Effects of thinning on soil microbial community and carbon fractions and their relationships in coastal protected forests

Thinning plays a critical role in soil carbon (C) sequestration by influencing forest density, soil microbial properties, and other factors. However, the relationship between soil microorganisms and soil organic carbon (SOC) fractions in coastal shelterbelts under thinning remains weak, especially regarding the effects of microbial-rich taxa on SOC fractions. This study examined Pinus thunbergii Parlatore (PTP) and Quercus acutissima Carruth. (QAC) forests after four years of thinning, analyzing changes in soil microbial communities and SOC fractions in the 0–20 cm surface layer. Besides, the SOC fractions affected by different abundance taxa of soil microbes during thinning were also analyzed. Thinning reduced Chao1 and ACE indices for soil bacterial communities in PTP and QAC forests, while fungal communities significantly increased. The relative abundance of abundant soil bacterial taxa rose by 3.33% (P < 0.05) in QAC, whereas abundant fungal taxa decreased by 7.22% (P < 0.05). Abundant fungal taxa, including Ascomycota and Basidiomycota, affected particulate organic carbon (POC), while bacterial taxa, such as Acidobacteriota and Proteobacteria, positively impacted mineral-associated organic carbon (MAOC). SOC fractions showed preferential changes, with POC decreasing by 21.61% (P < 0.05) in PTP and 24.41% (P < 0.05) in QAC, while MAOC rose by 22.95% (P < 0.05) in PTP and 6.53% (P > 0.05) in QAC. The carbon quality index (CQI) significantly increased by 56.76% (P < 0.05) in PTP and 38.89% (P < 0.05) in QAC. These findings demonstrate that thinning influences the active SOC fractions by altering the composition of the abundant soil microbial taxa, thereby promoting the stability of the soil carbon pool in protected coastal forests.