denitrification by Pseudomonas species bearing NirK and NirS

Article References: Chénier, N., Magyar, P. M., Zopfi, J., Frey, C., Kuhn, T., Lehmann, M. F., & Mohn, J. (2026). High nitrous oxide isotopic variability during denitrification by Pseudomonas species bearing NirK and NirS. Biogeosciences, 23(18), 6705-6724. https://doi.org/10.5194/bg-23-6705-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-6705-2026

Keywords: nitrous oxide, denitrification, isotopocules, site preference, Pseudomonas, NirK, NirS, oxygen isotope exchange, greenhouse gas, nitrogen cycle, biogeochemistry, QCLAS

The central challenge the researchers set out to solve is deceptively simple to state and notoriously hard to measure. When water carries too much nitrogen, in the form of nitrate from fertilizer runoff or wastewater discharge, aquatic ecosystems suffer. Water quality degrades, algal blooms erupt, and when those blooms die and decay, the decomposition consumes oxygen and can create low-oxygen dead zones that suffocate fish and other aquatic life. Microbes offer a natural cleanup service: through processes such as denitrification, they convert reactive nitrogen in the water into nitrogen gas, which escapes into the atmosphere and leaves the nutrient pool. The trouble is that the atmosphere itself is already dissolved in every body of water on the planet. Any sample of groundwater, lake water or seawater carries an enormous background load of atmospheric nitrogen gas, and the comparatively small amount produced by microbes is buried inside it like a whisper in a hurricane.

“The problem is that our water naturally has a huge amount of nitrogen gas that is dissolved from the air, so the gas that microbes produce can be very difficult to see,” explained first author Jiarui Liu, who conducted the research as a postdoctoral fellow at UC Santa Barbara’s Marine Science Institute and at UCLA. “The answer is written in the way nitrogen atoms are paired inside the nitrogen molecule.” That answer hinges on isotopes, the naturally occurring heavy and light variants of the same element. Nitrogen gas, or N2, consists of two nitrogen atoms joined by a strong triple bond. Those atoms come in two stable weights: the abundant nitrogen-14, with seven neutrons in its nucleus, and the heavier nitrogen-15, which carries one extra neutron and is roughly a hundred times rarer. Most N2 molecules in nature therefore contain two nitrogen-14 atoms. Some contain one nitrogen-14 and one nitrogen-15. And a vanishingly small fraction contain two nitrogen-15 atoms joined together, the molecule written as 15N15N.

Here is where the chemistry becomes genuinely surprising. If isotopes paired up purely at random, the frequency of the double-heavy molecule would simply reflect the product of the two individual isotope abundances. In atmospheric nitrogen gas, however, the two heavy nitrogen-15 atoms pair up far more often than chance alone would predict, a phenomenon scientists call isotopic clumping or mass-independent anomalous abundance. The physical origins of that atmospheric anomaly trace back to photochemical reactions high in the atmosphere, where ultraviolet light processes nitrogen compounds in ways that preferentially assemble the heavy-heavy pairs. Microbial nitrogen production, by contrast, is governed by enzyme kinetics that treat the isotopes nearly indiscriminately, so the nitrogen gas that microbes generate has its atoms paired essentially at random. When microbially produced nitrogen mixes into a water sample dominated by atmospheric nitrogen, it dilutes the excess of heavy pairs. The size of that dilution is a direct, quantitative record of how much microbial gas is present, even though the total amount of nitrogen gas in the sample looks unremarkable.

Reading that record, however, demands extraordinary analytical power. The team measured nitrogen gas extracted from water and sediment samples using UCLA’s Panorama mass spectrometer, one of the largest instruments of its kind in the world. A mass spectrometer uses electric and magnetic fields to separate molecules according to their mass and charge, allowing researchers to count molecules of different isotopic composition. The difficulty is that the rare heavy-heavy nitrogen molecules differ in mass from ordinary nitrogen gas by only the tiniest of margins, and other molecules with almost exactly the same mass can masquerade as the signal. Panorama’s unusually large magnets and flight paths give it the resolving power to separate species with these minuscule mass differences, making it one of the few instruments on Earth capable of the measurement. “At UCLA we discovered the anomalous pairing of heavy nitrogen atoms in Earth’s atmosphere and are now making use of this signature of nitrogen in air as a powerful and unique geochemical tool,” said geochemist Edward Young, a co-author of the paper and Liu’s postdoctoral advisor at UCLA.

