Unimodal soil respiration—nitrogen rate relationship is ubiquitous in terrestrial ecosystems

• There is a widespread unimodal response pattern of Rs to N addition across the globe. • A humped-back model predicts Rs-N rate relationship for five or more N levels. • Unimodal Rs-N rate relationship is confirmed by comparison of N response efficiency among N rates for 3 and 4 N-level experiments. • The rate-dependent Rs response is explained by the unfavorable environments for metabolic respiration under limited or excessive N status. Soil respiration (Rs), as the second largest exchange flux between the atmosphere and terrestrial ecosystems, is vulnerable to global nitrogen (N) enrichment. However, previous meta-analyses, assuming a linear Rs-N rate relationship, may bias the real changes of Rs with continuously increasing N input. In this study, by compiling a global database of Rs measurements at least 3 N addition levels from 175 field N manipulative experiments, we analyzed the relationship between Rs and N addition rate. The results indicated that a unimodal pattern dominated the Rs-N rate relationship. Specifically, for the experiments with five or more N levels, the unimodal pattern accounts for the largest proportion of all response patterns, at 45 %. For 3 and 4 N levels, we calculated the N response efficiency (NRE; Rs response per unit of N addition) to test the nonlinearity in Rs response, and the results also revealed a commonly unimodal pattern across studies. This pattern is likely attributed to the fact that moderate N additions can enhance the activities of plants and soil microbes, while limited or excessive N is unfavorable to their activities. The above findings suggest that the unimodal Rs-N rate relationship needs to be considered in Earth system models for more accurately projecting soil carbon flux under global N enrichment.