The study, led by Jesse Bash of the US Environmental Protection Agency, now at the Norwegian Meteorological Institute, together with colleagues from the EPA, Environment and Climate Change Canada, and partner institutions, scrutinized the Surface Tiled Aerosol and Gaseous Exchange option, known as STAGE, in version 5.3.2 of the Community Multiscale Air Quality model, or CMAQ. STAGE is a unified framework that estimates both dry deposition, the uptake of gases by surfaces, and bidirectional exchange, the two-way traffic of gases between the atmosphere and soils or vegetation. What makes STAGE distinctive is that it applies the same resistance model to every modeled gas, whether that gas only deposits, like nitric acid, or can flow in both directions, like ammonia. That consistency matters, because in earlier versions of CMAQ, ammonia bidirectional exchange and the deposition of all other species were handled by two separate, parallel resistance schemes with different structural assumptions.

The mathematical heart of STAGE is a resistance analog borrowed from electrical circuit theory, following the widely used parameterizations of Nemitz and colleagues from 2001 and Massad and colleagues from 2010. The flux of a trace gas is driven by the difference between the ambient concentration and a compensation point concentration at the surface, divided by the aerodynamic resistance. For ammonia, the compensation point is built from a two-layer canopy and soil model: gases diffuse through leaf boundary layers, stomata, cuticles, and in-canopy air, with each pathway assigned its own resistance. The model even solves for the leaf-level compensation point using a system of equations analogous to Kirchhoff’s current law, balancing exchange between the atmosphere, stomata, cuticular surfaces, and the ground. When the compensation points are set to zero, the framework collapses gracefully into a conventional dry deposition velocity, which is precisely what allows one model to serve both unidirectional and bidirectional species.

Several technical refinements were introduced to make this framework work at regional scale. Deposition to wet leaf surfaces, previously treated as instantaneous diffusion into canopy water, now uses a mass transfer formulation adapted from CMAQ’s aqueous chemistry module, accounting for gaseous diffusivity and bulk accommodation into roughly spherical droplets sitting on hydrophobic leaves. The depth of surface moisture is derived from relative humidity using Brunauer-Emmett-Teller adsorption theory calibrated against leaf wetness observations, and droplet radius follows from that water depth and the leaf area index. In-canopy aerodynamic resistance is computed by integrating eddy diffusivity through the canopy using a momentum attenuation approach. Soil processes were also updated: the diffusive length for soil gas exchange was increased to two centimeters to match the measurements underlying the model, and soil ammonium availability is now estimated with a nonlinear sorption curve rather than a fixed solution fraction.

To test whether all this machinery reflects reality, the team turned the regional model into a field-scale box model written in the R statistical language, using identical resistance parameterizations, and ran it against micrometeorological flux measurements from two North Carolina sites. The first was a managed, unfertilized 15-hectare grass field at Duke Forest, where fluxes of ammonia, nitric acid, and sulfur dioxide were measured with a MARGA instrument during the second half of 2012. The second was a 200-hectare fertilized corn field near Lillington, where ammonia gradients were measured with a continuous-flow wet denuder system in 2007. When the box model was fed observed soil and vegetation ammonia emission potentials, the compensation points that quantify how much ammonia the soil and plant tissues are ready to release, the modeled fluxes of all three gases agreed well with observations, with mean biases within or near the reported measurement uncertainty.

The trouble emerged when the model was run with the default tabular values built into CMAQ. At the Duke Forest grassland, the defaults for soil and vegetation emission potentials, a soil gamma of 20 and an apoplast gamma of 247, produced a simulated mean net deposition of minus 1.3 nanograms per square meter per hour, while the towers actually recorded a mean evasive flux of 8.4 nanograms per square meter per hour in the opposite direction. In plain terms, the model said the grassland was absorbing ammonia when it was actually emitting it. Sensitivity experiments showed the modeled flux was most sensitive to the vegetation emission potential, and that perturbing resistances alone could not reverse the sign of the error. The measured compensation points at the site simply exceeded the tabular assumptions, which were drawn from parameterizations based on annual nitrogen deposition fields for non-agricultural land.

The corn field told a more encouraging but nuanced story. Using median measured soil ammonium, soil pH, and an apoplast emission potential of 153.5 reported in earlier work, the model captured the observed fluxes with a normalized mean bias of minus 23.4 percent, rising to 40.5 percent with the default parameterizations. The site also displayed a persistent mid-morning emission peak that the model could not fully explain. Dew on the leaves carried a high emission potential, but there was not enough ammonium in the dew, and the dew dried too early, to account for emissions that continued between eight and eleven in the morning on a dry canopy. The authors speculate that wetting and drying of ammonium-rich exposed soil between the rows, a process the soil moisture probes missed, may drive the morning pulse, contrasting with earlier dew-evaporation explanations at a Colorado grassland.

The decisive test came at continental scale. The team compared an annual 2016 CMAQ simulation against ammonia retrievals from the Cross-Track Infrared Sounder, a satellite instrument capable of detecting ammonia down to roughly half a part per billion under typical conditions. The model captured the broad magnitude and spatial patterns of satellite-observed ammonia, but the biases were revealing. CMAQ overestimated concentrations by 0.5 to 0.9 parts per billion on average in intensely agricultural regions such as California, the Upper Midwest, and eastern North Carolina, while underestimating observations across the Great Plains and other vegetated areas, with a broad shortfall of roughly one to two parts per billion over non-agricultural land. That pattern mirrors the grassland box model result almost exactly, pointing to a common cause: default emission potentials for natural vegetation and soils that sit at the low end of what field measurements show.

