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 scale of the salinity problem is well documented. Salt-affected soils now cover vast expanses of every inhabited continent, a situation catalogued most recently in the Food and Agriculture Organization’s global assessment of salt-affected soils published in 2024. Estimates compiled by researchers using remote sensing and global datasets suggest that roughly a billion hectares of land are affected by salinity or sodicity, and the extent continues to grow as irrigation, climate change and poor drainage conspire to concentrate dissolved salts in the root zone. In irrigated agriculture, which supplies a disproportionate share of the world’s food, the risk is acute: every pass of irrigation water delivers a small load of dissolved salts, and without adequate leaching and drainage those salts accumulate season after season.

Salinity damages plants through well-understood physiological mechanisms. Excess salts in the soil solution raise the osmotic pressure of the soil water, making it harder for roots to extract moisture, a stress that resembles drought even in wet fields. Specific ions, particularly sodium and chloride, become toxic at high internal concentrations, disrupting enzyme function, photosynthesis and membrane transport. Plants respond by closing their stomata, slowing growth and diverting energy into defensive osmotic adjustment. The result is reduced biomass, lower yields and, crucially for the nitrogen argument, a diminished capacity of the crop to take up and assimilate nutrients from the soil.

It is this last consequence that Tarolli and Masin place at the centre of their commentary. When crop growth is suppressed by salt stress, the plant’s demand for nitrogen falls, but the nitrogen supplied to the field does not. Fertilizer applied at rates calibrated for healthy, unstressed crops meets a root system that can no longer absorb it efficiently. The surplus nitrogen does not simply wait in the soil for better times; it is mobile, and it moves. Nitrate leaches downward with percolating water toward groundwater, while microbial processes convert ammonium and nitrate into gaseous forms, including nitrous oxide, a greenhouse gas nearly three hundred times more potent than carbon dioxide over a century. Saline soils, with their altered microbial communities and often impaired structure, can be particularly prone to these gaseous losses.

The commentary draws on a growing body of evidence linking salinity to nitrogen dynamics. Recent work cited by the authors includes a 2025 review in Environmental Research Letters by Ghirardelli and colleagues examining the interactions between salt-affected soils and nutrient cycling, and a study published in Nature Food by Wen and colleagues in 2026 that quantifies how salinity depresses nitrogen-use efficiency and elevates reactive-nitrogen losses. Earlier research, including analyses published in Global Change Biology, had already established that salt stress reduces nitrogen uptake by crops and shifts the balance of nitrogen transformations in the soil. Taken together, these studies sketch a feedback loop with troubling implications: salinization degrades nitrogen-use efficiency, degraded nitrogen efficiency demands either more fertilizer or acceptance of yield losses, and additional fertilizer in salt-stressed fields leaks into water and air, compounding the environmental burden.

The concept of reactive nitrogen is central to understanding why this matters beyond the farm gate. Reactive nitrogen refers to all the chemically active forms of the element, including ammonia, nitrate and nitrogen oxides, that drive a cascade of environmental problems known as the nitrogen cascade. A landmark 2023 analysis in Nature by Gu and colleagues quantified the full costs of reactive nitrogen losses, tracing how a single molecule of nitrogen fertilizer can contribute in sequence to air pollution, ecosystem acidification, eutrophication of waterways, stratospheric ozone depletion and climate warming. Global synthetic nitrogen fertilizer use now exceeds one hundred million tonnes per year, and only a fraction of that nitrogen, often less than half, ends up in harvested products. Anything that pushes that efficiency lower, as salinity demonstrably does, multiplies the environmental and economic waste embedded in every bag of fertilizer.

Tarolli and Masin’s argument also carries a monitoring dimension. Their own research programme has explored how remote sensing and geospatial technologies can detect and map soil salinity across landscapes, work published in iScience in 2024 and extended in a 2026 study in the ISPRS Journal of Photogrammetry and Remote Sensing with Xue, Ghirardelli and Chen. Satellite and drone-based sensors can pick up the spectral signatures of salt accumulation and of vegetation stress, offering a way to identify fields where the salt-nitrogen interaction is likely to be eroding efficiency. The commentators suggest that such tools could be integrated into nutrient management, allowing farmers and advisers to adjust nitrogen applications in real time as salinity stress develops, rather than applying fertilizer on a fixed schedule that assumes ideal soil conditions.

