Contents and Movement of Phosphorus in Soil of Long Term Effluent Irrigated Land Forest
长期污水灌溉后林地土壤中磷的含量与移动

Treated sewage effluent from Rotorua City, New Zealand, has been spray-irrigated into Whakarewarewa Forest to minimize effluent-derived nitrogen (N) and phosphorus (P) entering Lake Rotorua. To assess the capacity of the ecosystem to assimilate the effluent derived nutrients, a long term field trial was established within the Rotorua land treatment system. The objective of this study was to determine the accumulation, redistribution and movement of P in the volcanic soil after 11year effluent irrigation. The soil samples were analyzed. Soil pH, total phosphorus (TP), Olsen P, Mehlich-3 P(M3P), M3Ca and M3Mg had increased significantly (p<0.05) in the top soil (0-10cm) in all effluent treated plots compared with the control plots. Some significant increase had extended deeper soil layer (20-40cm) or much deeper soil layer. Most effluent-derived P had accumulated in top 40cm. Concentrations of soil Olsen P and M3P in the medium and high effluent treatments were over environmental threshold levels(Olsen P, 60 mg·kg-1; M3P, 150 mg·kg-1) in the top 20cm. The values of soil saturation ratio (M3PSR) show that it was nearly saturated of soil P sorption in top soil. It was suggested that there was a potential environment crisis for soil P to move downward with drainage water. Regular soil sampling and Olsen P measurement using 60 mg·kg-1 as an environment threshold could provide a solution for monitoring the movement of P down the soil profile. Phosphorus saturation ratio index (M3PSR) calculated as the molar ratio of M3P to (M3Al + M3Fe) appeared to be inadequate as an indicator for downward movement of soil P in such land treatment system. A new Phosphorus saturation ratio index for this system will be one of future study work.