A process-based approach reveals organic materials and mineral fertilizer effects on phosphorus dynamics in a tropical Ferralsol

Phosphorus (P) availability in highly weathered tropical soils is strongly constrained by low orthophosphate concentrations in soil solution ( C P ) and rapid sorption processes. This study investigated the effects of organic materials and mineral fertilizers (OM-MF) on plant-available P in a Malagasy Ferralsol by quantifying both C P and the dynamics of diffusive P-ions at the soil solid-solution interface ( P r ). A microcosm experiment was conducted using 16 OM-MF and a control soil, incubated for seven months under controlled conditions. The dynamics of P r were assessed using isotopic exchange kinetics (IEK) based on the isotopic dilution principle. The resulting data were described using the Freundlich kinetic equation, yielding three parameters (v, w, p) that represent immediate P reactivity, sensitivity to C P , and temporal dynamics, respectively. C P remained very low in most treatments (<0.01 mg P L⁻ 1 ), even after fertilization, confirming the strong P sorption capacity of Ferralsols. P r increased with both C P and time and was accurately described by the Freundlich kinetic function. The parameters v and w varied substantially across treatments and were primarily associated with soil solution composition, particularly dissolved organic carbon and nitrogen, and mineral elements, while p showed lower predictability. The effectiveness of OM-MF strongly depended on their biochemical composition and their interactions with the soil environment. Notably, Hyperfos, a phosphate rock fertilizer, did not enhance P availability despite its high P content because of its low solubility. Overall, the results demonstrate that P availability is not directly controlled by total P inputs, but rather by soil-fertilizer interactions and transformation processes. This study highlights the value of process-based approaches for assessing plant-available P and emphasizes the need to adapt fertilization strategies to both soil properties and fertilizer characteristics to improve P-use efficiency in tropical agroecosystems.