The deeper message of the commentary is a call for integration. Soil salinity, water management and nutrient management have traditionally been treated as separate technical domains, addressed by different specialists, different policies and different parts of the agricultural research establishment. The nitrogen costs of salinity expose the weakness of that fragmentation. Drainage and leaching strategies that control salt accumulation also control the pathways by which nitrate escapes to groundwater. Irrigation scheduling that avoids waterlogging and salt concentration also protects the microbial processes that govern nitrogen availability. Fertilizer recommendations that account for salinity stress, rather than ignoring it, could simultaneously protect yields and cut reactive-nitrogen losses. The authors argue that managing soil, water and nutrients together is not merely desirable but necessary if the world is to feed a growing population without pushing the nitrogen cycle further out of balance.

The stakes are considerable. Salt-affected soils are expanding by millions of hectares each year according to global assessments, driven by seawater intrusion into coastal aquifers, melting permafrost releasing stored salts, unsustainable irrigation in arid basins and the over-extraction of groundwater that draws saline water upward. Each newly salinized hectare represents not only lost productive capacity but also, on the argument advanced in this commentary, a new source of nitrogen inefficiency and pollution. Conversely, reclaiming salt-affected soils through improved drainage, gypsum amendments, salt-tolerant crops and precision irrigation would deliver a double dividend: restored yields and improved nitrogen-use efficiency, with corresponding reductions in nitrous oxide emissions and water pollution.

For policymakers, the commentary lands at a moment when both nitrogen and salinity are climbing international agendas. The Kunming-Montreal Global Biodiversity Framework includes a target to halve nutrient pollution by 2030, and the FAO’s salt-affected soils assessment has prompted calls for national action plans. Tarolli and Masin’s analysis suggests that these two agendas should be pursued jointly rather than in parallel silos. Every investment in salinity control, they imply, is also an investment in nitrogen efficiency, and every nitrogen policy that ignores salinity risks overestimating the nutrient uptake that farmers can realistically achieve. The hidden nitrogen bill of soil salinity is now on the table, and settling it will require the kind of joined-up thinking that agricultural science has long preached and rarely practised.

Subject of Research: The impact of soil salinization on nitrogen-use efficiency and reactive-nitrogen losses in agricultural systems

Article Title: The nitrogen costs of soil salinity

Article References: Tarolli, P., & Masin, R. (2026). The nitrogen costs of soil salinity. Nature Food. https://doi.org/10.1038/s43016-026-01434-w

Image Credits: AI Generated

DOI: 10.1038/s43016-026-01434-w

Keywords: soil salinity, nitrogen-use efficiency, reactive nitrogen, soil salinization, agriculture, nitrate leaching, nitrous oxide, irrigation, soil management, remote sensing, food security, nutrient pollution

The study, led by Wen Wen and Baojing Gu of Zhejiang University together with colleagues at the International Institute for Applied Systems Analysis and the University of Maryland, set out to close a conspicuous gap in the global nitrogen accounting. Scientists have long known that salinity disrupts the microbial processes that govern nitrogen cycling in soil, but the combined effect of salinization on crop productivity and nitrogen losses had not been comprehensively measured at the global scale. To do so, the team built a spatially explicit nitrogen budget for the world’s saline croplands, combining a global soil salinity map derived from Hassani and colleagues with cropland extent data from the European Space Agency’s Climate Change Initiative land-use product. Nitrogen flows were modelled at a resolution of 0.5 degrees by 0.5 degrees using the IMAGE integrated assessment model and the CHANS nitrogen budget model, while satellite-derived vegetation indices from MODIS helped characterize crop conditions across affected regions.

The headline findings are stark. The researchers applied a pairwise comparison framework, matching observations from salt-affected and non-affected fields to isolate the salinity signal, and found that soil salinity raises nitrogen inputs on saline croplands by 13 percent while reducing the nitrogen actually harvested in crops by 7 percent. That mismatch, more nitrogen going in and less coming out as food, has to go somewhere. The analysis shows it escapes into the environment: reactive nitrogen losses from saline croplands are amplified by 61 percent compared with what would be expected from non-saline conditions. In practical terms, farmers on salt-affected land are applying more fertilizer to compensate for poor yields, yet the stressed soil microbial communities and impaired crop uptake mean a far larger share of that nitrogen ends up as ammonia volatilization, nitrous oxide emissions, nitrogen oxides and nitrate leaching.

