Dynamic Transformation of Different Phosphorus Fertilizer Sources and Their P Availability in Acidic and Calcareous Soils

Phosphorus (P) is a vital non-renewable macronutrient that is frequently immobilized by chemical fixation in acidic and calcareous soils. However, the specific transformation processes of different P sources across these different soil types remain poorly understood. A 120-day incubation experiment was conducted in a completely randomized design with three replicates using acidic and calcareous soils from Myanmar to evaluate six treatments: single superphosphate (SSP), diammonium phosphate (DAP), triple superphosphate (TSP), ammonium polyphosphate (APP), calcium phytate (CP), and an unfertilized control (CK). The modified Hedley fractionation method was employed to investigate the dynamic transformation of soil P fractions, the rate of available P transformation, and the relationship between soil transformation and soil P availability under different P-fertilizer sources. In both soils, SSP, DAP, and TSP provided high initial P release (249.7%, 239.9%, and 234.1%) in acidic soil and (159.5%, 170.8%, and 161.4%) calcareous soil, which peaked on day 10 compared to CK (p < 0.001) before rapidly declining. P transformation in calcareous soil declined more sharply than in acidic soil from the 10th day to the 15th day (especially in SSP and TSP, −11.3 and −16.9 mg kg−1), indicating a higher rate of P immobilization in calcareous soil. Conversely, the delayed peak of APP (day 60) and the stability of CP indicate resistance to immediate soil fixation. Notably, overall effectiveness depended on soil type: SSP performed best in acidic soil and DAP in calcareous soil.