Phosphorus Release and Transformations in Contrasting Tropical Paddy Soils Under Fertiliser Application

Abstract

Purpose: Inconsistent yield responses to inorganic phosphorus (P) fertilisers in tropical rice paddy soils remain a challenge. This study investigated the contributions of applied P fertilisers to soluble soil P and P transformation mechanisms in P-added paddy soils.

Methods: An incubation study was conducted on three rice-growing soils (Ultisol, Alfisol, and Entisol) in Sri Lanka with and without single superphosphate (SSP), triple superphosphate (TSP), and urea. Dissolved reactive phosphorus (DRP) was measured over 112 days of submergence. Thermodynamic modelling and chemical P fractionation were employed to assess soil P transformations.

Results: Phosphorus-fertilised soils had significantly higher DRP concentrations (1.1–8.0 mg L−1) compared to controls at 7 days after submergence but DRP declined beyond 21 days (0.024–0.300 mg L−1). Single superphosphate increased DRP more than TSP, short-term. Urea did not affect DRP concentration. Ultisols exhibited the lowest DRP, while Alfisols maintained higher DRP than Ultisol which was near or above the critical concentration for rice (0.1 mg L⁻1) after 28 days. In Entisol, only SSP maintained DRP above 0.1 mg L−1. Modelling suggested Ca phosphates and Fe oxy(hydr)oxides dissolved during submergence. Released P may be resorbed by Fe/Al oxy(hydr)oxides and Ca minerals, with evidence of downward movement of dissolved P and its resorption onto Fe/Al and Ca minerals possibly due to saturation of P sorption sites in the topsoil layer.

Conclusion: Low dissolved P in porewater may be linked to inherent soil characteristics, including low organic matter and high amorphous Fe and Al oxides.