Sloping Farmlands Conversion to Mixed Forest Improves Soil Carbon Pool on the Loess Plateau

Vegetation restoration is considered a potentially useful strategy for controlling soil erosion and improving soil organic carbon (SOC) in arid and semiarid ecosystems. However, there is still debate regarding which vegetation restoration type is the best choice. In this study, four vegetation restoration types (i.e., grasslands, shrubs, forests and mixed forests) converted from sloping farmlands were selected to explore the SOC variation among the four types and to investigate which soil factors had the greatest effect on SOC. The results showed while the magnitude of effect differed between vegetation restoration type, all studied systems significantly increased SOC and labile organic carbon contents ( p < 0.01), soil nutrients such as total nitrogen (TN) ( p < 0.01), available nitrogen (AN) ( p < 0.01), total phosphorus (TP) ( p < 0.05) and available phosphorus (AP) ( p < 0.05), soil enzyme activities such as phosphatase ( p < 0.01), soil microbial biomass carbon (MBC) ( p < 0.05), and basal respiration (BR) ( p < 0.05), but had significant negative correlationswith polyphenol oxidase ( p < 0.05). However, the effects of vegetation restoration of farmland converted to natural grasslands, shrubs, forests and mixed forests varied. Among the types studied, the mixed forests had the largest overall positive effects on SOC overall, followed by the natural grasslands. Soil nutrients such as N and P and soil microbial activities were the main factors that affected SOC after vegetation restoration. Mixed forests such as Robinia pseudoacacia and Caragana korshinskii are the best choice for farmland conversion on the central of the Loess Plateau.