The fate of nitrogen from dairy factory effluent and dairy cow urine in land treatment systems

Abstract

Land application is widely used in New Zealand for treatment of Dairy Factory Effluent (DFE). It is important to ensure that the application of DFE to land does not cause adverse environmental effects, e.g. nitrate leaching and nitrous oxide emissions. Research on the fate of nitrogen in DFE applied to land is limited and publications are few. The aim of this study was to determine the fate of nitrogen from DFE and dairy cow urine applied to a Templeton fine sandy loam (Udie Ustrochept) and pasture mix of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Dairy factory effluent was applied at rates of 25 and 50 mm at three weekly intervals until the amount of DFE applied was equivalent to approximately 300 and 600 kg N ha⁻¹ yr⁻¹, respectively. Measurements of leaching losses of nitrogen averaged 12.5 and 4.5 kg N ha⁻¹ yr⁻¹ for Control 25 and Control 50 treatments, 13.0 and 6.5 kg N ha⁻¹ yr⁻¹ for 25 and 50 mm 'cut and carry' treatments, and 88.4 and 60.7 kg N ha⁻¹ yr⁻¹ for 25 and 50 mm 'grazed' treatments, respectively. Pasture uptake of nitrogen averaged 150, 263, 304, 285, 351, and 375 kg N ha⁻¹ yr⁻¹ for Control 25, Control 50, DFE 25 and DFE 50 'cut and carry' treatments, and DFE 25 and DFE 50 'grazed' treatments, respectively. Denitrification losses of 3.8, 2.1, 44.3, 48.1, and 62.7 kg N ha⁻¹ yr⁻¹ for Control 25, Control 50, DFE 25 and DFE 50 'cut and carry' treatments, and the DFE 25 'grazed' treatment, respectively. Isotopic nitrogen studies found that 38.5 and 32.3 % of applied DFE nitrogen was immobilised in the DFE 25 and DFE 50 'cut and carry' treatments. Less than 1% of applied nitrogen was lost by volatilisation. Immobilisation and denitrification measurements indicate that soluble organic carbon in the DFE influences the fate of applied nitrogen. Leaching, denitrification, and volatilisation losses were higher in 'grazed' than 'cut and carry' treatments. This work highlights the need for Regional Councils to differentiate between the source of nitrogen and type of treatment systems when developing regional rules for land treatment systems.