The mechanisms behind this leakage are increasingly well documented in the underlying literature. Salt stress suppresses the genes and microbial communities responsible for nitrification and denitrification, shifting the balance of nitrogen transformations in the rhizosphere. Previous meta-analyses have shown that salinization drives concomitant increases in soil ammonia volatilization and nitrous oxide emission, a particularly troubling combination because nitrous oxide is a greenhouse gas roughly 300 times more potent than carbon dioxide over a century. Salinity also impairs root development and water uptake, reducing the plant’s ability to capture nitrogen before it escapes, and altered soil chemistry changes how ammonium exchanges between soil particles and soil water in the root zone.

What distinguishes the new work is not merely the diagnosis but the prescription. The researchers synthesized ten categories of salinity-management practices, ranging from improved irrigation and drainage schemes that leach salts below the root zone, to the application of soil amendments such as biochar, to biological approaches including halophytoremediation and the use of salt-tolerant crop varieties. Their meta-analysis of field studies found that widespread adoption of these practices could reduce annual reactive nitrogen losses from saline croplands by 59 percent, equivalent to 2.5 million tonnes of nitrogen per year, while simultaneously increasing crop production. The scale of that reduction is comparable to the total nitrogen mitigation potential of some entire national agricultural systems, suggesting that saline soil management deserves a place alongside fertilizer optimization and manure management in the global toolkit for nitrogen pollution control.

The economics are equally compelling. The team’s cost-benefit analysis estimates net global benefits of 24.5 billion US dollars per year, with an uncertainty range of plus or minus 11.8 billion, once increased crop output, avoided nitrogen losses and implementation costs are all accounted for. Because saline soils are concentrated in some of the world’s most productive irrigated regions, the benefits are not evenly distributed. China, Pakistan and Indonesia emerge as the countries with the largest aggregate mitigation opportunities, reflecting the vast extent of salt-affected irrigated cropland in their major river basins. Egypt, by contrast, stands out as the leading hotspot for per-hectare benefits, a reflection of the intensity of its irrigated agriculture and the severity of salinity in the Nile Delta, where every hectare restored delivers outsized gains in both yield and pollution reduction.

These findings arrive at a moment of growing urgency. The Food and Agriculture Organization’s 2024 Global Status of Salt-Affected Soils report documented the accelerating spread of salinization, driven by climate change, sea-level rise, over-extraction of groundwater and unsustainable irrigation. Climate models suggest that arid and semi-arid farming regions will face increasing pressure to irrigate with water of declining quality, while coastal deltas from Bangladesh to Egypt face saltwater intrusion. Left unmanaged, this trajectory threatens both the yields on which growing populations depend and the environmental integrity of the watersheds that drain from salt-affected basins. The new analysis reframes salinization not only as a production problem but as a pollution problem, adding a substantial and previously uncounted flux to the global reactive nitrogen budget.

The study also carries implications for climate policy. Because nitrous oxide from saline croplands is a significant contributor to agricultural greenhouse gas emissions, the finding that management practices can cut reactive nitrogen losses by more than half positions saline soil restoration as a candidate for inclusion in climate-smart agriculture frameworks. The authors argue that interventions which simultaneously raise productivity, reduce emissions and deliver positive economic returns are precisely the kind of no-regret investments that international climate finance mechanisms are designed to support. The methodological foundation of the work, drawing on established nitrogen budget models previously used to assess cost-effective mitigation of nitrogen pollution from global croplands, gives the projections a degree of credibility that purely correlative studies lack.

Challenges remain, of course. The ten categories of management practices vary widely in cost, technical difficulty and suitability across different soil types, water regimes and farming systems, and the meta-analytic effect sizes drawn from field trials may not translate uniformly to every salt-affected landscape. Implementing drainage and leaching schemes requires water infrastructure that many smallholder systems lack, and some amendments such as biochar remain expensive at scale. Yet the study’s central message is difficult to ignore: the world’s saline soils, long written off as marginal land, represent one of the largest untapped opportunities to grow more food while polluting less. As nations prepare to strengthen their commitments on both food security and nitrogen pollution, the humble salt-affected hectare may prove to be among the most valuable real estate in global agriculture.

