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