Changes in plant, soil, and microbes in a typical steppe from simulated grazing: explaining potential change in soil C

Grazing can directly or indirectly influence carbon (C) inputs, turnover, and retention in grassland soil. However, relative to the plant response to grazing, belowground biota and process responses are more complex and often do not correlate with the aboveground responses. Ungulate grazing involves three mechanisms; defoliation (removal of plant shoot tissue), dung and urine return, and trampling. An evaluation of the relative roles of these mechanisms and their combinations in grazing can explain the causes of changes in grasslands, thereby explaining soil C sequestration in a steppe ecosystem. In this study, we examined changes in plant attributes, soil abiotic characteristics, and the soil microbial community in response to mowing (M), dung and urine addition (DU), simulated trampling (T), and their combinations by conducting a 3-yr experiment in a steppe ecosystem in Inner Mongolia, China. Most of the variation in the grazing effects on grasslands was explained by defoliation through decreased plant production and soil respiration and altered vegetation composition. Dung and urine return was second to defoliation in explaining grazing effects on grassland, and led to increasing plant C inputs to the soil, while simultaneously, potential loss of soil C due to the increase of the abundance of bacteria and soil respiration eventually accelerated soil C cycling. An interaction between defoliation and trampling on microbial growth was observed in our study: trampling increased the abundance of total bacteria, fungi, and arbuscular mycorrhizal fungi (AMF) only in the no-mowing plots. Trampling led to plant allocation to belowground tissues and increased the abundance of fungi and AMF, which are critical for soil C sequestration, and trampling with defoliation further decreased the abundance of soil microbes, which may decelerate soil C cycling and increase its retention time. These results indicate that defoliation and dung and urine return play major roles in explaining grazing effects on grassland systems, including plant, soil, and microbial parameters, but the trampling effects and the interaction between defoliation and trampling are two key factors that contribute to explaining the overall effects of grazing on soil C sequestration in a typical steppe ecosystem in Inner Mongolia.