Impact of litter decomposition driven by nitrogen deposition on the soil organic carbon fractions in a Moso bamboo forest.

Soil organic carbon turnover and stabilization are closely related to nitrogen deposition and litter decomposition. However, there are great uncertainties about how the decomposition of bamboo litter driven by nitrogen deposition affects soil organic carbon components. To investigate the effects of nitrogen deposition-driven litter decomposition on soil organic carbon components, we conducted an experiment at the Anji Moso bamboo ecosystem research station of Zhejiang A&F University with nitrogen treatments (N, 50 kg N·hm^-2·a^-1; CK, control with equal amount of water) and litter treatments (L, litter retention; LR, litter removal) to analyze changes in litter mass loss, soil physicochemical properties, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and soil extracellular enzyme activity (EEAs). The results showed that nitrogen application significantly reduced the mass loss of leaf litter. Nitrogen application significantly increased POC content and decreased MAOC content, but litter retention significantly increased the contents of POC and MAOC in soil. Nitrogen application significantly decreased the activities of β-1,4-glucosidase (BG), β-1,4-xylosidase (BX), cellobiohydrolase (CBH), β-1,4-N-acetyl-glucosaminnidase (NAG), phenol oxidase (POX), and peroxidase (PER), while litter retention significantly increased the activities of BG, POX, and PER. Results of correlation analysis and random forest analysis showed that the key factors affecting the decomposition of Moso bamboo litter under nitrogen treatment were BG, PER, pH, microbial biomass carbon (MBC) and POX. Through redundancy analysis (RDA) and regression fitting analysis, we found that POC was significantly negatively correlated with mass loss, MBC, BG, CBH, POX and PER, and significantly positively correlated with ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N). MAOC was significantly positively correlated with mass loss, pH, MBC, CBH, NAG, POX and PER, and negatively correlated with microbial biomass nitrogen (MBN). In conclusion, nitrogen deposition inhibits bamboo leaf litter decomposition by reducing extracellular enzyme activities, thereby increasing soil POC content and decreasing MAOC content.