Restoration of ecosystem carbon and nitrogen storage and microbial biomass after grazing exclusion in semi-arid grasslands of Inner Mongolia

The semi-arid grasslands in Inner Mongolia have been degraded as a result of long-term grazing and poor grassland management. A series of ecological restoration strategies have been implemented to enhance the ecosystem services of China's grasslands. To understand the response of ecosystem C and N to grazing exclusion within a nationwide conservation project-Returning Grazing Lands to Grasslands (RGLG), the C and N storages in different parts of the plant-soil system as well as soil microbial biomass carbon (MBC) and nitrogen (MBN) contents were determined and compared between three grazing exclusion sites (6 years) and the paired adjacent grazing sites in the Hulunbuir grassland of Inner Mongolia. Our results showed that the exclusion of grazing for 6 years resulted in significant increases of ecosystem biomass and soil C and N contents, leading to a substantial restoration of ecosystem C and N storage. The average C and N accumulation rates in the whole ecosystem were 173.6 g C m−2 year−1 and 19.8 g N m−2 year−1, respectively. The accumulation of C and N in soil contributed to more than 87.6% and 98.2% of the increases in ecosystem C and N, respectively, indicating that the restoration of ecosystem C and N were mainly due to the increase of C and N in soil after exclusion of grazing. The higher MBC and MBN in exclosure sites than grazing sites might be partly because of the enhanced organic matter inputs from plant litter and root exudates. The significant correlations between soil organic matter and microbial biomass suggested that soil microbial biomass could be a sensitive indicator of the soil C and N dynamics. These results indicated that exclusion of grazing pressure within the RGLG project is an effective restoration approach to control grassland degradation in this region. However, more comprehensive multidisciplinary studies are needed to evaluate the effectiveness of the RGLG project and to improve the management strategies for grassland restoration.