Subject of Research: Global quantification of soil salinity impacts on crop yields and reactive nitrogen losses, and the mitigation potential of saline soil management practices

Article Title: Global management of saline soils to boost crop yields and halve nitrogen losses

Article References: Wen, W., Cheng, L., Zou, Y., Wang, C., Zhang, X., Gao, Y., Cui, J., & Gu, B. (2026). Global management of saline soils to boost crop yields and halve nitrogen losses. Nature Food. https://doi.org/10.1038/s43016-026-01428-8

Image Credits: AI Generated

DOI: 10.1038/s43016-026-01428-8

Keywords: soil salinity, nitrogen pollution, crop yields, saline croplands, reactive nitrogen, nitrous oxide, climate-smart agriculture, irrigation, biochar, food security, Nature Food, cost-benefit analysis

The synthesis, led by Lei Liu and Xuejun Liu of China Agricultural University together with an international team spanning more than twenty institutions, compiles evidence from national monitoring networks and atmospheric chemistry models to reconstruct how reactive nitrogen deposition across China has shifted since 1980. The headline finding is striking: deposition climbed relentlessly for three decades, peaking at 16.4 teragrams of nitrogen per year during 2010 to 2012, before turning a corner. By 2020, total deposition had fallen 14 percent to 13.3 teragrams per year, a decline the authors attribute overwhelmingly to stringent industrial controls on nitrogen oxide emissions.

The mechanics of that turnaround deserve attention. Nitrogen oxides are produced when fossil fuels burn at high temperatures, in power plants, factories, cement kilns, and vehicle engines. Once emitted, they transform in the atmosphere into nitric acid and nitrate aerosols that are scavenged by precipitation or deposited dry onto canopies and soils. Beginning in the early 2010s, China rolled out aggressive emission reduction programs, including ultra-low emission standards for the power sector and tightening vehicle regulations. Satellite observations of nitrogen dioxide columns documented steep drops in pollution hotspots, and the deposition record followed. The review reports that oxidized nitrogen deposition declined by 34 percent, accounting for nearly all of the observed national reduction.

But here is the twist that gives the review its urgency: while oxidized nitrogen fell, reduced nitrogen, the ammonia and ammonium compounds largely traced to agriculture, kept rising. Reduced nitrogen now constitutes 60 to 70 percent of total deposition across China, a share that has transformed the chemistry of the problem. Ammonia escapes from fertilized fields, manure heaps, and livestock operations, and unlike nitrogen oxides it has faced almost no targeted regulation. Adding to the complexity, declining sulfur dioxide emissions have reduced the formation of ammonium sulfate aerosols, which in turn leaves more free ammonia in the atmosphere and can even enhance ammonia and ammonium deposition, a feedback known as the ammonia compensating effect.

The ecological consequences of this agricultural dominance are documented in sobering detail. Widespread soil acidification is depleting base cations such as calcium and magnesium from croplands and forests, with long-term measurements across Chinese forest ecosystems showing significant pH declines. Freshwater systems are suffering too; atmospheric nitrogen input to lakes such as Taihu contributes measurably to eutrophication, fueling algal blooms that degrade drinking water supplies. Biodiversity is under pressure as nutrient enrichment favors fast-growing species over the specialized plants of grasslands and other nutrient-poor habitats, with experimental nitrogen addition studies in Chinese grasslands and tropical forests documenting species losses and shifts in community composition.

The review quantifies the scale of the policy challenge with a critical loads analysis, the standard framework for assessing how much nitrogen an ecosystem can absorb before harm occurs. As of 2020, roughly 15 percent of China’s land area still receives reactive nitrogen deposition exceeding the critical load for eutrophication, meaning ecosystems in those zones are being over-fertilized beyond their capacity to cope. The authors warn that climate change will make matters worse, because warming and intensified precipitation extremes are projected to reduce ecosystem resilience, thereby expanding the terrestrial area where deposition exceeds critical loads even if emissions remain flat.