The implications extend beyond model diagnostics. The team estimates that of the 10.3 megatonnes of nitrogen applied as fertilizer to row crops in the 2016 simulation, about 0.5 megatonnes, an emission factor of 4.8 percent, was re-emitted as ammonia, a figure consistent with published ranges. Deposition peaked near 30 kilograms of nitrogen per hectare downwind of agricultural sources in eastern North Carolina, within measured values but higher than some earlier net deposition estimates for the region. The authors conclude that raising the grassland vegetation emission potential by about 30 percent and the soil surface emission potential roughly fivefold would better match both the Duke Forest observations and the satellite data, changes that remain well within the natural variability reported across measurement studies. Because non-agricultural land covers roughly three quarters of the terrestrial surface in the model domain, the path forward is clear: more micrometeorological flux measurements and soil and vegetation chemistry across different land uses, soil types, and growth stages. The multi-resolution evaluation strategy, spanning flux towers, monitoring networks, and satellites, offers the modeling community a template for building the next generation of air-surface exchange models on firmer empirical ground.

Subject of Research: Evaluation of bidirectional ammonia, nitric acid, and sulfur dioxide air-surface exchange in the CMAQ model's STAGE deposition option against field, network, and satellite observations

Article Title: Evaluation of HNO3, SO2, and NH3 in the Surface Tiled Aerosol and Gaseous Exchange (STAGE) option in the Community Multiscale Air Quality Model version 5.3.2 against field-scale, in situ and satellite observations

Article References: Bash, J. O., Walker, J. T., Wu, Z., Rumsey, I. C., Murphy, B., Hogrefe, C., Fahey, K. M., Pye, H. O. T., Jones, M. R., Appel, K. W., Shephard, M. W., Alnsour, N. I., & Cady-Periera, K. E. (2026). Evaluation of HNO 3 , SO 2 , and NH 3 in the Surface Tiled Aerosol and Gaseous Exchange (STAGE) option in the Community Multiscale Air Quality Model version 5.3.2 against field-scale, in situ and satellite observations. Geoscientific Model Development, 19(19), 9377-9393. https://doi.org/10.5194/gmd-19-9377-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9377-2026

Keywords: ammonia, air quality modeling, CMAQ, STAGE, dry deposition, bidirectional exchange, satellite observations, CrIS, grassland flux, emission potential, nitrogen deposition, particulate matter

The research team, led by Panpan Wu and Rong Mao of Jiangxi Agricultural University, set out to answer a question that has lingered in soil ecology for years: does the chemical form of nitrogen deposition change its effect on microbial necromass? Atmospheric nitrogen does not arrive in a single form. Industrial and agricultural activities release both inorganic nitrogen, dominated by ammonium and nitrate, and organic nitrogen compounds that travel through the atmosphere and settle onto forests. Earlier studies often treated these forms as interchangeable, or focused on just one. The subtropical forests of southern China, which receive some of the highest nitrogen deposition loads in the world, offered an ideal natural laboratory for testing whether form matters.

The experiment was conducted in a subtropical plantation, where the researchers applied three nitrogen treatments to field plots: ammonium chloride representing inorganic nitrogen, and urea and glycine representing organic nitrogen. They sampled soil at two depths, from zero to ten centimeters and from ten to twenty centimeters, and measured microbial necromass carbon using amino sugar biomarkers, molecules that persist in soil after microbial cells die and serve as fingerprints of fungal and bacterial remains. Alongside these measurements, the team tracked soil nutrient availability, microbial biomass, community composition, hydrolytic enzyme activities, and the mineralization of organic carbon into carbon dioxide.

The results were striking in their consistency. Despite the very different chemistry of the three compounds, all of them reduced soil microbial necromass carbon by similar magnitudes: ammonium chloride cut it by 20.7 percent, urea by 17.4 percent, and glycine by 20.1 percent. Just as notably, the total soil organic carbon content and the cumulative carbon dioxide emitted from the soil did not change. This decoupling is what makes the study so compelling. The overall carbon pool looked unchanged, yet a biologically meaningful and relatively stable fraction of it — the accumulated corpses of generations of microbes — had measurably declined. Necromass carbon is not a passive residue; it is a dynamic pool whose accumulation depends on the balance between microbial growth, death, and the recycling of dead cells by surviving organisms.

When the researchers separated the necromass into its fungal and bacterial components, a clearer picture emerged. The decline was driven almost entirely by reduced fungal necromass carbon, while bacterial necromass remained essentially unchanged after nitrogen addition. This distinction carries weight. Fungal cell walls are rich in chitin and other nitrogen-containing compounds, and fungal residues are generally considered more persistent and more important contributors to stable soil organic carbon than bacterial residues. Previous work has suggested that fungal necromass contributes disproportionately to soil organic carbon and is more sensitive to land-use intensity. A selective erosion of the fungal component therefore represents a loss of precisely the fraction of soil carbon that is best at sticking around.

Why would added nitrogen suppress fungal residue accumulation? The team’s measurements of the living community point toward a fundamental shift in microbial life-history strategy. Across all three nitrogen forms, microbial biomass actually increased, but the composition of the community changed, as indicated by rising ratios of fungi to bacteria and of gram-positive to gram-negative bacteria. These shifts suggest a move toward resource-conservative microbial characteristics — organisms that invest carefully, grow slowly, and recycle resources efficiently rather than building new biomass rapidly. In microbial ecology, this is often framed as a shift along a continuum from fast-growing, copiotrophic strategists to slow-growing, oligotrophic ones. When the community tilts conservative, the turnover of cells and the production of fresh residues slow down, and the existing necromass pool can be mined for nutrients faster than it is replenished.

