New insights into organic carbon mineralization: Combining soil organic carbon fractions, soil bacterial composition, microbial metabolic potential, and soil metabolites

Cover crops are an important component of sustainable agriculture, and additional carbon (C) source inputs can accelerate the soil C cycle. However, the specific processes and factors that control cover crop impact on soil organic carbon (SOC) mineralization are still uncertain. In this study, the ecological associations among SOC fractions, soil bacterial composition, microbial metabolic potential, and soil metabolites and their combination with SOC mineralization based on an eight-year cover crop field experiment (summer fallow-winter wheat (Fallow), buckwheat-winter wheat (Buckwheat), black bean-winter wheat (Black bean) and sorghum sudangrass-winter wheat (Grass)) were analyzed. The results showed that cover crops increased SOC mineralization, which was 10.61 mg CO2-C kg- 1 soil d- 1 for Fallow and 1.17, 1.20 and 1.34 times greater for Buckwheat, Black bean and Grass, respectively, and that the SOC fractions and microbial metabolic potential showed similar trends to those of SOC mineralization. Cover crops increased the labile C content and amino acid and carboxylic acid metabolic activity, altered the soil bacterial composition and increased the amount of differentially abundant metabolites (DAMs). Hierarchical analyses revealed that soil bacteria and metabolites explained the majority of the variation in SOC mineralization, and partial least squares path modeling (PLS-PM) further revealed that soil bacteria increase microbial metabolic activity by regulating metabolites to promote SOC mineralization. Thus, cover crops alter the soil bacterial composition and stimulate microbial catabolic activity to influence the SOC cycle, a process in which soil metabolites play a key role.