Five-Year Nitrogen Addition Alters Surface Mineral Soil Carbon Status and Respiration Partitioning in an Alpine Coniferous Forest

Atmospheric nitrogen (N) deposition is increasing N inputs to forests, but consequences for soil carbon (C) status and respiration partitioning remain uncertain. We conducted a five-year field N-addition experiment in an alpine coniferous forest in Xizang, China. We measured post-treatment soil properties, extracellular enzyme activities, estimated microbial C use efficiency (CUE), total soil respiration (Rs), root-exclusion heterotrophic respiration (Rh), and residual autotrophic respiration (Ra = Rs − Rh). Relative to the zero-N control (N0; 0 kg N ha−1 yr−1), N1 and N2 had higher post-treatment soil organic C, whereas all N-addition treatments had >60% lower available phosphorus and 19.12%–64% lower Rs. The Rs contrast was more strongly reflected in estimated Ra (35.93%–87% lower), whereas Rh responses varied among treatments and campaigns. N addition altered enzyme allocation and was associated with 6.70%–16.60% lower estimated CUE. Exploratory random forest and partial least squares path models indicated that Rh covaried more closely with microbial biomass, whereas Rs and estimated Ra covaried with soil physicochemical conditions and phosphorus availability. Thus, higher post-treatment soil C in some N-addition treatments co-occurred with lower Rs. However, the absence of pretreatment soil data and direct root measurements limits causal interpretation.