Composition of soil fungi and bacteria under different forms of nitrogen addition

Peatland soils, usually nitrogen (N)-limited, are important carbon (C) pools, which can be seriously affected by atmospheric N deposition. As the main forms of N deposition, nitrate (NO3-) and ammonium (NH4+) have significant spatial differences and affect C in soils variously. However, N forms altering soil organic carbon (SOC) in peatlands are relatively less studied. Therefore, this study explored the change in SOC content by adding different N forms and concentrations in the soil of Zoige peatland and explained how N forms affected SOC content. Within stimulating 0-400 kg N ha-1 yr-1 of NH4+-N deposition, the NH4+-N addition generally increased SOC, and the NH4+-N addition of 210 kg N ha-1 yr-1 might reach a maximum SOC of 73.19 mg g-1. Path analysis suggested soil and microbial properties were crucial factors affecting SOC under NH4+-N addition. After NH4+-N addition, soil pH was decreased, inhibiting the growth of microbes such as Nocardioides and Ascomycota responsible for soil organic matter (SOM) decomposition, thus causing C accumulation. Within stimulating 0-400 kg N ha-1 yr-1 of NO3--N deposition, the NO3--N addition decreased SOC but increased dissolved organic carbon (DOC), with a maximum DOC of 0.86 mg g-1 at 240 kg N ha-1 yr-1 of NO3--N addition. Microbial and ecoenzyme activities were important factors affecting SOC under NO3--N addition. The NO3--N addition increased enzyme activities and the abundance of microbes responsible for C decomposition, such as Proteobacteria and Actinobacteria, promoting SOC decomposition, thus causing C loss. These results aid the evaluation of SOC dynamics under different future N deposition scenarios in N-limited soils of peatlands.