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 en-richment (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 Cinital) but was inhibited by the N+ treatment (-129.81 to -16.49 mg C/g Cinital), 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 micro-bial 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 con-sidered in soil process models to better predict the response of the nutrient cycle to global changes.

Automated summary

The reduction of nitrogen availability significantly promotes soil organic carbon decomposition, especially the decomposition of recalcitrant carbon, while it does not affect or even decrease the decomposition of labile carbon.