Functional microorganisms in bioorganic fertilizers: strain screening, application validation, and characterization.

The efficacy of bioorganic fertilizers is strongly dependent on functional microbial strains; however, systematic frameworks for candidate strain acquisition, functional screening, application validation, and strain characterization remain insufficiently developed for bioorganic fertilizer research and product development. This review synthesizes the major functional microbial groups used in bioorganic fertilizers and their mechanisms of action, with particular emphasis on strain selection strategies, functional screening and application-validation pathways, and methods for strain identification and functional characterization. Current evidence indicates that conventional culture-based isolation, oligotrophic screening, extreme-habitat screening, and modern molecular and data-driven approaches each occupy distinct methodological niches. These strategies are respectively suited to pure-culture resource acquisition, discovery of strains adapted to resource-limited environments, identification of stress-tolerant and process-adaptive strains, and candidate discovery coupled with functional analysis. Importantly, optimization of functional strains should not rely solely on single in vitro trait-based screening. Instead, it should integrate formulation-related adaptability, robustness-oriented microbial phenotyping, and application-context validation into a tiered evaluation framework spanning strain acquisition, functional screening, application validation, and strain identification and functional characterization. This review provides a systematic methodological reference for the selection, evaluation, and product development of functional strains for bioorganic fertilizers (BOFs).