Variation of soil organic carbon components and enzyme activities during the ecological restoration in a temperate forest

Knowledge of soil organic carbon (OC) components and enzyme activities is essential to maintain soil health and for sustainable land use during long-term vegetation restoration. The effects of vegetation restoration on soil physicochemical properties have been widely documented; however, knowledge of the relationships among soil OC components, enzyme activities, and soil properties in restored ecosystems remains unclear. Here, the soil from four successional stages of restored Robinia pseudoacacia vegetation (10, 20, 30, and 40 years) were sampled in China's Loess Plateau. The soil water content (SWC), bulk density (BD), pH, total nitrogen (TN), soil microbial biomass C (MBC) and N (MBN) contents, OC components (labile and recalcitrant OC), and soil enzyme (hydrolytic and oxidase) activities were measured. We found that restoration time significantly affects soil physicochemical properties, OC components and enzyme activities. Across the restoration time, the SWC decreased from year 10 to year 30, then increased from year 30 to year 40. Soil OC and TN increased with restoration time, while soil clay content decreased with restoration time. Following 40 years of restoration, soil MBC and MBN significantly increased by 214% and 88%, respectively. For OC components, 40 years of restoration significantly increased recalcitrant C component rather than labile C component. Soil hydrolytic enzyme activities significantly increased by 76.9% and 80.3% in 40 years stand compared with that of in 10 years stand, respectively, while soil oxidase enzyme activities significantly decreased with restoration time. Correlation analysis demonstrated that the SOC and MBC content significantly influenced soil recalcitrant C component and hydrolytic enzyme activities, while MBC and clay content significantly influenced soil oxidase enzyme activities. These results provided useful insights on the responses of forest restoration on soil OC components and related OC degrading enzymes and the understanding of soil C sequestration in the restored ecosystems in such areas.