To put China’s situation in global perspective, the team calculated what reductions would be needed to match the average nitrogen deposition levels currently experienced in the United States and Western Europe, regions that themselves wrestled with and partially tamed this problem over recent decades. The answer is dramatic: China would need to cut ammonia deposition by 56 to 76 percent and nitrogen oxide deposition by 53 to 60 percent. Those numbers underscore that despite genuine progress, Chinese ecosystems remain bathed in nitrogen at rates several times higher than their Western counterparts, with the gap driven primarily by the unrestrained agricultural ammonia component.

Why has ammonia escaped regulation for so long? Part of the answer is practical. Agricultural ammonia emissions come from millions of smallholder farms spread across vast territories, making them far harder to monitor and control than a few hundred power plants. Fertilizer overuse remains endemic in parts of Chinese agriculture, and manure management is often rudimentary. Yet the review notes that cost-effective mitigation options exist, from optimized fertilizer application and enhanced-efficiency products to improved livestock housing and manure storage. Research on smallholder ammonia mitigation campaigns has shown that air quality can improve while cereal yields are maintained, and economic analyses suggest the societal benefits of halving agricultural ammonia emissions in China far exceed the abatement costs.

The authors argue that the way forward requires integrating agricultural ammonia management into the broader architecture of climate and air quality policy, rather than treating it as a separate agricultural issue. This means connecting nitrogen policy to food system reform, including improvements in nitrogen use efficiency across the entire chain from fertilizer production to livestock feed to human diets. It also means anticipating the interactions between pollution control and climate, since a warmer, wetter future will alter both the emissions of ammonia from soils and livestock and the atmospheric processes that deposit nitrogen back to the surface. The review’s framework positions nitrogen deposition abatement as inseparable from the sustainability of China’s food systems.

For the world beyond China, the study offers both a template and a warning. The 34 percent drop in oxidized nitrogen deposition proves that determined industrial emission control can bend the curve on one of the most stubborn forms of air pollution, a lesson relevant to rapidly developing economies across Asia and Africa where nitrogen oxide emissions are still climbing. But the simultaneous rise in reduced nitrogen shows that solving the industrial half of the problem while ignoring agriculture simply shifts the burden. As global food demand grows and nitrogen fertilizer use expands, the Chinese experience makes clear that comprehensive nitrogen management, spanning smokestacks, tailpipes, fields, and barns alike, is the only route to protecting ecosystems while feeding a nation.

Subject of Research: Drivers, trends and ecological impacts of atmospheric reactive nitrogen deposition in China

Article Title: Drivers, trends and impacts of nitrogen deposition in China

Article References: Liu, L., Liu, X., Wang, X., Xu, W., Tang, A., Du, E., Duan, L., Pan, Y., Zhang, L., Shen, J., Song, L., Li, K., Zhou, X., Lu, X., Zhao, Y., Yu, Q., Li, M., Zhang, X., Wen, Z., … Zhang, F. (2026). Drivers, trends and impacts of nitrogen deposition in China. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-026-00830-x

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00830-x

Keywords: nitrogen deposition, reactive nitrogen, ammonia emissions, nitrogen oxides, soil acidification, eutrophication, critical loads, China, air quality, agriculture, ecosystem health, emission controls

The central finding is stark: since the 1960s, the annual amount of reactive nitrogen entering the biosphere has increased roughly fivefold. This surge is driven overwhelmingly by human activity, above all the Haber-Bosch process that converts atmospheric N2 into ammonia for synthetic fertilizer, a technology that has underpinned agricultural expansion since the 1940s. Between 2015 and 2025, global bulk nitrogen inputs to terrestrial and aquatic ecosystems are estimated at 330 teragrams of nitrogen per year and 183 teragrams per year respectively, with wide uncertainty ranges. By comparison, equivalent inputs in the late 1950s stood at just 121 and 55 teragrams per year, a more than doubling of the flows that nourish the world’s ecosystems in barely two generations.

What makes the new analysis remarkable is what happens on the output side of the ledger. Emissions of nitrogen from the biosphere to the atmosphere increased by only about 12 percent between the oldest and the most recent estimates. In other words, humanity has poured vastly more reactive nitrogen into the Earth system than natural processes have been able to convert back into inert gas. The result is a terrestrial nitrogen accumulation on the order of 50 teragrams of nitrogen per year between 1959 and 2025, an amount sufficient to fundamentally alter soil chemistry, water quality and atmospheric composition across the globe.