To prove the technique’s versatility, the study brought together researchers with expertise spanning groundwater, lakes and marine environments, applying the method to an unusually diverse set of field sites. The team analyzed Texas groundwater, lakes in Antarctica and Minnesota, coastal basins off Southern California, the waters of the Bay of Bengal, and deep-sea sediments offshore from Alaska. Each of these settings poses a different question about nitrogen: aquifers ask whether subsurface microbes can strip nitrate from drinking water before it reaches wells and waterways; lakes and coastal basins ask how much nitrogen is removed before it can fuel algal blooms; and open-ocean and sediment environments feed directly into estimates of the global marine nitrogen budget. The fact that a single isotope-pairing measurement could be deployed across all of them suggests the approach is not a niche trick but a genuinely general tool for the nitrogen sciences.

The practical stakes are considerable. Human activity has become a dominant force in the modern nitrogen cycle, with fertilizer production, fossil fuel combustion and wastewater discharge adding fixed nitrogen to the environment at rates that rival or exceed natural sources. Excess nitrogen degrades water quality and fuels harmful algal blooms whose decay strips oxygen from the water, creating dead zones that threaten fisheries and aquatic ecosystems around the world. Understanding how much nitrogen microbes naturally remove, and where, helps scientists judge how people can intervene to reduce or remediate that harm. “We want to understand whether microbes in groundwater can mitigate nitrate pollution, and how much nitrogen is removed along the way before it can fuel algal growth in lakes, rivers and coastal waters,” Liu said. “This gives us a clearer picture of whether nitrogen stays in an ecosystem or is removed from the nutrient pool as N2 gas.”

The technique may also close a long-recognized blind spot in water monitoring. “Many routine groundwater-quality monitoring programs do not measure the N2 gas produced within an aquifer, which can create substantial biases in our accounting of where nitrogen comes from and where it goes,” said co-author Alan Seltzer, assistant professor of hydrogeology at University College Dublin. “This study, and this exciting new technique, opens the door to a much more complete picture of the sources and fate of nitrogen in groundwater systems.” Because the microbial gas is invisible to standard nitrate measurements once it has formed, conventional monitoring can overestimate the nitrogen remaining in an aquifer or misattribute its sources. Incorporating isotope-pairing measurements would allow water managers to distinguish nitrogen that has been genuinely removed from nitrogen that has merely changed form, sharpening the accounting on which regulatory and remediation decisions depend.

Beyond individual ecosystems, the broader ambition is to pin down the pace of the planet’s nitrogen cycle as a whole. Scientists construct nitrogen budgets that compare the rates at which usable nitrogen is supplied to the biosphere, through biological fixation, lightning and human industry, with the rates at which it is removed by microbial conversion back to gas. That balance influences the growth of the plants and microbes that sustain food webs worldwide, and yet the removal term has remained the least well constrained. “We study how nitrogen cycles on the planet, and we know that these processes of nitrogen removal occur,” said co-author David Valentine, the Norris Presidential Chair of Biogeochemistry at UC Santa Barbara and Liu’s postdoctoral advisor. “But it’s very difficult to figure out how quickly that’s happening in a given environment, which makes it hard to work out the global nitrogen budget. Our new approach gives us a direct measure of that loss.”

Combined with information about water transport and how nitrogen gas accumulates in different settings, the new measurements allow researchers to estimate nitrogen-loss rates at the level of whole ecosystems, and extending those estimates across environments can link local measurements to regional and global budgets. The heavy-heavy nitrogen molecule thus offers something the field has lacked: an independent way to test whether nitrogen inputs and losses actually balance across the planet, and how that balance shifts as human pressures on the cycle continue to grow. The sophisticated machinery required means the method will not appear in routine field monitoring kits any time soon, but as a research instrument it promises to redraw the maps scientists use to navigate one of Earth’s most consequential element cycles, one rare pair of atoms at a time.

Subject of Research: Isotopic clumping of nitrogen-15 pairs as a geochemical tracer of microbial nitrogen loss in the environment

Article Title: Where does Earth's nitrogen go? Two atoms offer a clue

Article References: Where does Earth's nitrogen go? Two atoms offer a clue. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: nitrogen cycle, nitrogen-15, isotopes, mass spectrometry, biogeochemistry, groundwater, water quality, denitrification, algal blooms, nitrogen budget, UC Santa Barbara, Science journal

The research team, led by Mengxue Zhang and Rong Sheng of the Institute of Subtropical Agriculture at the Chinese Academy of Sciences, built their experiment on a five-year field trial at the Taoyuan Agroecosystem Research Station in Hunan Province. There, plots had received straw at rates corresponding to 0, 50, 100, and 150 percent of the local rice straw yield. To isolate the biological mechanisms from the noise of weather and field variability, the scientists brought soil from each plot into the laboratory and ran controlled microcosm incubations lasting 91 days, with a 28-day flooded period followed by 63 days of drying—mirroring the wet-dry rhythm of a real rice season.

