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 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: Biogeochemical reactions in anaerobic digesters for biogas production yield enhanced ammonia emissions. (n.d.). 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