Heavy thinning reduces soil organic carbon: Evidence from a 9-year thinning experiment in a pine plantation

Abstract

Forest thinning is a common forest management practice and has complex effects on soil organic carbon (SOC). However, the mechanisms underlying SOC and its components (physical fractions and chemical compositions) in response to forest thinning remain poorly understood. Four thinning treatments including control (CK with no thinning), light thinning (20% of basal area removed, LT), moderate thinning (40% of basal area removed, MT), and heavy thinning (60% of basal area removed, HT) were applied in a pine plantation. After nine years of thinning, SOC physical fractions and chemical compositions, soil heterotrophic respiration, plant biomass, soil properties, soil microbial biomass, and soil enzyme activity were measured. The results showed that HT significantly reduced SOC by an average of 39.15% relative to CK, which was likely due to the decreases in litter production, soil nutrients, and fine root biomass. Compared with CK, HT significantly reduced particulate organic carbon (POC), which was correlated with the decreases in soil nutrients and microbial biomass nitrogen. Moreover, MT and HT significantly reduced the percentage of alkyl-C and alkyl-C: O alkyl-C, whereas only HT increased the percentage of labile functional group of O alkyl-C. The changes in SOC chemical compositions were related to plant biomass and the decomposition of soil labile organic matter. Collectively, combining SOC fractions, plant biomass, and microbial decomposition could provide a comprehensive understanding of SOC dynamics in response to thinning in managed forests. We recommend that less than 60% thinning intensity would be reasonable in view of increasing forest productivity and maintaining soil organic carbon.