Responses of growing‐season soil respiration to water and nitrogen addition as affected by grazing intensity

Abstract

Abstract Most grasslands in the world, including the semi‐arid steppe in China, are threatened by nitrogen deposition, precipitation change and livestock grazing, which greatly affect soil carbon processes (e.g. soil respiration). Although the individual effects of nitrogen deposition and precipitation change on soil respiration are well understood, how their effects on soil respiration are altered by different grazing intensities is unclear. To determine how the effects of nitrogen deposition and precipitation change on soil respiration are affected by grazing intensity, we conducted an experiment in a semi‐arid steppe involving areas that experienced 10 years of no, light, moderate or heavy grazing. These areas were treated with water addition (110 mm, 30% of the mean annual precipitation) and nitrogen addition (10.5 g m−2 year−1). Our results showed that relative to no grazing, grazing decreased growing‐season soil respiration by 10%–19%. The decline in soil respiration was mainly via its negative effects on above‐ground net primary productivity (ANPP) and the fungi:bacteria ratio with light grazing, mainly via its negative effects on ANPP and leaf nitrogen content with moderate grazing, and mainly via its negative effects on ANPP, root biomass, microbial biomass and the fungi:bacteria ratio with heavy grazing. Across all grazing intensities, both water and water+nitrogen addition increased growing‐season soil respiration, whereas nitrogen addition decreased growing‐season soil respiration. Water addition increased growing‐season soil respiration mostly via its positive effect on ANPP with no grazing and with low grazing, and mostly via its positive effects on both plant and microbial variables with moderate and heavy grazing. The pathways determining the nitrogen addition‐induced decline in growing‐season soil respiration were the same within each of the four levels of grazing and mostly resulted from its negative effect on microbial variables. Our results indicate that the effects of climate change on growing‐season soil respiration and other soil carbon processes in grasslands depend on grazing intensity. The findings suggest that grazing intensity should be considered in future manipulation experiments and should be included in carbon models to accurately simulate soil carbon dynamics under scenarios of climate change in grassland ecosystems. A plain language summary is available for this article.