Author(s): Nan Liu, Yingjun Zhang * , Shujuan Chang, Haiming Kan, Lijun Lin
Carbon sequestration in rangeland ecosystems has emerged as an important service to sequester greenhouse gases and mitigate climate change. Grazing, as the most geographically expansive land use, occurs over a third of the earth's land surface and may potentially influence the storage of 10 9 Mg year-1 of greenhouse gases as soil C . In recent years, extensive work has been conducted toward improving understanding of C reserves of grazing lands and quantifying pools and fluxes. Grasslands store considerably more carbon in soil than in the vegetation . Studies have shown that grazing can often promote C storage , . Thus, understanding the change of soil C storage under grazing intensity is important to reduce greenhouse-gas emissions and mitigate climate change.
Grazing can have a direct impact on plant production and thereby on soil C inputs, and has been extensively studied -. The change of vegetation composition has proved to be an important factor in influencing soil carbon sequestration in grazing ecosystems . Vegetation that has changed from a C 3 dominated, to a more C4 dominated plant community due to grazing, can lead to SOC accumulating closer to the soil surface, making it more vulnerable to being lost to the atmosphere . However, it is also reported that an increase in communities of C4 grasses which are tolerant of grazing and have more dense root systems and higher root-to-shoot ratios, at heavy grazing would result in increases in soil C and N , . Grazing also influences the amount and composition of soil organic matter (SOM) ,  through its effects on litter accumulation and decomposition , . Soil microorganisms play a central role in decomposition and respiration, and influence C storage in soil. Soil microbial biomass, the living part of soil organic matter, functions as a transient nutrient sink and is responsible for decomposition and transformation of organic materials which are mostly derived from above and below-ground plant residues, and releasing nutrient from organic matter which is used by plants , . Small changes in soil organic carbon in the short term are difficult to monitor, because of large background C concentrations and the natural variability of soils . Microbial biomass carbon (MBC) generally comprises 1-4% of soil organic matter  and is the most active component of soil organic carbon that regulates biogeochemical processes in terrestrial ecosystems . Soil MBC, as an important indicator of changes of soil quality and management practices , , is very sensitive to environmental changes . Microbial biomass also acts as a small but labile reservoir of nutrients that contributes to maintaining long-term soil sustainability. In grazing lands, organic input from vegetation and excreta of animal can contribute to increased soil organic matter content and consequently cause an impact on soil biological processes. Thus, soil microbial biomass plays a critical role in grazing ecosystems as there is a large input of organic residue.