The effect of the irrigation and urine application on phosphorus loss to subsurface flow from a stony soil

Diffuse losses of Phosphorus (P) from agricultural land via subsurface flow (viz leaching) can result in degradation of surface water quality. Factors such as irrigation and soil pH can increase solubility of P and its loss. Urine patches are known to alter pH by up to 3 units, but it is unknown if this increases P loss. This study investigated subsurface P loss from a stony soil subject to different rates of irrigation (12, 25, 50 mm hr-1) with or without the application of urine (600 kg N ha-1). Results showed that despite receiving urine, average TP, FRP and FURP concentrations were significantly lower (27, 3, 7 μg L-1) than non-urine treatments (40, 8, 11 μg L-1). Increasing the rate (not depth) of irrigation did not affect the load of P lost. Total P loads were generally low (0.070 to 0.167 kg ha-1) compared to similar studies. The lack of differences observed with increasing irrigation rates was thought to be due to attenuation as P moved from preferential flow pathways in the topsoil to matrix flow in the subsoil gravels. The lower P loss from urine application was due to: 1) the buffering effect of soil solution negating any significant pH-induced solubilisation of P; and 2) enhanced P uptake and decreased drainage (due to greater evapotranspiration) as dry matter production increased in response to nitrogen from the urine. However, despite this soil having only a moderate Olsen P concentration, typical of productive landuses in the region, FRP and TP concentrations still exceeded guidelines for good surface water quality. Strategies should be considered to decrease these losses, especially where drainage occurs via preferential flow pathways that negate attenuation of P losses, or are well connected to surface waters.