Phosphorus storage on effluent irrigated land

Land disposal of secondary secondary treated sewage effluent is being widel y adopted in Australia as an alternative to tertiary treatment. At the Clev eland Waste Water Pollution Control Works, effluent has been applied to a H apludalf at high rates (3000 mm yr(-1)) for more than 20 yr, adding an esti mated 4.5 t P ha(-1) to the soil. Thus, this site pro,ides an opportunity t o assess the retention of P under conditions typical of southeast Queenslan d, Laboratory chemical fractionation and P sorption studies of effluent irr igated and control sites Here undertaken to determine the amount and Forms of P retained within this soil and its: capacity to retain further P. Organ ic material (peat) which had accumulated on the surface of effluent irrigat ed plots, was also sampled and its P content determined. Accumulation of P was limited to the sandy surface soil, increasing total soil P content by a pproximate to 700 kg ha(-1), with this increase being approximately evenly distributed between inorganic and organic pools. The excessive irrigation r ate used on this site, and the management adopted, has resulted in a marked podzolization of the surface horizon, apparent in reduced extractable Fe a nd Al concentrations. The removal of these P adsorbing materials would cont ribute, along with P saturation and organic ligand competition, to the much lower P sorption capacity of irrigated sites relative to the control. The largest P pool Has found to be the pest layer, containing approximately 150 0 kg P ha(-1). On this site P has accumulated principally in organic forms, both within the soil and as peat on its surface, This finding throws into question the common practice of predicting disposal scheme life on the basi s of P adsorption curves.