Thinning affects microbial biomass without changing enzyme activity in the soil of Pinus densiflora Sieb. et Zucc. forests after 7 years

Key message: Thinning increased microbial biomass but did not alter enzyme activities in the soil ofPinus densifloraSieb. et Zucc. forests in South Korea. This effect of thinning was larger under a relatively heavy thinning intensity, but there was divergence in the magnitude between sites. Context: The balance between microbial biomass accumulation and enzymatic C and N assimilation determines the level of bio-available C and N. However, the effects of thinning on these parameters remain contradictory and unconfirmed. Aims: The effects of thinning intensity on microbial biomass and enzyme activity were assessed in the soil of Pinus densiflora Sieb. et Zucc. forests in South Korea. Methods: Un-thinned control and 15 and 30% basal area thinning treatments were applied to two 51- to 60-year-old P. densiflora forests with different management histories, topographies, rainfall amounts, and soils. Seven years after thinning, microbial biomass and activities of N-acetyl-glucosaminidase, β-glucosidase, cellobiohydrolase, β-xylosidase, phenol oxidase, and peroxidase were measured before and after seasonally concentrated rains and at 0–10 cm depth. Results: Microbial biomass was generally highest under the 30% basal area thinning and lowest under the control, and was positively correlated to total soil C and N. The increase in microbial biomass was lower at the site displaying sandier, drier, and more acidic soils and retaining smaller amounts of thinning residue. Conversely, thinning had no significant effect on activities of all enzymes at both sites in both periods. Conclusion: Thinning can promote accumulation of microbial biomass without significant change in enzyme activities participating in the assimilation of C and N. This effect of thinning tended to increase with thinning intensity but differed in magnitude between sites.