Acidification-driven soil phosphorus fractionation following long-term fertilization and cessation at ukulinga mesic grassland in South Africa

Long-term fertilisation can alter soil nutrient dynamics, yet the persistence and redistribution of phosphorus (P) fractions following the cessation of combined nitrogen (N) and P inputs remain poorly understood. This study investigated how 71 years of P-only and N + P fertilisation, followed by a 3-year cessation, affected soil inorganic P fractions and associated chemical properties in a mesic South African grassland. Nitrogen-containing treatments caused severe acidification (pH decline from 4.66 to 3.36) and depletion of exchangeable Ca and Mg, whereas P-only fertilisation did not alter soil pH but increased Ca availability. These chemical shifts produced distinct P redistribution patterns: N + P treatments accumulated Fe-P and reductant-P, while P-only treatments promoted Ca-P formation. Following cessation, soluble P equilibrated across treatments, but stable Fe-P and reductant P remained high (77-122% above the control), indicating strong legacy P persistence. Two complementary PCA approaches confirmed that soil pH and exchangeable Mg were the primary drivers (PC1 = 94.2% variance) of variation in soil chemistry and strongly correlated with shifts in Fe- and reductant-P. Overall, the findings highlight that nitrogen-induced acidification - not P inputs alone - controls the long-term fate of soil P in this grassland. Effective management of acidic grassland must therefore integrate soil pH regulation with P fertilisation strategies to enhance legacy P utilization and maintain soil fertility.