An overview of the environmental effects of land application of farm effluents

New Zealand dairy and pig farms generate significant amounts of effluents that contain high concentrations of nutrients such as nitrogen (N), potassium (K), and phosphorus (P), and various trace contaminants (e.g., heavy metals, organic compounds, and endocrine-disrupting chemicals). Land application is a preferred option for farm effluent management. Regulations have been imposed to limit the land application of farm effluent to 150–200 kg N ha⁻¹ to minimise potential leaching loss of nitrate to groundwater. However, focusing mainly on nutrient recycling from farm effluent application has resulted in the effects of other effluent constituents, such as microbial pathogens, heavy metals, odorants and oestrogens, on the receiving ecosystems being overlooked. In this literature review, we assess land-applied farm effluents and their beneficial and potentially adverse effects on the receiving environment. Long-term application of farm effluent based on N loading can lead to P and heavy metal accumulation in the soil. High concentrations of K in effluent are likely to cause pasture nutrient imbalance and induce animal health problems. Recently, there has been some research interest in the role of runoff P in eutrophication of receiving water, effluent-derived pathogen survival and movement in soil ecosystems, effect of effluent-borne dissolved organic matter on pesticide transport in soil profile, and degradation of oestrogens in land-applied effluent. Further research in these areas in New Zealand is needed to help sustain the agricultural industry.