The enzyme data add a second, complementary mechanism: intensified phosphorus limitation. Nitrogen addition enhanced the activities of both carbon-acquiring and phosphorus-acquiring enzymes, and increased the vector length and vector angle derived from ecoenzymatic stoichiometry, a widely used framework for diagnosing microbial resource limitation. In plain terms, the microbes were working harder to scavenge phosphorus from the soil while also ramping up carbon-degrading enzymes. Phosphorus is a notoriously scarce nutrient in highly weathered subtropical soils, and when nitrogen — the resource that was previously limiting — becomes abundant, demand for phosphorus intensifies. To obtain it, microbes may deploy enzymes that break down organic matter, including the very necromass residues that would otherwise accumulate. This creates a paradox: the added nutrient stimulates microbial activity while simultaneously destabilizing the carbon reservoir that microbial activity builds.

Further analysis by the team tied the decline in necromass carbon directly to these two threads — the community shift toward resource-conservative traits and the aggravation of phosphorus limitation. Together, they sketch a coherent mechanistic chain. Nitrogen arrives in whatever chemical form. The microbial community reorganizes, favoring conservative strategists and shifting the balance among functional groups. Phosphorus becomes the bottleneck. Enzymes are deployed to mine organic matter for scarce nutrients. Fungal residues, the most persistent building blocks of stable soil carbon, are consumed or fail to accumulate at their previous rate. The necromass pool shrinks, even as the bulk soil organic carbon and carbon dioxide emissions show no obvious change. It is a slow-motion rearrangement that standard carbon measurements would miss entirely.

The broader significance of the study lies in its message about form-independence. Because ammonium chloride, urea, and glycine produced nearly identical outcomes, the researchers conclude that atmospheric nitrogen deposition reduces the accumulation of soil microbial necromass carbon irrespective of its chemical form, acting through altered community composition and resource stoichiometry rather than through any compound-specific pathway. For modelers, this simplifies the challenge: nitrogen loads, not nitrogen speciation, may be the key variable to track when projecting soil carbon futures in nitrogen-polluted regions. For policymakers, it means that the growing organic nitrogen component of deposition — long understudied relative to inorganic forms — is not a benign alternative. It carries the same risk to the microbial carbon pump, the suite of processes by which living microbes convert plant carbon into persistent soil organic matter.

There are also cautionary notes for carbon sequestration strategies. Subtropical forests are among the most productive carbon sinks on Earth, and their soils hold enormous stores of organic carbon. Some proposed climate interventions involve fertilizing forests with nitrogen to boost tree growth and carbon uptake. This study suggests such approaches could carry an underappreciated cost below ground: while trees may grow faster, the microbial machinery that stabilizes carbon in soil may be quietly dismantled, with fungal residues — the glue of long-term soil carbon storage — eroding by roughly a fifth within the timeframe of the experiment. The work, supported by the National Natural Science Foundation of China, underscores a growing recognition in soil science that the fate of carbon in soils is written in the lives and deaths of microorganisms, and that human disruption of nutrient cycles rewrites that story in ways we are only beginning to measure.

Subject of Research: Effects of inorganic and organic nitrogen addition on soil microbial necromass carbon in a subtropical plantation

Article Title: Inorganic and organic nitrogen addition consistently reduces soil microbial necromass carbon in association with altered microbial community composition and intensified phosphorus limitation in a subtropical plantation

Article References: Wu, P., Li, W., Zhang, Y., Liu, Z., & Mao, R. (2026). Inorganic and organic nitrogen addition consistently reduces soil microbial necromass carbon in association with altered microbial community composition and intensified phosphorus limitation in a subtropical plantation. Plant and Soil. https://doi.org/10.1007/s11104-026-09106-1

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09106-1

Keywords: nitrogen deposition, microbial necromass carbon, soil organic carbon, subtropical forest, phosphorus limitation, microbial community composition, amino sugars, fungal necromass, enzyme activity, carbon sequestration, soil ecology, plantation

The research team, led by Enyue Zou and Miao Wang of Northeast Forestry University together with colleagues from Qufu Normal University and the Institute of Applied Ecology of the Chinese Academy of Sciences, worked in a boreal forest dominated by Larix gmelinii, the Dahurian larch. The site sits at the heart of one of Earth’s largest continuous boreal belts, a biome where cold temperatures slow decomposition and keep nitrogen locked away in soil organic matter. Because plant growth in these forests is typically constrained by nitrogen availability, scientists have long assumed that any extra nitrogen arriving from the atmosphere would be snapped up eagerly by trees and soil microbes alike. The new study shows the reality is far more nuanced.

To follow the nitrogen, the researchers used one of the most powerful tools in ecosystem science: isotopic labeling. They applied paired tracers of nitrogen-15, a rare heavy isotope of nitrogen, in the form of both 15NH4+ and 15NO3−. Because the isotope behaves chemically like ordinary nitrogen but can be detected with a mass spectrometer, every atom of labeled nitrogen that enters a leaf, a fine root, or a soil aggregate leaves a measurable fingerprint. The team established four long-term treatments—a control plus low, medium, and high nitrogen addition—and then traced the labeled nitrogen through five distinct pools: leaves, branches, fine roots, coarse roots, and soil.

