Availability of nitrogen and phosphorus under recycled water irrigation

Land application of recycled water is currently practised in many countries as an alternative to discharge directly into inland and coastal waters. In this field study, nitrogen (N) and phosphorus (P) availability in pasture plots that received recycled water was compared with that of plots which received single applications of superphosphate fertiliser (SSP). The treatments included recycled water applied continuously, recycled water applied alternately with fresh water, single application of SSP at the beginning of the experiment, and a non-amended control. Results from the study showed that the amount of N and P removed by pasture was significantly (P < 0.01) greater in plots that received both recycled water treatments than in the SSP-treated plot and the control. The amount of N (as nitrate and ammonium) leached below 1.0 m soil depth was significantly greater (P < 0.01) for the continuous recycled water treatment than the other treatments or the control. Nitrate was the dominant (>80%) form of N leached from all 4 treatments. In contrast to the findings for N, a greater proportion of applied P (47.9%) was leached below 1.0 m soil depth from the SSP-treated plots than from plots that received recycled water continuously (26.0%), alternately with fresh water (13.3%), and the control. Results from the study may be useful for developing best management practices for recycled water irrigation of pasture.