Stand density-induced tree species diversity alters microbial carbon fixation/degradation genes and soil organic carbon pool during forest rewilding

Abstract Stand density management can influence understory natural regeneration and plant diversity during forest rewilding. Changes in plant diversity drive shifts in soil microbial communities and soil organic carbon (SOC) sequestration. However, the response of soil microbes, especially microbial carbon (C) metabolic functions, and SOC fractions to changes in stand density and plant diversity remains poorly understood. This study examined naturally regenerated Chinese fir (Cunninghamia lanceolata) stands after 30 years under three sprout density treatments (1200, 850 and 500 stems ha−1). We investigated variations in plant diversity (tree species, structural and functional), SOC fractions (particulate and mineral-associated organic C), microbial diversity, CO2 fixation pathways and carbohydrate-active enzymes, as well as the linkage among these variables in the topsoil (0–10 cm) using metagenomic sequencing. Tree species, structural and functional diversities, as well as fungal alpha diversity, increased with decreasing sprout density, whereas bacterial alpha diversity remained unchanged. The abundances of most C fixation pathways and genes involved in labile C degradation increased with decreasing sprout density. Microbial diversity and C fixation/degradation genes were primarily influenced by tree species diversity. The contents of SOC fractions increased with reduced sprout density and exhibited positive correlations with plant and fungal diversities, most C fixation pathways (e.g. Calvin cycle, rTCA cycle, DC/4-HB cycle and 3-HP/4-HB cycle), and labile C (e.g. cellulose and peptidoglycan) degradation genes. These findings highlight that reducing stand density significantly enhances tree species diversity over the long term, which in turn promotes SOC accumulation by influencing microbial C fixation and degradation potential. Our study provides new insights into how tree species diversity mediates microbial regulation of SOC sequestration during forest rewilding.