The sinks themselves are expanding, but not nearly fast enough to keep pace. Terrestrial denitrification, the anaerobic microbial process that converts soil nitrate back into gaseous forms of nitrogen, is estimated at roughly 100 teragrams of nitrogen per year for the 2010 to 2020 period, up from 69 teragrams per year in 1955. Ocean denitrification estimates rose from 87 teragrams per year to approximately 200 teragrams per year over the same interval. Even these substantially larger losses, however, are dwarfed by the incoming flux, which is why reactive nitrogen continues to build up in soils, groundwater, vegetation and coastal sediments around the world.

This accumulation is not merely a bookkeeping curiosity; it carries real ecological and economic costs. Excess nitrogen availability pollutes soils, waters and air, driving eutrophication of lakes and coastal seas, hypoxic dead zones, acidification, loss of biodiversity and the release of nitrous oxide, a greenhouse gas nearly 300 times more potent than carbon dioxide over a century and now considered the dominant ozone-depleting substance emitted in the twenty-first century. The nitrogen cascade, the term scientists use to describe how a single atom of reactive nitrogen can cause harm sequentially in the atmosphere, on land and in water, means that each unit of nitrogen applied to a field can trigger a chain of environmental consequences far beyond the farm gate.

Part of the difficulty in managing the nitrogen cycle is that several of its largest components remain the most poorly constrained. The review identifies terrestrial biological nitrogen fixation and denitrification as the largest uncertainties in the global budget, and notes that improved estimates of anthropogenic nitrogen inputs to aquatic ecosystems are urgently needed to reduce the high uncertainties surrounding food systems in a changing world. Further quantification of the ocean biomass pool and of nitrogen release through rock weathering is also required. Processes only recently discovered, such as comammox, the complete oxidation of ammonia to nitrate by single microorganisms, and feammox, the anaerobic oxidation of ammonium coupled to iron reduction, are not yet represented in spatially explicit global models, representing a clear priority for future research.

The authors compiled their estimates by synthesizing published budgets across the atmospheric, terrestrial and aquatic reservoirs, drawing on datasets now available openly through the Zenodo repository. This assembly of historical fluxes allows, for the first time, a coherent view of how the modern nitrogen budget has evolved since the earliest global syntheses of the late 1950s. By comparing the earliest comprehensive estimates with the latest data, the review demonstrates that the gap between nitrogen inputs and nitrogen removal has widened dramatically, transforming the global cycle from a roughly balanced system into one with a persistent and growing surplus of reactive nitrogen stored in the terrestrial biosphere.

Closing the budget is not merely an academic exercise. Quantifying nitrogen pools and fluxes across reservoirs is critical for monitoring the imbalance and for evaluating mitigation strategies, from national fertilizer policies to international climate agreements. The review argues that better observations, measurement techniques and models are essential for identifying knowledge gaps and guiding the transition toward a more balanced and sustainable nitrogen cycle. Without such constraints, policymakers are effectively flying blind when attempting to design interventions, since it remains unclear how much of the applied nitrogen is retained, how much is lost to water and how much returns to the atmosphere in reactive or inert forms.

The good news embedded in the analysis is that a pathway toward rebalancing exists. The authors estimate that reducing fertilizer demand and use through dietary changes, deploying technological advances that enhance fertilizer use efficiency, and improving waste management and nutrient recycling could together support a net flux of 51 teragrams of nitrogen per year back to the atmosphere, a magnitude comparable to the current terrestrial accumulation. In practical terms, this means shifting diets toward less nitrogen-intensive protein sources, adopting precision agriculture and controlled-release fertilizers, and recapturing nitrogen from human and animal waste streams that would otherwise flow into rivers and coastal waters.

Nitrogen sits alongside carbon as a headline element of the Anthropocene, and its trajectory will help determine whether humanity stays within planetary boundaries. The review’s numbers make clear that the era of cheap, abundant reactive nitrogen has transformed the planet as profoundly as the rise of atmospheric carbon dioxide, and that correcting the imbalance will require coordinated action across agriculture, energy, sanitation and diet. What the new synthesis offers is a clearer map of where nitrogen is coming from, where it is going and how much remains unaccounted for, giving scientists and policymakers alike the baseline they need to begin steering the global nitrogen cycle back toward equilibrium.

