Nitrogen deficiency accelerates soil organic carbon decomposition in temperate degraded grasslands

The impacts of nitrogen (N) availability on soil organic carbon (SOC) decomposition were often explored based on N enrichment (N+) experiments. However, many natural and anthropogenic processes often reduce soil N availability. There is no direct evidence about how decreased N availability (N-) affects SOC decomposition, and the mechanisms of microbe-driven SOC decomposition in response to N availability remain unclear. Here, we used ion-exchange membranes to simulate N-. Soil samples from four temperate grassland sites, ranging from non-degradation to extreme degradation, were incubated with the N- and N+ treatments. We found that the total cumulative carbon (C) release was promoted by the N- treatment (8.60 to 87.30 mg C/g C_inital) but was inhibited by the N+ treatment (-129.81 to -16.49 mg C/g C_inital), regardless of the degradation status. N- dramatically increased recalcitrant C decomposition by increasing soil pH at all grassland sites; while did not affect or even decreased labile C decomposition by significantly increasing microbial C use efficiency and soil microbial biomass N. Interestingly, the effects of N- and N+ on SOC decomposition was asymmetric; with increased grassland degradation, the SOC decomposition was more sensitive to N- than to N+. Our results provide direct evidence for the different effects and mechanisms of N- on SOC decomposition and should be considered in soil process models to better predict the response of the nutrient cycle to global changes.