Mechanisms Of Carbon Transformation In Grassland Ecosystems Affected By Grazing

Grassland is an important component of terrestrial ecosystem, and plays an important role in the global climate control and the supply of livestock products. The selective in-take, trampling and the excreta by grazing animals have an influence on the vegetation composition and productivity, physical and chemical properties of soil as well as soil microbial communities. Litter is one of the important sources of soil organic carbon, and its quality determines the availability of carbon and nutrients, as well as the composition and activity of soil microorganisms. The changes of vegetation and soil nutrients caused by the combined effects of grazing on grassland affect the decomposition of litter and the accumulation and release of nutrients and carbon.This study originated from an investigation on impacts by grazing intensities on vegetation,soil FF-SOM, as well as microbial community. Then experiments were designed following two main paths including nutrients and vegetation change. By using stable isotope probing technique,species removal et al., experiments in lab and in situ were conducted to detect the responses of litter decomposition, transformation of carbon and nutrient transfer. The main conclusions are as follows:(1)Grazing intensity has an effect on vegetation productivity and diversity, microbial composition, soil organic carbon and nutrients. The increase of grazing intensity is not conducive to the accumulation of stable organic matter in the soil, but the relationship between the measurement of carbon, nitrogen and phosphorus in the stable fraction is not affected by the grazing intensity. Therefore, optimizing grazing management can not only improve the productivity and diversity of vegetation, but also increase the activity and biomass of microorganisms, and increase the stable storage of carbon and nitrogen and phosphorus in soil.(2)Changes in vegetation community structure and productivity attribute to the change of aboveground carbon input, and the transformation of microecosystem and the mechanism of solid storage will also be affected. It shows that dominant species play a key and unique role in community formation and carbon transformation. In contrast to different successional communities, the response of plant soil system caused by dominant species was excluded,indicating that the characteristics of dominant species and coexistence with non dominant species may be the original regulatory factors of community composition and carbon flow and transformation. From an applied perspective, these findings reveal the importance of different resilience and restrictive factors in the degraded grassland for the precise formulation of recovery strategies.(3)Our reciprocal transplant litter experiment by incubating 13C-labeled litter provides new insight to effects of litter type and decomposition sites on decomposition rates and the fate of litter-derived C. During decomposition, it exhibited higher decomposition rate of cellulose and more 13C incorporation in fungi, when each type of litter was incubated in its dominated sites.HFA index presented positive values that illustrate dominant litter had advantage in mediating decomposition process. These results provide evidences for revealing the mechanisms of leaf litter-soil feedbacks in grassland, including the return of carbon (C) to the soil through litter decomposition and fraction in functional microbial groups. Moreover, these findings indicated that grassland with a higher biodiversity will benefit the litter decomposition within the ecosystem.Once such litter-soil feedbacks process is considered, vegetation management in grassland will contribute to carbon cycle, formation of soil organic matter in ecosystem.(4)The plant removal experiment illustrated that dominant species played crucial and distinct roles in community assembly and C transformation. Across contrasting successional communities,cascading responses in the plant-soil system suggested that the traits of dominant species and their co-existence with subordinate species might regulate the community composition and C source.Overall, our findings might help for the development of effective restoration strategies of deteriorated grasslands.In summary, grazing can change grassland vegetation composition and soil nutrient status, and further affect the turnover of carbon and nutrient in litter decomposition process, and consequently regulate the ecological service level of grassland.