The first surprise came at the very start of the tracer’s journey. Ammonium-derived nitrogen entered fine roots more rapidly in the early stages, producing a higher short-term uptake rate than its nitrate counterpart. This makes physiological sense: ammonium is already in a reduced form and can be assimilated directly into amino acids inside the root, whereas nitrate must first be reduced by the enzymes nitrate reductase and nitrite reductase before it can be used—a process that costs energy and takes time. Conifers, in particular, have historically been described as ammonium specialists, and the early uptake pattern in these larch trees fits that classical picture.

But the story changed dramatically as time passed. Nitrate-derived nitrogen proved to be the long-distance traveler of the pair. During the middle and later stages of the experiment, especially under medium and high nitrogen addition, nitrate-derived nitrogen was transported more readily to the leaves and branches above ground. This suggests that once nitrate is taken up, it moves through the xylem stream with relative ease, delivering nitrogen to the canopy where it fuels photosynthesis. In other words, the two forms of deposited nitrogen follow fundamentally different routes through the tree: ammonium is captured quickly and processed locally in the roots, while nitrate behaves more like a mobile courier service running from soil to crown.

Perhaps the most consequential finding concerns what long-term nitrogen addition did not do. Contrary to the expectation that chronic fertilization would continuously boost the fine roots’ appetite for nitrogen, the study found no sustained enhancement of fine-root uptake of the labeled tracer. What changed instead was the plumbing. Nitrogen addition promoted the transfer of labeled nitrogen from fine roots—the short-lived, high-turnover organs that do most of the absorbing—into coarse roots and then into aboveground organs. Under elevated nitrogen deposition, the forest essentially rerouted its internal nitrogen traffic, pushing a larger share of newly acquired nitrogen upward through the woody transport network rather than letting it linger below ground.

The soil, meanwhile, played the role of a temporary vault. Immediately after both nitrogen forms were applied, the soil was the major short-term sink, holding the bulk of the labeled nitrogen through microbial immobilization, exchange with mineral surfaces, and physical fixation. This initial retention is critical for the global carbon cycle, because nitrogen retained in soils can influence how much carbon forests store; previous work, including a 2022 analysis in Nature Communications, has linked the retention of deposited ammonium and nitrate to the strength of the forest carbon sink. But the vault does not hold forever. Over the course of the study, accumulation and recovery of the labeled nitrogen in plant pools steadily increased while soil recovery declined—a clear signature of substantial redistribution of deposited nitrogen from the soil into the living biomass.

This shifting balance carries implications that ripple far beyond a single larch stand. Global nitrogen deposition has risen sharply over the past century, particularly across Asia, and models of future forest carbon sequestration depend heavily on assumptions about where deposited nitrogen goes. If soils initially lock up most incoming nitrogen but gradually release it to trees, then the timescale of observation matters enormously: a snapshot taken weeks after deposition tells a very different story than one taken months or years later. The study’s multi-pool, multi-stage design captures exactly this temporal evolution, which is why its authors argue that long-term deposition reshapes not just how much nitrogen trees use, but the relative use of different nitrogen forms and the long-term fate of exogenous nitrogen across the whole plant–soil system.

The work also speaks to a long-running debate in forest ecology. Classic experiments in the 1990s suggested that conifer roots discriminate strongly against nitrate, implying that boreal and temperate conifers rely almost exclusively on ammonium. More recent studies, including work showing that mature conifers can assimilate nitrate as efficiently as ammonium in several plantation settings, have chipped away at that dogma. The new larch data land squarely in the revisionist camp: nitrate is not merely tolerated by these trees, it is preferentially exported to the canopy over time, particularly when nitrogen supply is elevated. The physiological machinery for nitrate uptake, transport, and signaling appears fully operational in this boreal species, and chronic deposition may even upregulate it.

For a warming world, the timing could not be more relevant. Boreal forests store an enormous fraction of terrestrial carbon, and their response to nitrogen deposition is woven into every major Earth system model. By demonstrating that deposited ammonium and nitrate follow distinct pathways—rapid root capture for ammonium, delayed canopy delivery for nitrate—and that years of enrichment reroute nitrogen from soil sinks into wood and leaves, the study provides the kind of mechanistic detail that modelers need to predict how much anthropogenic nitrogen will actually boost carbon sequestration, and how much will simply cycle through soils and back into the atmosphere. The larch forests of northeastern China, it turns out, are not passive sponges for pollution. They are active accountants, balancing a ledger of two very different currencies and deciding, season by season, where every atom should go.

Subject of Research: Fate of ammonium- and nitrate-derived nitrogen in a nitrogen-enriched boreal forest traced with 15N labeling

Article Title: Long-term nitrogen addition alters the uptake, transport, and fate of ammonium- and nitrate-derived nitrogen in a boreal forest

Article References: Zou, E., Wang, M., Liu, G., Huang, B., Yin, L., Liang, C., Xing, Y., Wang, X., & Wang, Q. (2026). Long-term nitrogen addition alters the uptake, transport, and fate of ammonium- and nitrate-derived nitrogen in a boreal forest. Plant and Soil. https://doi.org/10.1007/s11104-026-09077-3

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09077-3

Keywords: nitrogen deposition, boreal forest, Larix gmelinii, 15N tracer, ammonium, nitrate, fine roots, nitrogen uptake, plant-soil system, carbon sequestration, forest ecology, soil nitrogen

The study, led by Jian-Kun Sun, Wei-Wei Feng, Ming-Chao Liu, Ji-Xin Chen, Fa-Zhao Qi, and Yu-Long Feng of Southwest Forestry University and Shenyang Agricultural University in China, focused on a dramatic head-to-head contest between two plants. On one side was Solanum rostratum, an aggressive invader native to North America that has spread widely across China and other regions, armed with spines and a voracious appetite for resources. On the other was Astragalus laxmannii, a native species that shares the same habitats. The researchers grew the two species together in mixed field plots, forcing them to compete directly under different nitrogen regimes, and then measured everything from total biomass to the microscopic economics of their leaves and roots.