The fertility results were unambiguous. Five years of straw return had left its signature on the soil: bulk density fell, while cation exchange capacity, total carbon, total nitrogen, available nitrogen, available phosphorus, and available potassium all climbed with increasing straw rates. Even the lowest input, the half-rate treatment, cut bulk density by nearly 3 percent and lifted available potassium by more than 38 percent, available phosphorus by about 15 percent, and available nitrogen by nearly 7 percent relative to the no-straw control. Micronutrients such as manganese, copper, and zinc also rose. In short, the soil was demonstrably better at holding water, nutrients, and cations—the hallmarks of fertility.

But the greenhouse gas picture told a sharply different story depending on dose. Carbon dioxide fluxes rose in proportion to the straw input, with the half-rate treatment emitting 66 percent more CO2 than the control and the full and 150-percent rates reaching 2.2 and 2.5 times the control level. Methane, the more potent concern, was even more dose-sensitive: cumulative emissions from the half-rate treatment stayed low at 4.41 grams per square meter, while the full-rate and 150-percent treatments belched out roughly 23 and 38 grams per square meter—four and eight times higher, respectively. When the researchers converted all three gases into a single global warming potential metric using the latest 100-year conversion factors, the half-rate treatment showed only a 3 percent, statistically insignificant increase over the control, whereas the higher rates inflated the warming potential by up to 151 percent.

The key to this asymmetry lay in nitrous oxide, a greenhouse gas nearly 273 times more powerful than CO2 over a century. Counterintuitively, the half-rate treatment emitted significantly less N2O than the unfertilized-with-straw control—just 1.63 grams per square meter. The explanation emerged from the microbial genes the team quantified. N2O is produced by denitrifiers carrying the nirS and nirK genes and consumed by microbes carrying nosZ-type genes encoding N2O reductase. The half-rate soil harbored the highest abundance of the nosZII clade and the lowest nirS-to-nosZII ratio of any treatment, around 0.81 compared with 1.11 to 1.36 elsewhere. That combination—modest production capacity paired with maximal consumption capacity—meant the soil was effectively eating its own N2O before it could escape.

The timing of emissions added further nuance. Two distinct N2O peaks appeared during the incubation: one about 11 days after fertilization under flooded conditions, and another during the late drying phase. At the first peak, the control plot emitted the most N2O, likely because urea nitrogen sat available for nitrifiers and denitrifiers, while in straw-amended soils proliferating microbes competed fiercely for that same nitrogen. At the second peak, during drying, the order reversed as oxygen penetrated the profile and organic matter mineralization released fresh nitrogen. Yet even here, the half-rate soil kept its N2O fluxes as low as the control’s, thanks to its superior N2O-reducing community.

Methane told its own mechanistic tale. Methanogens, flagged by the mcrA gene, multiplied in proportion to straw input, with average abundances during the flux period rising 32 percent at the half rate and 129 percent at the highest rate. But production alone does not determine emissions—consumption by methanotrophs carrying the pmoA gene matters just as much. The researchers identified a critical threshold: a soil methane holding capacity of roughly 190 millimoles per mole under anaerobic conditions. Below that ceiling, retained methane could be oxidized in place; above it, the excess escaped to the atmosphere. The half-rate treatment’s soil methane concentrations fluctuated right around 200 millimoles per mole, hovering at the edge, while the higher rates blew well past the capacity. Meanwhile, the half-rate soil boasted methanotroph abundances about 81 percent higher than the control, giving it the oxidative muscle to mop up most of what its methanogens produced.

The broader significance of these findings is considerable. Rice cultivation accounts for roughly 12 percent of global agricultural greenhouse gas emissions, and straw incorporation has been identified as one of the key drivers, with reported methane increases in prior studies ranging from 23 percent to a staggering 1,192 percent. By demonstrating that the dose of straw—not merely its presence—governs the climate outcome, the study offers farmers and policymakers a concrete, actionable target: return about half the harvest. Previous work had hinted at this, showing that half-rate incorporation cut methane emissions by 48 percent relative to full incorporation, but the new research goes further by tying the climate benefit to specific microbial mechanisms and by simultaneously confirming that fertility gains are preserved.

The study also resolves a long-standing controversy in the literature. Reports on how straw affects N2O emissions have swung wildly, from 73 percent reductions to 137 percent increases, and the abundances of N2O-related genes have been reported to rise, fall, or remain unchanged. The Hunan results suggest that much of this inconsistency may stem from differing straw application rates across studies, which shift the balance between N2O-producing nirS communities and N2O-consuming nosZII communities in opposite directions. Similarly, conflicting findings on methanotroph responses—some studies found no change in pmoA abundance, others increases of up to 59 percent—may reflect the same dose dependence.

