Rhizosphere Engineering by Root Exudates: High-Yielding Alfalfa Recruits Functional PGPR to Sustain Soil Nutrient Availability Under Long-Term Cultivation.

Abstract

Soil nutrient transformation capacity is a critical determinant of sustainable productivity in perennial cropping systems; however, the extent to which high-yielding crops actively regulate rhizosphere microbial assembly to maintain nutrient availability remains poorly understood. We investigated whether root exudates from high-yielding alfalfa (Medicago sativa L.) selectively recruit plant growth-promoting rhizobacteria (PGPR) to enhance nutrient transformation. In an 8-year continuous alfalfa system (2018-2025), high-yielding cultivars increased soil organic carbon by 8.64%, total nitrogen by 6.01%, and moderately labile phosphorus fractions by 1.62%. Rhizobox experiments demonstrated that root exudates enhanced growth only with an active microbiome. High-yielding alfalfa enriched PGPR communities, specifically Ensifer, Pseudomonas, and Bacillus. Isolated strains exhibited N fixation, P solubilisation, and IAA production. Metabolomic profiling revealed that exudates were enriched in specific sugars and amino acids. Maltopentaose, maltotetraose, taurine, N-acetyl-L-leucine, and asparagine functioned as chemoattractants, stimulating PGPR proliferation and biofilm formation. These findings demonstrate that root exudate-mediated, targeted recruitment of functional PGPR enhances N fixation and P transformation, thereby supporting sustained high alfalfa productivity. This study demonstrates a key rhizosphere mechanism underlying the long-term sustainability of high-yielding perennial legume systems and provides a mechanistic basis for microbiome-informed sustainable alfalfa production and management.