The central question was deceptively simple: when soils receive more nitrogen, and when that nitrogen arrives in different chemical forms, do invasive and native species respond in the same way? The answer, according to the data, is a resounding no. Across nearly all treatments, Solanum rostratum outperformed its native neighbor in total biomass, and the gap between the two species widened as nitrogen availability increased. Crucially, the mechanism behind that widening gap was not just faster growth but a coordinated shift in how the invader builds and deploys its tissues.

To understand that mechanism, it helps to think of plants as investors operating under a strict carbon budget. Ecologists describe this through the framework of the plant economics spectrum, a continuum that runs from acquisitive strategies—thin, cheap tissues built for rapid resource capture—to conservative strategies, which favor dense, durable structures that conserve resources over the long term. The leaves and roots of a plant tend to align along this spectrum, and the new study shows that the two species sit at opposite ends of it.

Solanum rostratum displayed the classic acquisitive profile. It had a higher specific leaf area, meaning it constructed more light-intercepting surface per gram of leaf tissue, and a higher net photosynthetic rate, converting sunlight into sugars faster. It also achieved a higher photosynthetic nitrogen-use efficiency, extracting more carbon gain from every unit of nitrogen invested in its photosynthetic machinery. Below ground, the invader paired these leaf traits with a higher specific root length—more foraging root length per unit of root mass—along with thinner roots and lower root tissue density. In other words, it built an inexpensive, high-throughput system for capturing both light and soil nutrients. Astragalus laxmannii, by contrast, showed the conservative syndrome: thicker, denser roots, a higher root-to-shoot ratio, and slower, more frugal resource economics.

The most striking finding emerged when nitrogen was added. For the invader, nitrogen addition pushed its acquisitive traits even further: specific leaf area, photosynthetic rate, photosynthetic nitrogen-use efficiency, and specific root length all increased. For the native species, those same responses were smaller or in some cases statistically indistinguishable from zero. Because the invader could flexibly scale up its resource-capture apparatus while the native could not, the growth difference between the two species grew larger under nitrogen enrichment. Nitrogen pollution, in effect, acted as an amplifier for a pre-existing competitive asymmetry rather than creating one from scratch.

The chemical form of the nitrogen mattered as well. Soils supply inorganic nitrogen primarily in two forms: nitrate and ammonium. Plants expend different amounts of energy assimilating each, and species differ in their preferences and transport capacities. The experiment revealed that adding nitrate was generally more effective at boosting the invader’s performance in some cases, while ammonium additions could more effectively increase biomass for the native Astragalus laxmannii. This detail carries real-world weight, because atmospheric nitrogen deposition changes not just the total amount of nitrogen entering ecosystems but also the relative proportions of nitrate and ammonium that plants encounter. Landscapes receiving nitrate-dominated deposition may therefore be tipping the competitive balance further in favor of invaders with nitrate-hungry, acquisitive physiologies.

Statistical analysis of the trait data confirmed that these leaf and root characteristics were not incidental correlates of the growth difference—they significantly contributed to it. The acquisitive traits of the invader and the conservative traits of the native together explained a substantial portion of why Solanum rostratum accumulated more biomass under competition. This coordinated leaf-root perspective is important because much earlier invasion research focused on leaves alone, treating below-ground economics as a black box. By measuring both organ systems simultaneously, the study demonstrates that invasion success rests on a whole-plant strategy, with roots and leaves working in concert to convert abundant nitrogen into rapid growth.

The findings arrive at a moment of mounting global concern. Nitrogen deposition has intensified dramatically over industrialized and agricultural regions, with East Asia among the most affected areas, and recent global assessments have documented rising impacts of plant invasions on terrestrial ecosystems worldwide. If nitrogen enrichment systematically favors acquisitive invaders over conservative natives, then the two environmental pressures—nutrient pollution and biological invasion—may be reinforcing each other, producing ecosystems that are simultaneously more eutrophic and more dominated by exotic species. That feedback could accelerate the displacement of native flora and erode the biodiversity that underpins ecosystem services.

There are also practical implications for land managers. Controlling invasive plants in nitrogen-enriched landscapes may require more than mechanical removal or herbicide treatment; it may demand interventions that address the underlying soil fertility, such as restoring low-nitrogen conditions, managing fertilizer runoff, or favoring native species with sufficient plasticity to respond to elevated nutrients. The study’s authors suggest that increasing soil nitrogen availability, especially in the form of nitrate, may facilitate invasions of exotic plants characterized by coordinated resource-acquisitive strategies—a conclusion that turns a global pollution problem into a predictive tool for invasion risk. As nitrogen continues to accumulate in soils around the world, the plants best positioned to exploit it may increasingly be the ones we least want.

