Phosphorus source shapes rhizosphere microbiome assembly and plant oxidative stress regulation in a volcanic Andisol:...

Volcanic Andisols in southern Chile severely limit pasture productivity due to their high phosphorus (P) sorption capacity. This study evaluated how contrasting P sources shape the rhizosphere microbiome of perennial ryegrass (Lolium perenne) and the pathways linking microbial community composition, plant biochemical responses, and biomass production under P-limiting conditions. A greenhouse experiment using an Andisol (Freire series) tested four P sources; Triple Superphosphate (TSP), Phosphate Rock (PR), Cattle Dung (CD), and Poultry Manure (PM); at two nitrogen (N) rates (100 and 200 mg kg^-1) alongside no-P control that received the same two N rates and basal nutrients but no added P. Rhizosphere communities were characterized by 16S rRNA amplicon sequencing coupled with functional gene prediction (PICRUSt2), antioxidant enzyme activity and lipid peroxidation markers (superoxide dismutase, catalase, malondialdehyde), P-starvation transporter and phosphatase gene expression (LpPHT1;1, LpPHT1;4, LpPAP1), and soil chemistry. P source was the dominant driver of both plant performance and community composition, with N rate exerting a secondary, modulatory role; attenuating P-starvation gene expression at higher doses and subtly influencing community beta-diversity. Soil P availability, pH, and exchangeable aluminum were the significant drivers of community structure. Path analysis revealed that the rhizosphere microbiome had no significant direct effect on shoot P uptake but was strongly associated with reduced foliar oxidative stress (β = -0.759, p < 0.001), with its indirect effect on shoot biomass statistically consistent with mediation through this antioxidant buffering response. Organic amendments enriched predicted genes related to P mineralization and biological N-P cycling while suppressing denitrification-associated genes. These findings suggest that in high P-fixing volcanic soils, rhizosphere community shifts benefit plant growth primarily by alleviating P deficiency-induced oxidative stress rather than by directly enhancing P uptake, highlighting the agronomic value of organic amendments beyond nutrient supply alone.