Nutrient leaching losses from undisturbed soil cores following applications of piggery wastewater

Land disposal of wastewater from intensive livestock industries can result in large amounts of nutrients and salts being applied to soils. When irrigated at rates to meet crop phosphorus (P) requirements, nitrogen (N), calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), chloride (Cl), and sulfate (SO4) applied in the wastewater often exceed crop demands, and are susceptible to leaching. Leaching of surface-applied piggery wastewater was investigated using large undisturbed soil cores (30 cm i.d. by 60 or 75 cm long) from 2 piggery wastewater disposal areas (Site 1, Vertosol; Site 2, Sodosol) in south-east Queensland. About 3% of the total wastewater P applied to the Vertosol, and about 10% of that applied to the sodosol, was leached. The magnitude of these losses was consistent with the chemical properties of each soil, and the availability of P sorption sites (i.e. hydrous Fe oxides). The major forms of P in the leachate included both molybdate reactive P (MRP) and unreactive P (UP, includes dissolved organic P, soluble organic P, particulate P, and non-reactive P). Phosphorus leached from the Vertosol was largely (≈80%) as UP because the MRP was sorbed by the soil colloids. Much of the P leached from the sodosol was present as MRP (≈70%) because the wastewater applied to this soil also contained about 70% MRP, and this soil had only a limited ability to sorb MRP. Losses of nitrogen (N) were found to be of a major environmental concern. Both wastewater samples contained very high levels of N, with ammonium (NH4-N) making up about 80% of the total Kjeldahl N (TKN) and organic N about 20%. Negligible amounts of applied NH4-N were detected either sorbed by the soil or in the leachate because it was converted to nitrate (NO3-N) within the soil core. This NO3-N was highly mobile, and was readily leached from the soil cores. Nitrogen represented the major limitation to the long-term use of land for disposal of piggery wastewater. For land disposal to be an effective management option, N applied in piggery wastewater may need to be limited to about 200 kg/ha.year. Significant amounts of Ca, Mg, K, and Na applied in the wastewater were leached from the soil cores. It is recommended that more attention be placed on the impact of N (TKN, NH4-N, and NO3-N), Ca, Mg, K, and Na on the receiving soil and water environments rather than focussing primarily on wastewater P. Management strategies should be developed for disposal sites to minimise leaching losses by maximising nutrient removal from the soil solution through crop uptake, reaction with the soil colloids, and efficient irrigation practices. nitrogen, phosphorus, cations, nitrification, piggery wastewater.