Phosphorus transport in subsurface flow from a stony soil under irrigated and non-irrigated lucerne

The extent of diffuse loss of phosphorus (P) from soils under deep rooting forage crops such as lucerne are currently unknown. This study used large (2.0 m by 1.5 m) monolith lysimeters to quantify the amounts and forms of P in drainage under lucerne from two sites, a non-irrigated dairy system and an irrigated dairy system that also received farm dairy effluent (FDE). Results showed that despite greater P inputs to the irrigated site compared to the non-irrigated site, there was no difference in total P concentrations in drainage. There were also no differences in the forms of P lost between sites, which were dominated by particulate P (48 to 52 %). More P was lost from the irrigated site (0.262 kg ha-1) than the non-irrigated site (0.164 kg ha-1). This larger P loss was a result of irrigation producing more drainage (42 %) than observed at the non-irrigated site. The amount and forms of P lost from lucerne were similar to or lower than losses reported for stony soils under grass-based pasture swards amended with P fertiliser and FDE. Because P leaching was regulated by drainage volume rather than P input, management of irrigation water to reduce drainage from the root zone will likely help minimise P leaching.