Severe nutrient depletion reshapes rhizosphere microbial succession toward necromass recycling under continuous monocropping of Coix lacryma-jobi

Continuous monocropping often leads to progressive soil functional degradation and yield decline, yet the mechanisms by which severe nutrient depletion and stoichiometric imbalance reshape microbial functional reorganization in the rhizosphere remain poorly understood. Here, using a pot-based consecutive cropping system of Coix lacryma-jobi as a model, we integrated soil metabolomics, metagenomics, and physicochemical analyses to elucidate the assembly and successional trajectories of the rhizosphere ecosystem. Our results show that severe nutrient depletion—characterized by the marked decline of bioavailable phosphorus and potassium—was a major stoichiometric constraint associated with microbial succession, rather than soil acidification or autotoxic effects alone. This multidimensional nutrient limitation was accompanied by a fundamental shift in microbial functional strategies from nutrient acquisition to resource conservation. Specifically, microbial functional convergence increased, characterized by the upregulation of membrane transport systems and nitrogen metabolism pathways. Network topology analysis further revealed that Mesorhizobium emerged under nutrient stress as a key hub taxon coordinating microbial necromass recycling. This hub taxon was tightly coupled with glucosamine, a marker of microbial cell wall turnover, and membrane transport-related functional modules, indicating a progressive transition of community metabolism toward necromass scavenging as a means to sustain function under nutrient-depleted conditions. Collectively, our findings show that consecutive cropping forces the rhizosphere ecosystem into a necromass-recycling-oriented adaptive state, providing a mechanistic framework for understanding how nutrient depletion and stoichiometric imbalance shape microbial functional evolution in agricultural systems.