The authors caution that their conclusions come from a single soil type, an acidic Ultisol derived from quaternary red clay under a subtropical monsoon climate. Different soils possess different methane holding capacities and nitrogen transformation dynamics, so field tests across diverse soil types remain necessary before the half-rate prescription can be generalized worldwide. Still, the core message stands as a rare piece of good news in agricultural climate science: with the right amount of straw, rice paddies can grow more fertile without growing warmer, turning an agricultural waste problem into a climate-smart resource.

Subject of Research: Optimal rice straw incorporation rates for balancing paddy soil fertility and greenhouse gas emissions

Article Title: Mechanisms of enhancing soil fertility without obviously elevating global warming potential under an optimal rice straw incorporation rate in a paddy soil

Article References: Zhang, M., Liao, R., Zhang, W., Fang, C., Guerrero-Cruz, S., Táncsics, A., Zhu, B., Wei, W., & Sheng, R. (2026). Mechanisms of enhancing soil fertility without obviously elevating global warming potential under an optimal rice straw incorporation rate in a paddy soil. SOIL, 12(2), 871-883. https://doi.org/10.5194/soil-12-871-2026

Image Credits: AI Generated

DOI: 10.5194/soil-12-871-2026

Keywords: rice straw, paddy soil, soil fertility, greenhouse gases, methane, nitrous oxide, methanotrophs, denitrification, global warming potential, climate-smart agriculture, soil microbiology, sustainable farming

The trouble with these emissions is that they are notoriously difficult to pin down. Nitrous oxide from treatment plants emerges from the biological machinery that removes nitrogen from sewage, and that machinery involves at least three distinct microbial production pathways operated by two different bacterial groups. Ammonium-oxidizing bacteria can release the gas as a byproduct of hydroxylamine oxidation, the first step in converting ammonium to nitrite, or through nitrifier denitrification, in which they reduce nitrite to nitrous oxide using ammonium as the electron donor. Meanwhile, ordinary heterotrophic bacteria perform classical denitrification, coupling the oxidation of organic matter to the stepwise reduction of nitrate and nitrite, with nitrous oxide appearing as an obligatory intermediate. Crucially, the same heterotrophs can also act as a sink, reducing nitrous oxide all the way to harmless dinitrogen gas. Which of these processes dominates at any given moment depends on oxygen levels, substrate availability and reactor operation, and until now the tools for watching them in real time simply did not exist.

A team of researchers from Empa, Eawag, Vito and the University of Münster, led by Hannes Keck of Empa’s Laboratory for Air Pollution and Environmental Technology, has now demonstrated a way to do exactly that. Writing in the journal Atmospheric Measurement Techniques, they describe the first long-term application of an off-axis integrated cavity output spectrometer, or OA-ICOS, for continuous isotopic analysis of nitrous oxide at a pilot-scale wastewater treatment plant. The instrument, equipped with a quantum cascade laser probing absorption lines near 2192.3 inverse centimeters, recorded the abundance of the rare nitrous oxide isotopologues carrying nitrogen-15 in the central alpha position, nitrogen-15 in the terminal beta position, and oxygen-18, at a temporal resolution of up to one spectrum per second. Over twelve months and 998 hours of measurement time at two eight-cubic-meter sequencing batch reactors fed with real municipal wastewater in Dübendorf, Switzerland, the setup delivered isotopic data fast enough to follow the daily rhythms of the microbial community itself.

The power of the approach lies in what the isotopes reveal. Because the nitrous oxide molecule is asymmetric, the position of a nitrogen-15 atom within it carries information about how the molecule was made. Each production pathway leaves a characteristic imprint on the difference between the two nitrogen positions, a quantity known as site preference, and on the bulk nitrogen and oxygen isotope ratios. Hydroxylamine oxidation, for instance, typically produces high site preference values, whereas denitrification pathways yield lower ones. Reduction of nitrous oxide to dinitrogen adds a further twist: enzymes preferentially cleave bonds between lighter isotopes, so as reduction proceeds the remaining nitrous oxide pool becomes progressively enriched in heavy isotopes. By plotting site preference against the oxygen isotope ratio, researchers can read off both the source of the gas and how much of it has already been destroyed by the microbes before it escaped to the atmosphere.

Getting such measurements to work outside the laboratory required considerable engineering. Laser-based isotope analyzers are sensitive to more than just the target gas: pressure broadening by the surrounding gas matrix, spectral interference from carbon dioxide and methane, and non-linear detector responses all distort the apparent isotope ratios. The team built a custom dynamic dilution system with five mass flow controllers that diluted the reactor off-gas with nitrous-oxide-free synthetic air to a stable target of twelve parts per million, a compromise between analyzer sensitivity and the need to cover real emission events. Off-gas was dehumidified by permeation drying, stripped of carbon dioxide by chemical absorption, and filtered before entering the spectrometer. Calibration gases bracketed every sample measurement to correct for instrumental drift, and dedicated experiments quantified the analyzer’s dependence on nitrous oxide mole fraction and on oxygen content, which varies as aeration bubbles through the reactors. The resulting combined uncertainties, between 0.5 and 1.1 per mil across all measured quantities, met the team’s target of one per mil for source attribution work.

