| Summary: | In Australia, approximately half of the estimated 275,000 dry tonnes of biosolids produced annually are used in agricultural land application programs as a source of plant nutrients, primarily nitrogen (N) and phosphorus (P). Application rates of biosolids typically range from 7 to 20 dry t/ha, with rates commonly determined by the N limiting biosolids application rate (NLBAR), a function of the type of biosolids, the crop N requirement, and in some instances, the N status of the soil. The ramification of applying biosolids based on the NLBAR is that the accompanying P applications are much higher than traditional sources of P fertiliser that are applied at much lower rates consistent with P removal by the crop. To date, there is very little research data to implicate P applied in biosolids as a source of nutrient pollution, even at the high loading rates. The National Biosolids Research Project (NBRP), which has operated across five states in Australia, has recently culminated four years of research into the impact of land application of biosolids on soil and plant nutrient concentrations in a range of soil types, crops and climates. In this paper, changes in total soil P, bicarbonate extractable soil P and mass balances are reported for a range of biosolids, soils and crops throughout Australia as an indication of the bioavailability of biosolid P and risk of off-site movement compared with inorganic fertiliser P.
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