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 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 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. [Display omitted] •Nitrogen (N) deficiency increased soil recalcitrant carbon (C) decomposition.•Microbial regulation strategies under N imbalance drove soil labile C release.•Soil organic C decomposition responded to changes in N availability asymmetrically.•Soil organic C decomposition was more sensitive to N scarcity than to N abundance.