Subject of Research: How soil nitrogen availability and form influence competitive trait differences between an invasive plant and a native species

Article Title: Increasing soil availability of nitrogen, especially nitrate, amplifies growth advantage associated with acquisitive leaf–root traits in Solanum rostratum compared with co-occurring native species

Article References: Sun, J.-K., Feng, W.-W., Liu, M.-C., Chen, J.-X., Qi, F.-Z., & Feng, Y.-L. (2026). Increasing soil availability of nitrogen, especially nitrate, amplifies growth advantage associated with acquisitive leaf–root traits in Solanum rostratum compared with co-occurring native species. Plant and Soil. https://doi.org/10.1007/s11104-026-09091-5

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09091-5

Keywords: biological invasion, Solanum rostratum, nitrogen deposition, nitrate, ammonium, plant traits, leaf economics, root economics, photosynthetic nitrogen-use efficiency, specific leaf area, specific root length, plant competition

The mountains in question form a roughly 500-mile-long arc stretching from the Adirondacks of upstate New York through the Green Mountains of Vermont and the White Mountains of New Hampshire to Mount Katahdin in Maine and the Gaspé Peninsula in eastern Quebec. Although these alpine zones cover only about 50 square miles in total, they punch far above their weight ecologically. They harbor numerous rare species of flowering plants found nowhere else in the region, making them recognized hotspots of biodiversity. They are also remarkably accessible: nearly 70 million people live within a day’s drive of at least one of these mountaintop habitats, a proximity that brings both public appreciation and mounting pressures from recreation, including the risk that hikers, hunters, bikers, and skiers inadvertently track in invasive plant species.

Jonathan Chipman, director of the Citrin Family GIS/Applied Spatial Analysis Laboratory at Dartmouth, initiated the study after noticing a conspicuous gap in the scientific literature. Greening trends, in which vegetation cover and productivity increase over time, had been documented in relation to climate warming in the Alps, the Rocky Mountains, and the Arctic, but no one had carried out a comparable regional-scale assessment of what was happening above tree line in northeastern North America. Chipman partnered with co-author Jordon Tourville, a terrestrial ecologist at the Appalachian Mountain Club, whose organization contributes extensive alpine ecology expertise from fieldwork and citizen science programs. The collaboration paired Dartmouth’s strength in remote sensing and satellite time-series analysis with the Club’s on-the-ground knowledge of the region’s fragile summit ecosystems.

The technical foundation of the study was the Landsat satellite record, the longest continuous archive of moderate-resolution Earth observation imagery available. The researchers used imagery spanning 1984 to 2024, a forty-year window long enough to separate genuine directional trends from year-to-year weather noise. They defined the alpine zones on every site south of the St. Lawrence River that was large enough to monitor reliably from orbit, identified 35 major alpine zones, and modeled the greenness of each landscape through time. Crucially, the analysis worked at two scales simultaneously: the team examined where greening was occurring across the region as a whole and within individual mountains, allowing them to detect patterns that a coarser regional average would have obscured.

Working in this particular region posed a distinctive data challenge. The Northeast is notoriously cloudy, which means far fewer clear-sky satellite observations are available than in arid or high-latitude regions where remote sensing studies are more commonly conducted. The sparsity is worst in the early part of the record, with relatively few usable images from the 1980s and 1990s. That imbalance creates a statistical hazard the researchers call observation frequency bias: an apparent trend can emerge simply because the limited older data happen to capture unusual years, rather than because vegetation is genuinely changing. To address this, the team developed a method that takes every sparse observation in the historical record and converts it into the best possible estimate of what conditions were at that time, effectively reconstructing a more complete and trustworthy time series from fragmentary inputs.

The pattern that emerged from the analysis was not the one the researchers might have predicted from simple climate logic. In regions such as the Alps, warming typically manifests as an upward advance of the tree line, with forest creeping into what was formerly alpine terrain. In the Northeast, Chipman explains, the situation is different. The study found significant greening and shrubification, a process in which low-growing alpine plants are replaced by shrubs, occurring inside the alpine zone itself, not merely at the advancing margin of the forest. This was especially evident in the Presidential Range of New Hampshire and on Katahdin in Maine, where greening was prominent at higher elevations well within alpine territory. Vegetation increases were observed across several distinct plant communities, including cliff plants that grow from rocky outcrops, cushion-tussock vegetation forming dense dome-shaped mats, and sedge meadows of hardy grass-like plants.

One of the most important physical factors shaping this pattern, the researchers argue, is wind. All of the region’s mountains experience very high winds, particularly in winter, and Mount Washington in New Hampshire’s White Mountains holds the record for the highest wind speed ever recorded by a staffed weather station: 231 miles per hour. These brutal winds suppress woody growth and push the alpine zone to lower elevations than it would occupy at this latitude elsewhere in the world. The study’s results suggest that the prevailing greening trend is being held back on the windward, exposed sides of the mountains, while on more sheltered sides the growth of shrubs and denser vegetation is being unleashed and proceeding faster. Wind exposure, in other words, acts as a spatial filter on how climate warming translates into visible ecological change across the rugged terrain.

Wind is not the only complicating factor. The researchers emphasize that the observed greening does not necessarily match the pattern expected from warming alone, and they suspect that additional influences are layered on top of the regional warming trend. Among these are declining snow cover, atmospheric nutrient deposition, soil chemistry, slope steepness and sun exposure, and a historical legacy of nitrogen pollution. In the late twentieth century, the region’s mountains were regularly doused with nitrogen emitted by Midwestern power plants and transported by prevailing winds, which acted as a fertilizer that benefited some plant species more than others. After the 1990 Clean Air Act amendments sharply reduced those emissions, the fertilizer effect diminished, but ecosystems do not adjust instantaneously. The result, the authors suggest, is a greening pattern that varies over both space and time as warming, nutrient history, topography, and wind exposure interact in different combinations on different peaks.