The first major result came from four months of monitoring under standard operating conditions, with dissolved oxygen held at two milligrams per liter. The isotopic signatures pointed clearly to denitrification, either nitrifier denitrification or heterotrophic denitrification, as the dominant source of nitrous oxide, consistent with earlier full-scale studies. Just as telling was what was absent: no elevated site preference values above thirty per mil appeared, indicating that hydroxylamine oxidation played no significant role during the measurement periods. The data also traced a clear reduction line, a linear relationship between site preference and oxygen isotopes with a slope of 0.37, falling within the range expected for progressive microbial reduction of nitrous oxide to dinitrogen. In other words, the plant was simultaneously producing and destroying the greenhouse gas, and the balance between the two determined what ultimately reached the sky.

To probe that balance directly, the researchers ran a paired experiment in which one reactor operated at high dissolved oxygen and the other at a low setpoint of 0.5 milligrams per liter, with nitrite added to stimulate nitrous oxide production. The low-oxygen reactor progressed much further along the reduction line, and calculations based on the isotopic enrichment showed that the fraction of nitrous oxide converted to dinitrogen increased by forty percent compared with the high-oxygen reactor. Over the course of a single aeration phase, the low-oxygen reactor’s reduction fraction climbed from thirty-five to fifty percent before settling near forty, while the high-oxygen reactor’s fell steadily from ten percent to zero. The explanation is enzymatic: the copper-sulfur cluster of nitrous oxide reductase, the enzyme that performs the final step of denitrification, is notoriously oxygen-sensitive, so suppressing aeration gives the microbes a chance to finish the job they started.

This finding carries immediate practical weight. Dissolved oxygen setpoints are among the most controllable parameters in plant operation, and the new data suggest that lowering them during critical periods could tip the microbial balance toward destruction of nitrous oxide rather than its release. The isotopic approach makes this actionable in near real time, because operators can see not just how much gas is escaping but which process is responsible and whether mitigation is working. The team also demonstrated a third capability: low-level nitrogen-15 labeling, in which small amounts of tagged ammonium are added to raise isotope values just above natural abundance. When they spiked a reactor with ammonium enriched to about one hundred per mil, the emitted nitrous oxide showed a corresponding jump in bulk nitrogen isotope values, from around minus forty per mil under standard operation to 6.5 per mil, tracing the labeled nitrogen through nitrite and nitrate into the gas phase and confirming the setup’s suitability for tracing specific microbial conversions.

The researchers are candid about the limitations. More than half of the raw data had to be discarded because nitrous oxide mole fractions drifted outside the narrow eleven-to-thirteen-ppm window where corrections are reliable, particularly during rapid emission spikes. Carbon dioxide traps occasionally broke through before reaching capacity, and suitable isotopic calibration standards at process-relevant concentrations are not yet commercially available. Yet the authors argue that the alternative, manual bag sampling followed by laboratory analysis, carries comparable costs while sacrificing the temporal resolution needed to capture the strong daily and seasonal dynamics of plant emissions. The spectroscopic platform is also scalable: measurement frequency can be increased, multiple reactors can be analyzed sequentially, and with minor adaptations the system could move from pilot scale to full-scale plants, where isotopic data could feed directly into operational control strategies.

What emerges from this year-long experiment is a template for turning wastewater plants from opaque emission sources into observable, optimizable systems. The isotopic fingerprints showed that denitrification, not hydroxylamine oxidation, drives the bulk of nitrous oxide production, that oxygen availability strongly controls how much of the gas the microbes manage to destroy themselves, and that operational levers like dissolved oxygen setpoints can be tuned with evidence rather than guesswork. As countries revise their emission inventories upward and plants face pressure to cut their climate footprint, tools that reveal the invisible microbial accounting of nitrogen in real time may prove essential. The laser in a Swiss pilot reactor has, for the first time, made that accounting visible, one five-minute measurement at a time.