Subject of Research: Fluxes and imbalances in the modern global nitrogen cycle

Article Title: Fluxes and imbalances in the modern global nitrogen cycle

Article References: Almaraz, M., Sun, X., Davidson, E. A., Zhang, X., Galloway, J. N., & Raymond, P. A. (2026). Fluxes and imbalances in the modern global nitrogen cycle. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-026-00821-y

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00821-y

Keywords: nitrogen cycle, reactive nitrogen, Haber-Bosch process, denitrification, nitrogen fixation, fertilizer, nitrous oxide, eutrophication, biogeochemistry, global nitrogen budget, comammox, nitrogen pollution

The research, led by Viney P. Aneja and Srijan Sengupta of North Carolina State University, together with Swarnali Sanyal of the University of Illinois Urbana-Champaign and William H. Schlesinger of the Cary Institute of Ecosystem Studies, examined ammonia emissions from swine operations that had retrofitted their waste management systems with anaerobic digestion. The team combined direct field measurements with outputs from a semiempirical mass-transfer model, a computational tool that estimates how quickly gaseous ammonia escapes from liquid manure surfaces based on chemical and physical conditions. Their central finding is striking: ammonia flux from so-called biogas secondary lagoons — open lagoons that receive digestate, the liquid residue left after anaerobic digestion — was statistically significantly higher than ammonia flux from conventional open-air lagoons used in traditional manure management, with the difference holding even after the analysis controlled for temperature, the single most important driver of volatilization.

To understand why this happens, it helps to follow the chemistry of nitrogen through a digester system. In a conventional lagoon, raw manure sits in open basins where organic nitrogen compounds are gradually mineralized to ammonium, and some of that ammonium escapes to the air as ammonia gas. In an anaerobic digester, by contrast, microbes decompose the organic matter in an oxygen-free environment, breaking down proteins and urea far more completely and rapidly. This process converts a large share of the organic nitrogen into total ammoniacal nitrogen, the pool of dissolved ammonia and ammonium ions from which gaseous ammonia can volatilize. At the same time, digestion consumes volatile acids and raises the pH of the digestate. Because the equilibrium between ammonium ions and free ammonia gas shifts strongly toward the gas phase as pH rises, digestate emerging from a digester is essentially primed for ammonia loss the moment it is exposed to air.

That exposure comes quickly. After digestion, the liquid effluent is typically transferred to a secondary storage lagoon before being pumped onto cropland as fertilizer. The study’s model simulations confirm that both during secondary storage and during subsequent land application under typical management practices, the elevated total ammoniacal nitrogen concentration and higher pH of digestate dramatically enhance the potential for volatilization. In effect, the digester concentrates the nitrogen problem: it converts slow, diffuse organic nitrogen into a chemically reactive, easily airborne form, then hands that material to an open lagoon where wind and warm temperatures can strip it into the atmosphere. The regression analysis, which statistically isolated the effect of the management system from confounding variables, found the difference between biogas secondary lagoons and conventional lagoons to be highly significant, with a p-value below 0.001 — a level of statistical confidence rarely achieved in environmental field studies.

The consequences extend well beyond the fence lines of the farms themselves. Ammonia is classified as a reactive nitrogen compound, and once released, it does not stay ammonia for long. In the atmosphere it reacts with acidic species such as nitric and sulfuric acids to form fine particulate matter, microscopic particles known as PM2.5 that penetrate deep into human lungs and are linked to asthma, cardiovascular disease, and premature death. Deposited back onto land and water, ammonia and its reaction products acidify soils, over-fertilize sensitive natural ecosystems in a process called eutrophication, and contribute to biodiversity loss in nitrogen-limited habitats. Some of the deposited nitrogen ultimately converts to nitrate, which can leach into groundwater and contaminate drinking water supplies — a cascade of impacts the study’s authors explicitly flag as a risk of degraded air quality and water quality relative to manure managed in systems open to the atmosphere.