Whether this greening is good news is an open question, and the co-authors urge caution. Accelerated plant growth might sound benign, even positive, but in alpine zones it may not be. If aggressive, faster-growing flora begin to crowd out the rare, slow-growing flowering plants that make these summits biodiversity hotspots, the net effect could be a loss of the very species that define the habitat. Shrubification, in particular, represents a structural transformation of the ecosystem rather than a simple increase in life. The researchers hope that their regional analysis will serve as a practical tool for land managers and stewardship organizations, helping them target monitoring and conservation efforts at the zones and slopes where change is happening fastest. Chipman framed the project as a model of collaboration between a university and a regional nongovernmental organization, aimed not at research in the abstract but at helping managers, stakeholders, and the public understand what is happening in the mountains around them.

The study also had an educational dimension. A pilot version of the work was conducted by co-author Irene Ko, a Women in Science Project intern in 2024 and a Citrin Lab research assistant from 2024 to 2025, who examined 22 alpine zones and demonstrated that the topic merited full-scale investigation. Funding came from Dartmouth’s Women In Science Program, the William H. Neukom Institute for Computational Science, and the Jeffrey and Rona Citrin family. As satellite archives continue to grow and analytical methods for handling sparse, cloud-obscured records improve, the Northeast’s alpine summits, long studied plot by plot on foot, now have a four-decade regional baseline against which future change can be measured, and against which the fate of some of eastern North America’s rarest plants may ultimately be decided.

Subject of Research: Long-term satellite analysis of vegetation greening and shrubification in the alpine zones of the northeastern United States and Canada

Article Title: Study finds alpine peaks are greening in Northeast U.S., Canada

Article References: Study finds alpine peaks are greening in Northeast U.S., Canada. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: alpine zone, greening, shrubs, Landsat, remote sensing, northern Appalachians, climate change, tree line, biodiversity, nitrogen deposition, wind exposure, Ecosphere

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

Nitrogen is the nutrient that most often limits plant growth, and human activities—from fossil fuel combustion to intensive agriculture—have more than doubled the amount of reactive nitrogen cycling through the global environment. When this nitrogen rains down on forests, it can fertilize trees, acidify soils, and shift the delicate balance of elements such as carbon, nitrogen, and phosphorus within living tissue. Ecologists have long predicted that forests receiving chronically high nitrogen inputs should eventually reach

The concept of nitrogen saturation, formalized in influential syntheses of temperate forest research in the late 1990s, describes a sequence of stages through which a forest ecosystem passes as chronic nitrogen inputs accumulate. In the earliest stages, added nitrogen is captured efficiently by plants and soil microbes, stimulating growth and enhancing nutrient uptake. As deposition continues, however, the system’s capacity to retain nitrogen becomes exhausted: excess nitrate leaches into streams, soils acidify, base cations are depleted, and the availability of other nutrients, particularly phosphorus, becomes the principal constraint on plant productivity. Under this framework, forests in regions with decades of elevated deposition were expected to exhibit symptoms of saturation, including diminished growth responses to further nitrogen inputs and declining foliar nitrogen relative to phosphorus. The new study from western China complicates this tidy progression, finding that a dominant tree in a high-deposition subtropical forest continues to respond nutritionally to added nitrogen rather than showing the plateau or decline that saturation would predict.

The setting matters enormously for interpreting this result. Subtropical China receives some of the highest rates of atmospheric nitrogen deposition anywhere on Earth, driven by dense industrial activity, intensive fertilizer use, and rapid urbanization in the region. Yet the soils and vegetation of these humid, warm forests differ in fundamental ways from the temperate and boreal systems where the saturation model was originally developed. Subtropical forests tend to be phosphorus-limited rather than nitrogen-limited, with highly weathered, acidic soils that hold relatively little labile phosphorus. In such systems, nitrogen deposition can act as a partial fertilizer even at high background rates, because the trees have evolved under conditions where nitrogen availability fluctuates and phosphorus scarcity, not nitrogen scarcity, sets the ceiling on productivity. The evergreen secondary forest where Michelia wilsonii grows represents exactly this kind of environment, where the interplay between abundant nitrogen and constrained phosphorus shapes every aspect of plant nutrient strategy.

Stoichiometry, the study of the ratios of elements such as carbon, nitrogen, and phosphorus in living tissue, provides a powerful lens for reading these nutrient dynamics. Leaf carbon concentrations are typically quite stable across environmental gradients, reflecting the structural and metabolic constancy of the photosynthetic apparatus. Nitrogen and phosphorus, by contrast, vary considerably with supply, because both are essential to proteins, nucleic acids, and the energy-transfer machinery of cells. The ratio of nitrogen to phosphorus in leaves is widely used as an indicator of which nutrient limits plant growth at a given moment, while carbon-to-nutrient ratios reflect how efficiently plants convert assimilated carbon into nutrient-rich tissue. When nitrogen deposition alters these ratios, it signals a shift in the internal economy of the plant, with cascading consequences for herbivores, decomposers, and the recycling of nutrients through the ecosystem.

The experimental design employed in the study followed a now-standard approach in deposition research: plots received supplemental nitrogen at rates of zero, twenty, and forty kilograms of nitrogen per hectare per year, spanning the range of additional inputs that forests in the region might plausibly experience. By measuring leaf carbon, nitrogen, and phosphorus concentrations alongside soil and microbial biomass pools, the researchers could trace how nitrogen moved through the ecosystem and where its effects originated. This multi-tiered sampling is critical because leaf chemistry does not respond to deposition in isolation; it reflects the integrated outcome of soil nutrient availability, microbial competition for nutrients, and the tree’s own physiological regulation of uptake and internal recycling.

