Different mechanisms of phosphorus transformation in calcareous and acid soils affected by ammonium polyphosphate

The effects of ammonium polyphosphate (APP, (NH4)n+2PnO3n+1, n < 20) on soil phosphorus (P) availability vary depending on polymerization distributions and the soil type, yet the mechanisms driving these differences remain unclear. This study explored the availability and transformation of P affected by APP1 (P species of P1-P2) and APP2 (P species of P1-P7) in two different soils, in comparison with conventional ammonium phosphates (APs). APP application increased Olsen-P by 10.7–24.8% in calcareous soil, but decreased it by 2.6–10.8% in acid soil relative to APs. In calcareous soil, APP significantly increased soluble-P, adsorbed-P, and Fe-associated P, as reflected by CaCl2, NaHCO3, and NaOH extractable Ps, while decreased more stable Ca-associated P and occluded P indicated by NH4Ac and Na3C6H5O7-Na2S2O4-NaOH extractable Ps. The changes in the composition of CaCO3 and Fe/Al oxides together with/without the reduced organic carbon loss mainly contributed to the decrease in P sorption/precipitation and the increase in P desorption/dissolution. In acid soil, APP significantly increased microbial biomass P, leading to reduced labile inorganic P and elevated labile organic P. Meanwhile, APP increased both oxalate-extractable and complex Fe/Al oxides, which affected P adsorption–desorption to a certain extent. Compared to APP1, APP2 resulted in P existing in a more labile adsorbed state, thereby increasing P availability in both calcareous and acid soils. The main processes affecting P availability in the calcareous soils were abiotic transformations, while biotic transformations played the key role in the acid soils.