Impact of farm-dairy effluent application on the amounts and forms of phosphorus loss by leaching from irrigated grassland

Sufficient evidence exists to support the hypothesis that the eutrophication of surface waters is accelerated by increased transfer of nutrients (e.g., phosphorus, P) from landscape to water. The objective of this paper is to combine the results of a lysimeter experiment that used intact soil monolith lysimeters (50 cm diameter, 70 cm deep) and a field experiment that used field plots (10 × 2 m 2 ) of a Lismore stony silt loam soil (Udic Ustochrept). The soils received mineral P fertiliser alone at 45 kg P ha −1 yr −1 or in combination with farm-dairy effluent (FDE) at 200 or 400 kg N ha −1 yr −1 . Annual mean total P concentrations and losses were two- to threefold higher from the soil amended with both FDE and P fertiliser than P fertiliser alone. The higher losses as particulate unreactive P during irrigation seasons are attributed to the regular inputs of high intensity flood irrigation, which increased the transfer of soil particles from the soil profile. Greater than 60% of P loss occurred immediately following FDE application over a 2-year period, highlighting the importance of preferential flow in this soil. Phosphorus-31 nuclear magnetic resonance analysis showed that FDE was rich in inorganic orthophosphate (86%) while leachate contained only 12% of inorganic orthophosphate. This indicated that inorganic P applied in FDE was sorbed because of the high P fixation capacity of Lismore subsoil, which is due to higher amounts of Fe and Al. On the other hand, orthophosphate monoesters and diesters were only 13% of P in FDE, compared with 88% in leachate suggesting that organic P forms are mobile in the soil profile, and are selectively transported through soil. The Olsen P and isotopic exchange parameters (concentration of inorganic P in the soil solution (C P ) and the quantity of P that is immediately available to plants (E 1min )) of Lismore soil increased with increase in P application from mineral P fertiliser and FDE. This suggests that P fixation sites in soil may be getting saturated with applied P inputs.