One of the most intriguing patterns in the results is the non-linear response of leaf nitrogen and phosphorus, which rose at moderate nitrogen addition and then declined at the highest rate. This initial-increase-then-decline trajectory suggests that moderate nitrogen inputs relieve a nutrient constraint and allow the tree to enrich its foliage, but that heavier inputs trigger compensatory mechanisms or stress responses that pull nutrient concentrations back down. Possible explanations include increased leaching of nutrients from soils under heavier loading, soil acidification that reduces phosphorus availability, or physiological downregulation of uptake when the tree has accumulated sufficient nitrogen. The corresponding seasonal shifts in carbon-to-nitrogen and carbon-to-phosphorus ratios in summer samples reinforce the picture of a tree actively recalibrating its tissue chemistry as inputs change, rather than passively accumulating nitrogen.

Nutrient resorption, the process by which trees withdraw nitrogen and phosphorus from leaves before they are shed, is another key thread in the study. Resorption efficiency is a central component of nutrient conservation in evergreen species, which must sustain their foliage for multiple growing seasons in nutrient-poor environments. When soil nitrogen is abundant, trees typically reduce their reliance on resorption and instead draw more nitrogen directly from the soil, a shift that can loosen the tight internal cycling characteristic of infertile sites. The finding that resorption efficiencies followed the same rise-and-fall pattern as leaf nutrient concentrations indicates that deposition is reshaping not just what the leaves contain but how the tree manages its nutrient capital over time. Changes in resorption feed back into litter quality, which in turn alters decomposition rates and the release of nutrients back into the soil, closing a loop that connects deposition to the entire biogeochemical cycle of the forest floor.

The soil and microbial measurements add an important belowground dimension to the story. Soil organic carbon and microbial biomass carbon emerged as the primary determinants of variation in leaf stoichiometry, implicating the microbial community as a gatekeeper controlling nutrient flows to tree roots. Microbes and plant roots compete directly for nitrogen and phosphorus in the soil, and the elemental composition of microbial biomass determines whether immobilized nutrients are locked up in microbial tissue or released for plant uptake. Nitrogen deposition is known to shift microbial communities, favoring some groups over others and altering the balance of fungal and bacterial dominance, with consequences for carbon storage and nutrient turnover. The strong correlations observed between microbial biomass composition and leaf chemistry in this forest suggest that belowground responses are not merely a side effect of deposition but an active mediator of how trees experience added nitrogen.

The absence of nitrogen saturation in this system deserves particular attention. Several factors could explain why the forest has not crossed the saturation threshold despite high background deposition. The humid subtropical climate supports rapid plant growth and high nutrient demand, allowing trees and microbes to absorb substantial nitrogen inputs. Deep soils and abundant organic matter may provide large exchange and retention capacities. Moreover, if phosphorus availability, while low, is sufficient to support continued growth, then added nitrogen can still be converted into biomass rather than accumulating as excess. The authors’ conclusion that high deposition continues to enhance the nutrition and growth of dominant species implies that these forests remain in the fertilization stage of the saturation sequence, a finding that extends the applicability of the saturation framework by revealing how differently it can unfold in subtropical versus temperate settings.

The ecological implications of this continued fertilization are far-reaching. If dominant species such as Michelia wilsonii gain a nutritional advantage under sustained deposition, they may outcompete subordinate species that are less able to exploit the extra nitrogen, simplifying forest composition and altering canopy structure. Shifts in leaf chemistry also propagate upward and downward through the food web, affecting insect herbivores whose foliar diets become more nitrogen-rich, and decomposer communities whose litter inputs change in quality. Over longer timescales, the combination of enhanced growth, altered litter chemistry, and modified microbial activity could change how much carbon these forests store, a question of global relevance given the role of subtropical forests in the terrestrial carbon sink. At the same time, the non-linear responses observed here caution against assuming that fertilization benefits will persist indefinitely; the decline in leaf nutrients at the highest addition rate hints that thresholds may exist beyond which negative effects emerge.

More broadly, the study underscores the value of examining plant responses to pollution through the integrated framework of ecological stoichiometry, which links leaf chemistry, soil processes, and microbial ecology into a single analytical picture. Rather than treating nitrogen deposition as a simple dose of fertilizer or toxin, this approach reveals it as a force that reorganizes the flow of multiple elements through an ecosystem, with effects that depend on season, soil properties, and the identity of the organisms involved. For forests across subtropical Asia, where deposition rates remain high and may continue to rise, understanding these element-coupled responses will be essential for predicting which species thrive, which decline, and how the structure and function of some of the world’s most biodiverse ecosystems will be reshaped in the decades ahead.

Subject of Research: Effects of nitrogen addition on leaf carbon, nitrogen, and phosphorus stoichiometry of the dominant tree Michelia wilsonii in a high-nitrogen-deposition subtropical forest in western China.

Article Title: No N saturation, N addition still alters leaf stoichiometry of Michelia wilsonii in a high-N input forest

Article References: Liu, S., Zheng, X., Xiao, Y., Wang, L., Li, H., You, C., Xu, L., Xu, H., Xu, Z., Tan, B., Yuan, Y., & Zhang, L. (2026). No N saturation, N addition still alters leaf stoichiometry of Michelia wilsonii in a high-N input forest. Plant and Soil. https://doi.org/10.1007/s11104-026-09111-4

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09111-4

Keywords: nitrogen deposition, nitrogen saturation, leaf stoichiometry, Michelia wilsonii, ecological stoichiometry, soil organic carbon, microbial biomass, nutrient resorption, subtropical forest, phosphorus limitation, forest ecology, Plant and Soil