Soil Nitrogen Cycle Has Not Been Stimulated by Nitrogen Enrichment in Forests Under Atmospheric Nitrogen Loading

Soil nitrogen (N) status is pivotal information for evaluating the role of the forest N cycle in forest function evolution with environmental changes. However, soil N levels and processes in forests receiving high anthropogenic N inputs remain less investigated, which hinders understanding the effects of high N pollution on forest N status and related functions. This study investigated soil N concentrations, net N rates, and their relationships with environmental factors among 26 temperate forests under the total N deposition of 28.7−69.0 kg-N/ha/yr in an N-polluted region of northern China. Compared with other forests globally, we found distinctly high levels of soil inorganic N (NH4+, 27.1±0.8 mg-N/kg; NO3−, 7.0±0.8 mg-N/kg) but low net N mineralization (Nmin, 0.5±0.1 mg-N/kg/d) and nitrification rates (Nnit, 0.4±0.1 mg-N/kg/d) among the study forests. Atmospheric N inputs stimulated soil nitrification but did not enhance soil microbial biomass N and N mineralization. Still, soil moisture and C availability were found as dominant factors regulating the variation of soil N mineralization. By budgeting the atmospheric N inputs and differences in soil total N densities between the 2000s and 2010s, we found that atmospheric N inputs into study forests (56.7 kg-N/ha/yr) were more retained than lost proportionally (84% vs 16%). We concluded that soil N enrichment did not stimulate mineralization among forests under high N deposition. These results clarified soil N levels under high N pollution and identified major controlling factors of soil N dynamics, which is useful for evaluating fates and ecological effects of atmospheric N pollution