Contrasting Soil Chemical and Microbial Responses to Moderate Thinning in a South Swedish Spruce Forest

Abstract

Abstract Small-scale forest owners frequently implement thinning to enhance stand productivity, improve timber quality, and maintain structural diversity; however, its short-term effects on soil physicochemical and biological properties remain insufficiently understood. This study evaluated the short-term effects of moderate thinning on selected soil physicochemical properties, microbial indicators, and wood decomposition by comparing adjacent thinned and unthinned spruce stands in southern Sweden. Soil moisture, pH, bulk density, organic carbon (OC), total nitrogen (N), mineral N forms, cation exchange capacity (CEC), major nutrients, microbial biomass indicators, and wood decomposition were assessed. Two years after thinning, no significant differences were detected in bulk soil physicochemical properties, nutrient pools, or carbon stocks between the stands, suggesting limited short-term effects on soil fertility and carbon storage. In contrast, wood decomposition rates and microbial biomass nitrogen (MBN) were significantly higher in the thinned stand, indicating an early biological response to thinning despite no significant differences in total microbial, fungal, or bacterial biomass. Pearson correlation analysis further revealed significant associations between MBN and several soil physicochemical properties, highlighting the close linkage between microbial N dynamics and soil conditions. Overall, the findings suggest that moderate thinning had limited short-term effects on bulk soil properties but stimulated wood decay processes and microbial N-immobilization. Given the short monitoring period and the case-study nature of this work, the results should be interpreted with caution, and long-term studies across replicated stands are needed to evaluate the persistence of these responses and their implications for forest soil functioning.