Abstract: Soil extracellular enzymes play an important role in soil nutrient cycling. However, the mechanism underlying the changes of soil enzymes during vegetation succession is not clear. In this study, on the basis of substituting space for time, cropland and Quercus liaotungensis forests after 30, 60 and 120 years restoration were selected as research objects to examine the changes and driving factors (vegetation and soil physicochemical characteristics, and microbial biomass) of soil enzyme activities and their stoichiometry in the process of secondary succession in Fuxian County, Shaanxi Province. Across the chronosequence, the activities of β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG), and leucine aminopeptidase (LAP) showed an increasing trend, that of alkaline phosphatase (AP) first increased, then decreased and then increased, reaching a maximum value after 120 years of vegetation restoration. The activities of BG, LAP+NAG and AP ranged 25.96-40.96, 57.10-128.05, 65.41-126.60 nmol·g^-1·h^-1, respectively. BG/(LAP+NAG) and BG/AP in the cropland were higher than those in the 30-year-old forest, while (LAP+NAG)/AP showed an opposite trend. In the cropland, soil microbial metabolism was mainly limited by C and P. With vegetation succession, BG/(LAP+NAG) increased gradually, and (LAP+NAG)/AP and BG/AP increased first and then decreased, indicating that the limitation of C and P on microbial metabolism was alleviated and shifted to N limitation. RDA results showed that soil enzyme activities and their stoichiometry were mainly affected by soil microbial biomass, ammonium nitrogen, and root biomass during the secondary succession. In conclusion, vegetation succession alters microbial metabolism by affecting soil nutrient content, with consequences on soil nutrient cycling.

Key words: soil enzyme activity, enzymatic stoichiometry, nutrient limitation, vector analysis.