Thinning increased the contribution of microbial necromass to the soil organic carbon in shelter forests

Thinning is an essential forest management practice that improves the quality and carbon sequestration capacity of shelter forests, and affects the soil organic carbon (SOC) pool mainly by altering understory hydrothermal conditions and litter inputs. The idea that SOC is divided into two pools, plant and microbial, according to where it comes from, has received general attention. The limited quantitative research on microbial necromass carbon (MNC) and plant necromass carbon(PNC) may hinder our understanding of the mechanisms by which thinning affects the soil carbon pool and forest quality in shelter forests. In this study, we selected shelter forests (mainly Quercus acutissima Carruth and Pinus thunbergii Parl) within 5 years of thinning and used two common biomarkers (e.g. amino sugars and lignin phenols) to quantify the relative changes in MNC and PNC under thinning. Moreover, the mantel-test and random forest models were used to further reveal the relationships among the soil physicochemical properties, microbial community, MNC and PNC. Our results indicated that thinning had a similar effect on the soil carbon pools of Pinus thunbergii Parl (PTP) and Quercus acutissima Carruth (QAC) within 5 years, specifically increasing the contents of the fungal necromass carbon of 10.3 % and 7.3 %, and MNC of 9.2 % and 5.8 %, respectively. PNC did not change within 5 years of thinning, but the monomer content of cinnamyl (0.06 g kg⁻¹ and 0.08 g kg⁻¹), vanillyl (0.17 g kg⁻¹ and 0.21 g kg⁻¹) and syringyl (0.10 g kg⁻¹ and 0.08 g kg⁻¹) in PTP and QAC significantly increased with thinning, respectively. Furthermore, the mantel-test and random forest models showed that the MNC and PNC had different drivers. For MNC, the carbon-to‑nitrogen ratio and pH were the dominant factors, while PNC mainly depended on the regulatory effects of the phylum Firmicutes and total phosphorus. In summary, this study suggested that the effect of thinning on soil carbon sequestration capability in shelter forests is mainly mediated by the regulation of MNC accumulation.