Subject of Research: Real-time isotopic analysis of nitrous oxide production and reduction during biological nitrogen removal in wastewater treatment

Article Title: On-line analysis of N2O isotopic composition during biological nitrogen removal in wastewater treatment to disentangle production and reduction processes

Article References: Keck, H., Strubbe, L., Magyar, P. M., Joss, A., Froemelt, A., Kupferschmid, A., Knorr, K.-H., & Mohn, J. (2026). On-line analysis of N 2 O isotopic composition during biological nitrogen removal in wastewater treatment to disentangle production and reduction processes. Atmospheric Measurement Techniques, 19(19), 6311-6326. https://doi.org/10.5194/amt-19-6311-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-6311-2026

Keywords: nitrous oxide, wastewater treatment, stable isotopes, laser spectroscopy, denitrification, nitrification, greenhouse gas emissions, site preference, dissolved oxygen, OA-ICOS, nitrogen removal, isotopocule analysis

The research, led by corresponding author Lili Wang and colleagues including Qiannan Yang, Guilong Zhang, Jie Li, Hu Li, and Lukas Van Zwieten, examined an alkaline paddy field in Tianjin, China, where rice-straw biochar had been applied annually for more than five years. Rather than sampling only the familiar topsoil layer where most agronomic studies stop, the team collected soil from four depths extending from the surface down to 80 centimeters below ground. This vertical perspective proved essential, because the study’s central finding is that biochar’s influence on nitrogen cycling changes dramatically with depth.

Using a nitrogen-15 tracer technique, the researchers simultaneously quantified three major nitrate reduction processes that operate in waterlogged paddy soils. The first is denitrification, in which microbes convert nitrate step by step into nitrogen gases that escape to the atmosphere. The second is anaerobic ammonium oxidation, or anammox, a process in which ammonium and nitrate react to produce dinitrogen gas, again removing reactive nitrogen from the system. The third is dissimilatory nitrate reduction to ammonium, known as DNRA, a fermentation-like pathway that reduces nitrate all the way back to ammonium. From a farmer’s standpoint, these pathways could hardly matter more: DNRA keeps nitrogen in the soil where roots can potentially access it, while denitrification and anammox effectively bleed nitrogen out of the field.

The study revealed a striking depth-dependent pattern in how these three pathways partition the nitrate supply. Across the soil profile, the contribution of DNRA increased from 25.7 percent in the surface layer to as high as 91.1 percent at depth, while the contribution of denitrification fell from 67.9 percent to just 7.2 percent. In other words, the deeper the soil, the more nitrate reduction shifted from a nitrogen-losing process to a nitrogen-retaining one. Biochar amplified this trend. Under the biochar treatment, DNRA became increasingly favored below the surface, and in the deepest layer sampled, 60 to 80 centimeters below ground, biochar increased the DNRA contribution by 37.0 percent while reducing the contribution of denitrification by 27.8 percent.

Corresponding author Lili Wang emphasized that this depth sensitivity is the study’s key message. As she put it, the effect of biochar on nitrogen cycling cannot be understood by looking only at the surface soil. As soil depth increased, biochar shifted nitrate reduction toward the pathway that retains nitrogen as ammonium, while suppressing the pathways associated with nitrogen loss. For years, biochar research has concentrated overwhelmingly on the top 20 or 30 centimeters of soil, where most sampling protocols end. The new results suggest that this focus may have missed some of the most consequential chemistry happening far below the plow layer.

Why would biochar favor one microbial pathway over another, and why would that preference change with depth? To answer this, the researchers examined the environmental conditions and microbial communities at each depth. Statistical modeling identified several important controls on the pathway balance: soil pH, the ratio of soil organic carbon to nitrate, dissolved organic carbon, and ferrous iron. These factors are known to shape the competitive hierarchy among nitrate-reducing microbes. Organisms capable of DNRA tend to thrive when carbon is abundant relative to nitrate, while denitrifiers often dominate under different carbon and oxygen regimes. Iron chemistry adds another layer of complexity, because ferrous iron can participate in chemically coupling nitrate reduction to iron oxidation in flooded soils.

Biochar altered these environmental conditions differently at different depths, and those alterations cascaded through the nitrogen-transforming microbial communities and the functional genes they carry. In the surface soil, biochar stimulated several nitrate reduction processes at once, apparently by improving carbon availability and modifying pH in the zone where the amendment was concentrated. Deeper in the profile, however, the picture changed. The soil became more alkaline and carbon availability declined with depth. These conditions increasingly suppressed denitrification and anammox while allowing DNRA to account for a larger and larger share of nitrate transformation. The result is a vertical gradient in nitrogen fate that no surface-only study could have detected.

The practical implications extend well beyond the paddy fields of Tianjin. Nitrogen transformations below the plow layer can determine whether nitrate remains in the soil, moves downward toward groundwater, or is converted into gaseous products that contribute to greenhouse gas emissions and the loss of fertilizer value. In alkaline rice-growing regions, where high pH already shapes microbial activity in distinctive ways, the finding that long-term biochar application may help conserve nitrogen in deeper horizons offers a potential strategy for improving nitrogen use efficiency. More ammonium retained in the profile means less fertilizer needed, fewer nitrate pulses into aquifers, and potentially reduced emissions of nitrous oxide, a potent greenhouse gas produced during denitrification.

