The trend of soil organic carbon fractions related to the successions of different vegetation types on the tableland of the Loess Plateau of China

Different growth stages of vegetation are vital factors controlling soil organic carbon (SOC) sequestration on the tableland of the Loess Plateau. To ascertain the SOC sequestration process and direction, we studied the changes in SOC fractions and the relationships with enzyme activities in different stages of restoration with different vegetation types. Soils from different stages of farmland conversion into grassland or forestland were selected, in which the space sequence and sampling time were combined to replace time series. SOC, particulate OC (POC), light fraction OC (LFOC), dissolved OC (DOC), and soil enzyme activities of catalase, urease, alkali-phosphatase, and inverse were measured. The change process of labile organic carbon (LOC) was performed to study the direction of SOC in the succession of different vegetation types. The effect of SOC–soil enzyme activity interactions was evaluated by RDA variance decomposition to illustrate the role of soil enzyme activity in SOC sequestration. With the extension of the planting time, the SOC fractions and total nitrogen under Hippophae rhamnoides (HR) increased rapidly from 0 to 16 and 16 to 21 years and decreased rapidly from 21 to 30 years, while LOC showed greater variation. LOC under Pinus tabulaeformis (PT) changed more slowly than that under HR. The sensitivity of different LOC to land use varied. From the change rate of soil LOC, we concluded the DOC changed the fastest, followed by the LFOC, the CPOC in POC was the slowest in the extension of vegetation growth time. By RDA variance decomposition, fine POC was mainly affected by urease; coarse POC in POC was mainly affected by alkaline phosphatase, which was promoted by invertase; LFOC was mainly promoted by catalase; DOC was mainly affected by alkaline phosphatase. Therefore, during the progression of SOC change, the initial stage was obviously affected by alkaline phosphatase. Catalase played a leading role during the gradual decrease in SOC activity. When SOC became stable, it was instead mainly affected by urease. Land use changes and different growth stages of vegetation altered the SOC sequestration process and direction. The long-term return of farmland to grassland would greatly promote the concentrations of SOC fractions, and the short-term return of farmland to PT or HR would also encourage an increase in these fractions, while the long-term would have the opposite effect. Therefore, in order to effectively improve SOC concentrations, during vegetation restoration, it should be properly matched in vegetation species and stages.