Changes of soil properties regulate the soil organic carbon loss with grassland degradation on the Qinghai-Tibet Plateau

Grassland in the Qinghai-Tibet Plateau (QTP) provides tremendous carbon (C) sinks and is the important ground for grazing. Grassland degradation, the loss of plant coverage and the emergence of sand activities, results in substantial reduction in soil organic carbon (SOC). To demonstrate the pattern of SOC loss and to elucidate underlying mechanisms, vegetation, soil microclimate, soil properties and respiration of grasslands with different degradation severity over the QTP were investigated. The survey and laboratory data were analyzed by three structural equation modeling (SEM) analyses, which based on three conceptual understandings. The black box model (M1) directly related the abiotic and biotic factors to SOC without consideration of any mechanisms. The biological understanding model (M2) developed the structure of SEM mainly considering ecological processes that regulate the soil SOC. The overall model (M3) developed the SEM structure with the inclusion of both physical and biological processes. Soil moisture (θ), the above and the below-ground plant productivity, and SOC significantly decreased while soil temperature (Tsoil) maintained with the development of land degradation. All the three models successfully fitted the data with R2 about 0.50. Significant pathways from latent variables to SOC were only observed from soil microclimate and soil properties in the M1. In the M2, three mechanisms can explain the SOC change. The decrease in θ and the consequent adverse effect on soil respiration suggest suppressed C output through microbial decomposition, thus lead to the less SOC loss. The decline in aboveground net primary productivity (ANPP) resulted from a decrease in coverage or due to the change in relative abundance of sedge, forbs, and grass directly or indirectly reduced the C input, and finally lead to the 40–50% loss in SOC. In the M3, only the change in soil properties can explain the SOC reduction. Our results suggest that changes in soil abiotic factors like soil bulk density and pH are the primary factors control the SOC change with land degradation.