The authors are careful to note that biochar’s effects are not universal. They caution that the outcomes depend strongly on soil depth, initial soil pH, the aging of the biochar over successive seasons, and changes in available carbon. Biochar is not chemically static; over years in a flooded field it weathers, its labile carbon fraction diminishes, and its surface chemistry evolves. The Tianjin experiment captured the consequences of more than five years of annual application, but the researchers stress that future studies should compare fresh and naturally aged biochar directly, and should measure nitrogen leaching and gaseous nitrogen losses explicitly rather than inferring them from pathway partitioning alone. Such measurements would close the loop between microbial process rates and actual field-scale nitrogen balances.

What the study delivers now is mechanistic evidence for a principle that soil scientists have increasingly advocated: biochar-based nitrogen management should consider the entire soil profile rather than focusing only on topsoil. The nitrogen-15 tracer approach allowed the team to watch three competing pathways respond in real time to five years of amendment, and the answer that emerged is unusually clear. In deep alkaline paddy soil, biochar nudged the microbial economy toward conservation—toward ammonium that stays put rather than gases that drift away. For a crop grown on roughly 160 million hectares worldwide, much of it in alkaline or calcareous soils, that shift could translate into meaningful gains in fertilizer efficiency and meaningful reductions in nitrogen pollution. The deeper soil, long treated as a black box beneath the sampling auger, turns out to be where some of biochar’s most valuable work is done.

Subject of Research: Long-term biochar effects on nitrate reduction pathways and nitrogen retention in deep alkaline paddy soils

Article Title: Long-term biochar use may help conserve nitrogen deep in alkaline paddy soils

Article References: Long-term biochar use may help conserve nitrogen deep in alkaline paddy soils. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: biochar, nitrogen cycling, paddy soil, DNRA, denitrification, anammox, nitrogen-15 tracer, alkaline soil, soil depth, nitrogen retention, rice straw, soil microbiology

The physics of the problem is deceptively simple. Gas diffusion through water is roughly 10,000 times slower than through air, so the moment soil becomes saturated, the oxygen supply to roots and microbes collapses. Root respiration and microbial metabolism rapidly consume whatever oxygen remains, pushing the rhizosphere from hypoxic to fully anoxic conditions within hours. As oxygen disappears, the soil’s redox potential, a measure of its capacity to accept or donate electrons, plummets. This single chemical variable cascades through the entire soil ecosystem, forcing microorganisms to abandon oxygen as their terminal electron acceptor and turn to alternatives such as nitrate, ferric iron, sulfate, and ultimately carbon dioxide itself.

Plants are not passive victims of this suffocation. The review details how roots deploy a suite of adaptive strategies governed by an elegant signaling network. Trapped ethylene, which accumulates because its diffusion out of submerged tissues is blocked, acts as an early warning signal. It induces PHYTOGLOBIN1, which lowers nitric oxide levels, and stabilizes a family of transcription factors called ERFVIIs, whose stability is controlled by the oxygen-dependent N-end rule pathway. This molecular priming prepares the plant for low-oxygen stress before conditions become lethal. In parallel, reactive oxygen species generated by NADPH oxidase enzymes drive developmental changes such as aerenchyma formation, the creation of air channels within root tissue that allow oxygen to travel internally from shoots to roots, and barriers that prevent that precious oxygen from leaking into the surrounding reduced soil.

Meanwhile, root metabolism undergoes a fundamental reprogramming. Deprived of oxygen, roots shift from aerobic respiration to fermentation, activating enzymes such as pyruvate decarboxylase, alcohol dehydrogenase, and lactate dehydrogenase to squeeze out whatever energy they can. The byproducts of this anaerobic metabolism, including ethanol, lactate, succinate, malate, and alanine, accumulate in root tissues and are released into the rhizosphere. The authors are careful to note a critical methodological distinction that has muddied the literature: metabolites detected inside roots or in xylem sap reflect internal metabolism and transport, whereas only compounds recovered from collected exudate fractions provide direct evidence of release from roots. Treating these compartments as interchangeable, they argue, has led to widespread misinterpretation.

Those released fermentation products are anything but metabolic waste. In the oxygen-starved rhizosphere, compounds such as ethanol and lactate serve as readily metabolizable carbon sources and electron donors, fueling anaerobic food webs and selectively favoring microbial groups equipped to exploit them. Sulfate-reducing bacteria and methanogenic archaea proliferate in this carbon-rich, oxygen-poor environment. Alanine plays a particularly intriguing role: produced by alanine aminotransferase, it acts as a stable reservoir of carbon and nitrogen that can be metabolically neutralized during reoxygenation, directly linking the hypoxic phase to post-flood recovery. The review cautions, however, that the causal contribution of specific root-derived metabolites to specific microbial functional shifts remains a plausible interpretation that still requires direct experimental validation.

