Interaction Between Nitrogen Forms, Phosphorus Forms, and Mineral Phases Drives Phosphorus Limitation and Uptake by Ryegrass in a Model System

Background Nitrogen (N) and phosphorus (P) cycles significantly influence ecosystem functioning, yet the interactions between their chemical forms and mineral phases in determining P limitation remain poorly understood. Aims We investigated the impact of interactions between P forms (organic vs. inorganic), N forms, and mineral types on plant P uptake and aimed to understand how these interactions affect rhizosphere processes governing P availability, including pH modifications and phosphatase activity. Methods Ryegrass (Lolium multiflorum) plants were grown in RHIZOtest‐like devices in the presence of three P compounds (inositol hexakisphosphate—IHP, glycerophosphate—GLY, and inorganic P—IP) adsorbed onto two mineral phases (goethite and kaolinite), combined with two nitrogen forms (NO 3 ‐N and NH 4 ‐N). Results N addition shifted plant nutrition from N to P limitation, with the degree of this shift depending on P form and mineral type. Organic P sources induced stronger P limitation than IP. NO 3 ‐N enhanced P acquisition more effectively than NH 4 ‐N across all treatments. Mechanistically, N addition increased phosphatase activity but simultaneously depleted available P pools, with enzymatic P mobilization being the dominant process rather than pH‐mediated effects. Clay minerals (kaolinite) showed lower P limitation than Fe oxyhydroxides (goethite) due to contrasting adsorption mechanisms. Conclusions In this model system, NO 3 ‐N promoted greater P acquisition than NH 4 ‐N through enhanced phosphatase activity rather than pH changes, and P limitation was more severe on Fe oxyhydroxides than on kaolinite. These findings suggest that selecting nitrate‐based fertilization and adapting P management to soil mineralogy could help alleviate P limitation.