Effects of long-term irrigation on soil phosphorus under temperate grazed pasture

The effect of 62 years of different irrigation regimes on the amounts, forms and distribution of phosphorus (P) in the soil profile to 100 cm under grazed pasture was determined. Irrigation treatments included nil irrigation (base average rainfall of 740 mm year(-1); control), together with flood irrigation applied when gravimetric moisture (GM) in the topsoil decreased below 10 or 20% (i.e. rainfall + 260 mm year(-1) of supplemental irrigation [irrigation(10%GM)] and rainfall + 770 mm year(-1) [irrigation(20%GM)], respectively). Phosphorus fertilizer in the form of single superphosphate was applied annually at the rate of 23 kg P ha(-1) to all treatments. Total soil profile P was smallest under the large rate of irrigation (5054 kg P ha(-1)) compared with the small rate (5908 kg P ha(-1)) and the control (6423 kg P ha(-1)), which was attributed mainly to changes in inorganic P. These data indicated that markedly more P was removed, transferred or lost under irrigation. Substantial increases in pasture production and animal grazing capacity occurred in response to irrigation, which in turn resulted in increased removal and transfer of P in animal products, animal excreta (stock camp transfer and irrigation outwash) and by leaching. Thus, combined annual removal of P in animal products, internal dung transfer and loss in irrigation outwash were directly related to irrigation frequency and increased from 8 to 18.6 kg P ha(-1) for the irrigation(10%GM) and irrigation(20%GM) treatments, respectively. Highlights Examined effect of 62 years of irrigation on phosphorus in the soil profile to 100 cm. Phosphorus fertilizer was applied annually at the same rate to all treatments. Total soil profile P decreased with irrigation and was smallest under the large rate of irrigation. P removal and transfer or loss increased with irrigation frequency.