Long-Term Nitrogen Addition Shifts Soil Microbial Community Structure and Potential Carbon-Nitrogen and PAH Metabolic Functions

This is the authors' abstract. We don't add key points for this paper.

Nitrogen is an essential plant nutrient that also shapes soil properties and microbial communities, but its interactions with residual organic pollutants remain poorly resolved. In a 15-year field experiment, we evaluated four nitrogen (N) fertilization rates and collected surface soil (0-20 cm) to determine their effects on soil physicochemical properties, microbial community structure, exploratory co-occurrence networks, metagenomic functional profiles, and residual polycyclic aromatic hydrocarbons (PAHs). Relative to the control (CK), total N and Ca concentrations in the high-N treatment (N3) were 2.25 and 1.38 times higher, respectively. N addition was associated with differences in exploratory microbial network topology: in N3, average degree increased by 15% for bacteria and 31% for fungi, whereas in the low-N treatment (N1), modularity decreased by 12% and 30%, respectively. High N also showed a higher relative representation of genes assigned to carbon metabolism and changes in the relative abundance of genes in the PAH-degradation pathway (ko00624). Mantel tests indicated treatment-dependent associations between microbial community composition and residual PAH profiles, particularly under low N. Overall, long-term N fertilization was associated with shifts in soil microbial communities and their functional potential related to carbon, nitrogen, and PAH metabolism. These association-based results do not demonstrate pathway activity, PAH degradation rates, or environmental risk; direct process-rate and bioavailability measurements would be required for those inferences. These findings support site-specific N management that accounts for both crop nutrient demand and the persistence of legacy organic contaminants.