The timing of this research matters. Across the United States, the livestock industry is actively retrofitting existing animal waste management systems at CAFOs to collect biogas, driven by a combination of renewable energy incentives, carbon credit markets, and corporate climate pledges. In states such as North Carolina and Iowa, major utility partnerships have funneled hundreds of millions of dollars into covering swine lagoons with anaerobic digesters. The shift from using animal waste solely as crop fertilizer toward producing biofuels is routinely advocated as an environmentally friendly strategy because of its potential to reduce greenhouse gas emissions, particularly methane, which traps far more heat per molecule than carbon dioxide over a twenty-year time horizon. The new findings do not dispute the climate accounting for methane. Instead, they reveal a blind spot in how the environmental performance of digesters is evaluated — one focused almost exclusively on carbon while ignoring the reactive nitrogen consequences of the same technology.

The study’s synopsis is blunt: anaerobic digesters used for biogas production from animal manure processing carry negative consequences for society and the environment and may not represent the cleaner energy future they are advertised to be. The authors caution that retrofitting digesters without integrated nitrogen management may simply shift environmental impacts from methane to reactive nitrogen, exchanging a visible, well-regulated climate problem for a diffuse air and water pollution problem that is harder to measure and easier to overlook. This kind of pollution shifting is a familiar trap in environmental engineering; solutions that solve one problem in isolation frequently create or amplify others when the full system is examined. The digester-ammonia connection is a textbook example, hidden in plain sight because the emissions occur at secondary lagoons and applied fields rather than at the digester itself.

Importantly, the researchers do not argue that biogas should be abandoned. Their conclusion is conditional but constructive: the environmental risk can be substantially reduced if digester systems are paired with mitigation technologies for animal waste management. Such technologies exist. Solid-liquid separation and acidification of digestate can lower pH and suppress the ammonia equilibrium shift. Covering secondary lagoons, capturing or scrubbing emitted ammonia, and applying digestate with injection or rapid incorporation into soils rather than surface spraying can all cut volatilization losses substantially. The key insight is that these controls must be designed into the retrofit from the beginning, not bolted on after air quality problems emerge. A digester project that captures methane but leaves hot, high-pH digestate evaporating in an open lagoon has, by the study’s measurements, made local air quality worse, not better.

For policymakers, the study lands at a sensitive moment. Regulatory frameworks in the United States reward digesters primarily for their methane capture, and renewable natural gas credits are priced on carbon metrics alone. If ammonia penalties were factored into the environmental accounting — as they increasingly are in European assessments of biogas systems — the calculus of which waste management strategies deserve public support could shift. The authors’ field-based evidence, strengthened by independent model confirmation, gives regulators a quantitative basis for requiring nitrogen management plans as a condition of digester incentives. It also gives communities near CAFOs, many of which are already overburdened by odor and particulate pollution, a new line of evidence in ongoing debates about the true cost of industrial livestock production.

Ultimately, the research is a reminder that environmental virtue in one dimension does not guarantee virtue in all. Capturing methane from manure is genuinely valuable for the climate, and the study does not challenge that. But the biogas story told to the public has been incomplete. The full lifecycle of digested manure — from the microbial transformations inside the digester to the moment the resulting liquid drifts off a secondary lagoon on a summer afternoon — determines the real environmental footprint of this technology. As the United States accelerates its retrofit of livestock operations in the name of renewable energy, the message from Biogeochemistry is clear: measure the nitrogen, manage the digestate, and do not assume that a covered lagoon means a cleaner farm. Without that vigilance, the cleaner energy future may arrive with an invisible plume of ammonia attached.

Subject of Research: Ammonia emissions from anaerobic digesters used for biogas production at swine concentrated animal feeding operations.

Article Title: Biogeochemical reactions in anaerobic digesters for biogas production yield enhanced ammonia emissions

Article References: Aneja, V. P., Sengupta, S., Sanyal, S., & Schlesinger, W. H. (2026). Biogeochemical reactions in anaerobic digesters for biogas production yield enhanced ammonia emissions. Biogeochemistry. https://doi.org/10.1007/s10533-026-01372-6

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01372-6

Keywords: anaerobic digestion, biogas, ammonia emissions, reactive nitrogen, CAFOs, swine manure, digestate, methane capture, air quality, water quality, volatilization, renewable energy