Soil bacterial community shifts and metabolic pathway changes in Pinus massoniana forests under varying thinning densities: Ecological restoration of degraded red soils

Soil bacteria are crucial indicators of soil quality and play a key role in the ecological restoration of degraded sites. This study examines the effects of thinning retention density and stand structural variations on soil bacterial communities and metabolite composition in Pinus massoniana forests within degraded red soil regions of Changting, Fujian. High-throughput sequencing and non-targeted metabolomics were used to analyze bacterial diversity, metabolites, and their functional interactions with stand structure. The results showed that with increasing thinning retention density, bacterial richness (Sobs, Chao1, ACE), evenness (Pielou), and diversity (Shannon) followed the trend: T2000 > T1600 > T2700 > T1200 > T0. The relative abundance of Acidobacteria and Proteobacteria first increased and then decreased, whereas Chloroflexi declined. Bacterial community composition varied by thinning density: Chloroflexi dominated unthinned stands, Acidobacteria were most abundant in moderately thinned stands, and Proteobacteria were prevalent in higher-density thinned stands. Metabolomics identified 1188 metabolites, including lipids (18.7 %), organic acids (7.8 %), and phenols (7.8 %). Differential metabolites, such as L-selenomethionine, scopolamine, and isocitrate, were enriched in key pathways related to phenylpropanoid biosynthesis, secondary metabolite synthesis, and amino acid alkaloid production. Correlation analysis revealed that adenosine 3′-phosphate 5′-phosphosulfate and isocitrate were positively associated with Chloroflexi, while Tropinone and Seleno-l-methionine correlated with Proteobacteria. Overall, thinning alters microbial composition and metabolic pathways, reducing oligotrophic bacteria while promoting copiotrophic bacteria, and the decomposition functional metabolites such as L-selenomethionine and sinapyl alcohol decreased. These changes influence soil nutrient cycling and provide insights into sustainable forest management in degraded red soil regions.