The microbial consequences are dramatic and well documented. In waterlogged wheat systems, researchers have recorded enrichment of anaerobic taxa including Firmicutes, a decline of beneficial Actinobacteria and Proteobacteria, and an increased abundance of potential pathogens. The review frames these shifts through the conceptual lens of the redox tower, the thermodynamic hierarchy of electron acceptors that governs microbial competition. Once oxygen is depleted, denitrifiers gain the advantage, reducing nitrate stepwise to nitrous oxide or dinitrogen gas. As redox potential continues to fall, iron-reducing and sulfate-reducing bacteria take over, driving coupled sulfur and iron cycling. In strongly reducing soils, methanogenic archaea become the terminal consumers, generating methane from acetate, hydrogen, and carbon dioxide, a process especially pronounced in rice paddies and organic-rich wetlands with direct implications for greenhouse gas emissions.

Against this backdrop, the review examines plant growth-promoting rhizobacteria, or PGPR, as potential allies for flooded crops. Strains producing the enzyme ACC deaminase can degrade the immediate precursor of ethylene, moderating the ethylene-driven growth inhibition, accelerated senescence, and chlorosis that accompany waterlogging. Other beneficial traits include the production of extracellular polymeric substances that stabilize soil aggregates and create localized microenvironments at the root surface that are oxygen-limited but not fully anoxic. PGPR can also activate the plant’s own antioxidant defenses, boosting enzymes such as superoxide dismutase, catalase, and ascorbate peroxidase; basil plants inoculated with Pseudomonas or Azotobacter strains showed significantly higher superoxide dismutase activity under flooding than uninoculated controls. Yet the authors highlight a persistent problem: single-strain inoculants often fail to persist in saturated soils, where rapidly shifting redox conditions and fierce competition from indigenous anaerobes overwhelm introduced strains.

Their proposed solution is a shift from single-trait optimization toward synthetic microbial communities, or SynComs, that combine complementary functions such as ACC deaminase activity, exopolysaccharide production, nitrogen fixation, phosphate solubilization, and facultative anaerobic respiration. Such functionally redundant consortia, the review argues, are more likely to remain active across the fluctuating redox gradients that define flooded rhizospheres. Metagenome-informed strain selection and machine learning approaches could improve the odds of persistence by prioritizing traits associated with environmental adaptation. Interestingly, short-term flooding may even offer a temporary nutritional bonus: in heavily weathered soils, microbial iron reduction dissolves iron oxides and releases bound phosphorus, with studies documenting simultaneous increases in dissolved iron, phosphate, and dissolved organic carbon during the initial flooding phase.

Perhaps the most underappreciated insight concerns what happens after the water recedes. The re-aeration phase, the review argues, is a critical window of opportunity comparable in importance to the initial flooding response, yet it remains poorly studied. As oxygen returns, root exudation shifts from fermentation products back toward the sugars, amino acids, and organic acids of aerobic metabolism, and this changing chemical signature drives microbial reassembly, favoring aerobic taxa such as Actinobacteria, Proteobacteria, and mycorrhizal fungi while obligate anaerobes decline. Repeated flooding may also build biological memory: plants can retain biochemical and regulatory imprints that enhance antioxidant capacity in subsequent events, while microbial communities adapted to redox swings can be reassembled more quickly, a phenomenon known as the microbiome legacy effect that may even influence the stress responses of following plant generations.

The authors conclude that meaningful progress will require abandoning single-component fixes in favor of integrated plant-microbe-soil system engineering. They call for multi-omics approaches combining root metabolomics, transcriptomics, and metagenomics with high-frequency redox monitoring, alongside stable isotope tracing and spatially resolved imaging to map the actual flux of metabolites between roots and microbes. Predictive tools, including rhizosphere digital twins and process-based ecosystem models, could help translate laboratory insights into field applications, though long-term validation in flood-prone agroecosystems remains essential. With the IPCC projecting a 1.5 to twofold increase in extreme precipitation by mid-century, the ability to stabilize rhizosphere function across flooding and recovery cycles may become a defining challenge for global food security, and the invisible chemistry of root exudates may hold more of the answer than anyone suspected.

Subject of Research: Root exudate redox dynamics and rhizosphere microbial responses to flooding

Article Title: Root exudate redox dynamics under flooding: emerging insights into rhizosphere microbial interactions and plant–soil responses

Article References: Kang, S.-M., Khan, I., Woo, J.-I., Park, Y.-S., & Lee, I.-J. (2026). Root exudate redox dynamics under flooding: emerging insights into rhizosphere microbial interactions and plant–soil responses. Plant and Soil. https://doi.org/10.1007/s11104-026-09129-8

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09129-8

Keywords: rhizosphere, flooding stress, root exudates, redox potential, hypoxia, PGPR, methanogenesis, denitrification, microbiome, SynComs, plant stress memory, ACC deaminase