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

• APP application increased Olsen-P in calcareous soil, but decreased it in acid soil. • APP increased labile P i while decreased more stable P i in calcareous soil. • APP application reduced labile P i and elevated labile P o in acid soil. • Abiotic and biotic factors affected P availability in the calcareous and acid soil, respectively. • APP with more P species coexistence could increase soil P availability. The effects of ammonium polyphosphate (APP, (NH 4 ) n+2 P n O 3n+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 P 1 -P 2 ) and APP2 (P species of P 1 -P 7 ) 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 CaCl 2 , NaHCO 3 , and NaOH extractable Ps, while decreased more stable Ca-associated P and occluded P indicated by NH 4 Ac and Na 3 C 6 H 5 O 7 -Na 2 S 2 O 4 -NaOH extractable Ps. The changes in the composition of CaCO 3 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.