Autotrophic respiration is more sensitive to nitrogen addition and grazing than heterotrophic respiration in a meadow steppe

A quantitative understanding of temporal patterns of soil respiration (SR) and its components, as well as of their controlling factors, are key to the estimation of grassland C sequestration under different scenarios of land-use change and global change. However, the sensitivity of seasonal patterns and magnitude of SR and its components to N addition (N) and grazing (G) and their interactions remains poorly understood. We carried out an experiment with N, G and N addition plus grazing (NG) treatments in temperate meadow steppe located in the easternmost of the Eurasian steppe. We measured SR and its components, i.e., autotrophic respiration (AR) and heterotrophic respiration (HR), along with a suite of other biotic and abiotic factors over 3 years. N addition increased AR when compared to the control treatment (CK), whereas moderate grazing decreased AR, and N addition and grazing had additive effects on AR. We found that aboveground biomass was the predominant controlling factor of AR. However, the N, G and NG treatments had no significant effects on HR. Although N addition and grazing caused changes in a series of biotic and abiotic factors (e.g. total PLFAs, soil fungal/bacterial ratio and bulk density) that controlled soil microbial metabolic processes, collectively these effects on HR appeared to offset each other. Moreover, compared to the CK and N treatments, G and NG treatments reduced the apparent temperature sensitivity (Q(10)) of AR and HR. Grazing dampened the positive response of Q(10) of HR to N addition. Our study suggests that AR and HR are regulated by different underlying mechanisms and respond differently to N and G, and therefore should be modeled separately to improve models and predictions of ecosystem C cycling. Our study also indicates that moderate grazing could produce beneficial effects on meadow steppe carbon sequestration under future scenarios of nitrogen deposition and climate warming.

Automated summary

Understanding the temporal patterns and controlling factors of soil respiration (SR) and its components is crucial for estimating grassland carbon sequestration under different land-use and global change scenarios. However, little is known about the sensitivity of seasonal patterns and magnitude of SR and its components to nitrogen addition (N) and grazing (G) and their interactions. In this study, we conducted an experiment with N, G, and N addition plus grazing (NG) treatments in a temperate meadow steppe. We found that autotrophic respiration (AR) was increased by N addition, decreased by moderate grazing, and had additive effects with N addition and grazing. Aboveground biomass was identified as the main controlling factor of AR. On the other hand, N, G, and NG treatments had no significant effects on heterotrophic respiration (HR). Our study suggests that AR and HR are regulated by different mechanisms and respond differently to N and G, highlighting the importance of modeling them separately for accurate predictions of ecosystem carbon cycling. Additionally, moderate grazing may have beneficial effects on meadow steppe carbon sequestration under future scenarios of nitrogen deposition and climate warming.