[Response Characteristics and Influencing Mechanism of Soil Enzyme Activity During Secondary Forest Succession in...

Soil extracellular enzymes are regulated by a range of biotic and abiotic factors, such as soil moisture and nutrients. In vegetation restoration, due to the heterogeneity of ecosystems and vertical spatial environments of soils, the differences in the response of soil surface and substrate enzyme activities to different stages of succession and the driving mechanisms are unclear. Therefore, using the method of "space instead of time," we analyzed the characteristics of four extracellular enzymes and their influencing factors in the soil surface and bottom layers during the succession of secondary forests in loess hilly areas. The results showed that： ① The activities of β-1,4-glucosidase （BG）； β-D-cellobiose hydrolase （CBH）； and β-1,4-N-acetamido-glucosidase （NAG） were significantly lower in the soil subsoil layer （40-100 cm） compared with those in the 0-20 cm soil layer； however, the activities of alkaline phosphatase （ALP） in the late successional period in the 0-20 cm soil layer were significantly lower than those in the 20-40 cm and 40-60 cm soil layers. Compared with the early and late successional stages, the soil enzyme activities reached the maximum value in the middle stage of succession. ② The succession of secondary forest changed the distribution characteristics of plant community and soil nutrient profile. In the middle stage of succession, the Gleason （G） richness index, Shannon-Wiener diversity index （H）, and Pielou evenness index （E） were the lowest. Compared with that in the early succession stage, the nitrate nitrogen content in the topsoil （0-40 cm） increased significantly in the middle succession stage. Compared with that in the middle succession stage, the soil water content and organic carbon content in the bottom soil layer （40-100 cm） increased significantly in the late succession stage. ③ Redundancy analysis and the least squares path model showed that soil nutrients were the most important factor affecting the soil enzyme activity in the surface layer （0-40 cm） of secondary forest, among which nitrate nitrogen had the highest explanation rate for the change of soil enzyme activity, and soil water content was the key factor to regulate the soil enzyme activity in the bottom layer （40-100 cm）. The results showed that the soil enzyme activity decreased significantly with the increase in soil depth in secondary forest, and the soil enzyme activity was the highest in the middle stage of succession. In the surface layer of soil, the enzyme activity was mainly affected by the content of nitrate nitrogen, and in the bottom layer of soil, and the enzyme activity was mainly regulated by soil water content. The results clarified the profile characteristics of soil enzyme activities at different succession stages and the driving factors of topsoil and subsoil, which provided a basis for formulating scientific forest soil protection policies.