Nitrogen Pollution Quietly Strips Carbon From Forest Soils, No Matter Its Form

Deep in the soils of a subtropical Chinese plantation, an invisible reservoir of carbon is quietly shrinking. A new field experiment published in Plant and Soil shows that adding nitrogen to forest soil — whether as inorganic ammonium chloride or as organic compounds such as urea and glycine — consistently reduced the amount of carbon locked away in dead microbial cells, known as microbial necromass carbon. The finding matters because microbial residues are increasingly recognized as one of the largest and most stable contributors to soil organic carbon, the vast carbon store that helps regulate the planet’s climate. If nitrogen pollution erodes this reservoir, the implications ripple outward for carbon accounting, forest management, and global climate models.

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