Effects of irrigating dairy-grazed grassland with farm dairy effluent on nitrous oxide emissions

Abstract

While irrigation of farm dairy effluent (FDE) to land is becoming popular in New Zealand, it can lead to increased emissions of the greenhouse gas nitrous oxide (N2O). This paper reports the results from trials on N2O emissions from irrigation of FDE to two dairy-grazed pastures on two poorly drained silt-loam soils located at Waikato and Manawatu, New Zealand. These pasture soils were periodically irrigated with FDE under contrasting soil moisture conditions with water-filled pore-space (WFPS) ranging between 26% and 94%. Nitrous oxide emissions were measured from the FDE irrigated and unirrigated sites using large numbers of static chambers (12–20). Irrigation of FDE generally increased N2O emissions compared to the control. N2O emissions varied with changes in climatic conditions and soil WFPS. Overall N2O emissions from effluent-derived N ranged between 0.01% and 4.93% depending on irrigation time and soil WFPS. Lower N2O emissions from FDE were attributable to very low soil WFPS conditions during the dry seasons. Higher N2O emissions were measured from application of FDE to a recently grazed pasture on wet soil. Our results suggest strategic application of FDE during dry summer and autumn seasons can reduce N2O emissions from application of FDE. Delaying effluent-irrigation after grazing events could further reduce N2O emissions by reducing the levels of surplus mineral-N.