Root Exudate Chemotypes and Soil Improvement in Orchard Green Manures: Species-Specific Associations Reflecting Family-Level Phylogenetic Differences

Green manures differ widely in their capacity to modify the rhizosphere soil environment. However, the link between root exudate chemistry and changes in soil properties remains poorly understood. We grew alfalfa (Medicago sativa L.), white clover (Trifolium repens L.), smooth bromegrass (Bromus inermis Leyss.) and perennial ryegrass (Lolium perenne L.) in pear orchard inter-rows, sampling rhizosphere soil (operationally defined here as the 1–4 mm root-adhering layer) and inter-row non-rhizosphere background soil at four stages within a single growing season. Root exudates (112 compounds) were profiled by GC-MS and analyzed with soil properties using compositional data approaches. The four species formed distinguishable chemotypes. Rhizosphere effects diverged: perennial ryegrass showed the largest relative increase in extractable P versus its own inter-row non-rhizosphere background (+37.0%), whereas absolute rhizosphere available P was highest in alfalfa (48.3 mg kg−1), which also accumulated the most nitrogen (total N +19.6%), and white clover buffered pH best. This increase in extractable P was accompanied by lower rhizosphere TN and AN and does not by itself demonstrate enhanced p acquisition, which in calcareous soils depends on sorption and Ca-phosphate precipitation. PCA separated species along phosphorus–organic, matter–salinity, and nitrogen–potassium gradients (PERMANOVA R2 = 0.365, p = 0.001). After center log-ratio (CLR) correction, the chemotype remained coupled to soil properties (Mantel r = 0.246, p = 0.005), but fell to r = 0.062 (p = 0.236) when family was controlled, indicating that the coupling was largely attributable to family-level background (two legumes versus two grasses). Among compound classes, amino acids showed the strongest coupling with the soil matrix (r = 0.32, p < 0.01). Among individual compounds, tryptophan showed the strongest uncorrected correlation with rhizosphere nitrogen (r = +0.83), although this association was not significant after multiple-testing correction (minimum q = 0.056) and should only be used for hypothesis generation purposes. Entropy-weighted TOPSIS and gray relational analysis (GRA) anchored to nitrogen function ranked alfalfa first in all scenarios, its lead robust to index composition. Overall, these findings are exploratory: the chemotype–soil association reflects family-structured multivariate covariation rather than single-molecule causation, and alfalfa is a tentative recommendation for this nitrogen-limited orchard, pending validation by controlled causal experiments.