Assessment of legacy phosphorus speciation under long-term tillage and phosphate fertilizer management in a tropical soil

This study used a combination of analytical techniques – chemical fractionation, XANES and ³¹P NMR spectroscopies – to investigate and quantify the effects of different long-term tillage practices [conventional tillage (CT) with plowing at 20 cm-depth and harrowing, and no-tillage (NT) systems] and phosphate fertilizer inputs (triple superphosphate-TSP and Gafsa reactive phosphate rock-RPR applied in the sowing furrow or broadcast) on the nature and bioavailability of phosphorus (P) in the topsoil (0–20 cm) of a strongly weathered Oxisol in Brazil. The soil was sampled after 17 years of phosphate application (35 kg ha^-1 yr^-1) and after eight years of maize (Zea mays L.) cropping without P input. Results showed that soil legacy P accumulated from P inputs was mainly present as non-labile inorganic P adsorbed on iron (P-Fe) and aluminum (P-Al) oxides (average of 79 %). Accumulation of soil legacy P in labile pool was greater under NT (65 mg kg⁻¹) compared with CT (32 mg kg⁻¹), while organic P as monoester and diester was also greater in soil maintained under NT than CT (81 vs 61 mg kg⁻¹). Maize grown after cessation of P inputs mobilized and depleted both labile (average of -8 mg kg⁻¹) and non-labile (average of −47 mg kg⁻¹) pools of legacy P. Calcium-bound P acted as labile reserve of P, especially when RPR was applied under NT, while P-Fe and P-Al were less available to maize, increasing their proportion under P suppression. Long-term tillage and phosphate fertilization strategies influence the bioavailability of legacy P in tropical soil, and conservation systems enhance the potential for crops to access and utilize the legacy P reserves.