Accumulation and distribution of phosphorus in the soil profile under fertilized grazed pasture

The fate of phosphorus originating from long-term fertilizer inputs in the soil profile is of great agronomic and environmental concern but remains poorly understood. Sequential fractionation was used to investigate accumulation and distribution of phosphorus in the soil profile to 100 cm (0-7.5, 7.5-15, 15-25, 25-50, 50-75, 75-100 cm) under fertilized and grazed pasture. The trial site had received annual superphosphate fertilizer inputs at different rates (0, 188 and 376 kg superphosphate ha−1 year−1; control, 188PA and 376PA, respectively) for 57 years. While there was significant accumulation of total phosphorus in the topsoil (0-7.5 cm), no corresponding differences were observed in organic or residual (non-extracted) phosphorus between treatments. However, fractionation data showed that inorganic forms of phosphorus were significantly higher in the 376PA treatment for all soil depths to 50 cm compared with the other treatments. Furthermore, the relative magnitude and significance of differences between the 376PA and other treatments in soil to 50 cm were greater for readily-soluble inorganic phosphorus (NaHCO3 and NaOH I extractable) compared with more recalcitrant inorganic phosphorus (HCl and NaOH II extractable). As such, phosphorus applied in excess of agronomic requirements and soil retention capacity was transferred below the topsoil and root zone by leacing, and most of the transfer of phosphorus was in readily-soluble inorganic forms. This presents an enhanced risk of elevated phosphorus transfer to groundwater with associated adverse impacts on water quality under fertilized and irrigated pastures. Our findings highlight the need to carefully assess and manage fertilizer phosphorus inputs in accordance with agronomic demand and soil retention capacity to minimize downward phosphorus movement and phosphorus transfer to water bodies.