Soil microbial community and enzymatic activity in soil particle-size fractions of spruce plantation and secondary birch forest

Community composition and activity of soil microbes are responsive to changes in physiochemical and biological attributes during forest succession. However, whether soil microbial community structure and activity in soil particle-size fractions under different restoration approaches (regeneration through secondary succession and afforestation) during a chronosequence ranging from 20 to 70 years following deforestation of natural forest is still unclear. 24 soil samples from the top 10 cm of soil profiles were collected for four stand ages of spruce plantations and natural birch forests from three different sites. Soil aggregates were fractionated with dry-sieving technique and soil microbial communities in soil particle-size fractions were characterized by PLFAs profiling and enzymatic activity assay. The effects of restoration approach, soil aggregate-size and stand age on soil microbial biomass were depended on microbial group identity. Differences in microbial biomass were correlated to soil organic carbon, total nitrogen and pH. Additionally, differences in soil organic carbon, total nitrogen, pH and C:N ratio across restoration approaches, soil aggregate-sizes and stand ages jointly regulated soil microbial community composition. Soil aggregate-size and stand age also exerted significant effects on the potential activities of β-1, 4-glucosidase (βG), β-1, 4-N-acetyl-glucosaminidase (NAG), acid phosphatase (ACP) and polyphenol oxidase (POX), while the effect of restoration approach on enzyme activities depended on the enzyme identity. Soil organic carbon, total nitrogen were major predictors of enzymatic activity in particle-size fractions. The carbon quality index (CQI=lnPOX/(lnβG+lnPOX)) rather than soil quality index (SQI=(βG*NAG*ACP*POX4)) was a promising indicator of the response of soil microbial activity to restoration approach and stand age. In conclusion, the effects of restoration approach on soil microbial biomass and enzymatic activity vary depending on aggregate-size and stand age. Soil microbial biomass, community structure and enzymatic activity provide evidence that natural restoration enhancing soil quality in comparison with artificial restoration in the fragile subalpine forest ecosystem.