Microbes Drive Straw Decomposition and Microbial Metabolism in Mollisols with Different Straw Return Rates

The process of straw decomposition is highly complex and is regulated by a multitude of interacting factors. However, how the straw return rate influences the metabolic byproducts, extracellular enzymes, and microbial communities during straw decomposition remains an unresolved question. To address this, we integrated straw and soil chemistry, amplicon sequencing, untargeted metabolomics, and enzyme assays across four straw return rates (no return, 1/3, 1/2, and the Full treatment) at 30 and 90 d. Straw mass loss was greatest under the 1/2 return treatment at 90 d, reaching 53.7%, whereas soil organic carbon (SOC) and total nitrogen (N) were highest under the 1/3 return, exceeding the control by 17.6% and 17.2%, respectively. This indicates that decomposition and short-term soil C and N accumulation were decoupled. Microbial communities underwent clear temporal turnover, and hydrolytic enzymes increased from 30 to 90 d and were positively associated with mass loss. In contrast, oxidative enzymes showed no positive association with decomposition. Metabolomic profiles shifted from early-stage labile compounds to later-stage aromatic and phenolic compounds, and straw and soil metabolomes were closely coupled in a stage-specific manner (Procrustes M2 = 0.15, p = 0.001; Mantel r = 0.69, p = 0.001). Mantel tests further indicated that the metabolome was significantly associated with return rate and SOC. Partial least squares path modeling revealed that the direct and indirect pathways linking return rate, metabolome, enzymes, and decomposition were reorganized between 30 and 90 d. Overall, straw return rate influenced decomposition through stage-dependent biochemical and microbial changes, and straw mass loss and short-term soil C and N accumulation were decoupled, responding nonlinearly and peaking under different return rates.