Phosphorus sorption and nitrogen transformation in two soils treated with piggery wastewater

Abstract

Land application of piggery wastewater has the potential to contaminate receiving water bodies due to the presence of elevated concentrations of nitrogen (N) and phosphorus (P). This paper investigates P sorption and N transformation using soil from 2 wastewater disposal sites in south-east Queensland. Soil from Site 1 was classified as a clay (Vertosol) and soil from Site 2 was classified as a sandy loam (Sodosol). Soil was collected from wastewater disposal (irrigated) and non-disposal (non-irrigated) areas of each site. Nutrient sorption kinetics and transformation were studied by reacting each soil with either wastewater or an inorganic salt solution over a period of 21 days. Solution P concentrations decreased with time for all soils. These changes were greatest during the early stage of the study (<10 days), after which time solution P concentrations remained relatively stable. Concentrations of solution P tended to remain higher in irrigated than non-irrigated soils. This was attributed to a loss of high-affinity sorption sites due to past wastewater additions. Cation exchange (for times <3 days) and nitrification (for times >3 days) were found to be the primary mechanisms responsible for decreases in solution ammonium (NH₄-N) over the 21-day period. Phosphorus and NH₄-N sorption isotherms were determined using a leaching procedure, and the data were adequately described (r² >0.95) by the Freundlich equation. Irrigated samples generally sorbed less P than non-irrigated soils. This was attributed to the loss of high-affinity P sorption sites due to previous wastewater additions. Cation exchange and competition between added and resident cations for the exchange sites was found to govern NH₄-N sorption by these soils. Results from this study suggest that long-term land application of piggery wastewater may encourage leaching of N and P from the plant root-zone in soils with limited capacity to retain these nutrients. Appropriate management strategies to minimise these losses need to be developed to avoid degradation of the receiving soil and/or water environments.

sorption kinetics, nitrification, ammonium, nitrate.