Absence of nitrogen threshold effect on soil respiration leads to an underestimation of global soil carbon sequestration in model

Abstract. Soil respiration (Rs), a key component of the global carbon cycle, plays a pivotal role in regulating atmospheric CO2 concentrations and climate. Yet, the global responses of Rs and its components, including heterotrophic respiration (Rh) and autotrophic respiration (Ra) to varying levels of nitrogen (N) deposition remain poorly understood. By synthesizing global data from 931 paired observations of 226 experimental sites, this study indicated that Rs responses generally decline with increasing N inputs: low to moderate additions exert negligible or slightly positive effects, while higher additions consistently suppress Rs, mainly through reductions in Rh. Based on an N addition threshold (Nth) that indicates whether Rs increases or decreases, we incorporated this response pattern into the Community Land Model version 5 (CLM5). We demonstrate that ignoring N's variable impacts may cause Rs to be overestimated or soil carbon sequestration to be underestimated by up to 20 %. Our results provide a robust global assessment of Rs-N relationships and demonstrate the importance of representing N-induced reductions in soil respiration in Earth system models, to improve predictions of terrestrial carbon-climate feedbacks.