Phosphorus dynamics in a sandy soil amended with ph-modified pig slurry: insights from a rhizobox study using 33P isotopes

Abstract Background and aims This study assessed the impact of pH modification of pig slurry on the dynamics of slurry- and soil-derived phosphorus (P) after surface application, as a potential alternative to inorganic P fertilizers. Methods Mineral acidification (pH 5), bio-acidification (pH 5), and alkalinization (pH 9.5) were the strategies used to modify the pH of pig slurry using paper industry by-products and additive combinations. Maize seedlings were grown in rhizoboxes to monitor root and shoot traits over 23 days. The labile P pool in the sandy loam soil was labeled with 33 P to distinguish between soil- and fertilizer-derived P. Post-harvest analyses included acid phosphatase activity and mycorrhizal root colonization. Results All (bio)-acidified treatments significantly increased slurry water-extractable P by 70% but spent acid by-product reduced maize root growth and total P uptake. Alkalinized slurry did not affect root growth, P uptake, or soil enzymatic activity. Root branching and length were diminished in all acidified treatments, accompanied by a 59% reduction in enzymatic activity and mycorrhizal root colonization remaining below 10% across treatments. Conclusion While acidified slurry improved P solubility, gains in plant P uptake and biomass were limited, likely due to inhibitory effects on root development and symbiotic microbial functions, reflecting trade-offs between P availability and plant-soil biological interactions.