Influence of farm dairy effluent ammonium concentrations on soil N2O emissions

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) and the single-most ozone (O₃) depleting substance. Agriculture is the dominant source of anthropogenic N₂O emissions globally, and especially in New Zealand. Urine, synthetic nitrogen (N) fertiliser and farm dairy effluent (FDE) are the main sources of N₂O emissions from agricultural soils in New Zealand. Urine and synthetic N fertiliser have received considerable research attention to minimise their contribution to soil N₂O emissions due to the high N loadings and greater emission factors (EF) of these inputs. However, as the land application of Farm Dairy Effluent (FDE) is a less significant contributor to New Zealand’s overall N₂O emissions profile, research on this N-input is limited. The mass of FDE applied to land increased from, 18kt in 1990 to 39kt in 2013, and a recent increase in popularity of herd homes will further increase the mass of FDE produced. Thus, FDE requires further research. Limited data is available on the EF’s from the land application of FDE. In addition, an analysis of the literature suggests the NH₄⁺-N concentration of FDE is highly variable. The aim of this study was to determine the influence of FDE NH₄⁺-N concentration on soil N₂O emissions. A 35-day field trial was conducted, where 10 mm of FDE was applied to pasture at NH₄⁺-N concentrations of either 90, 150, 200, 300 or 400 mg NH4⁺-N L-1. The 150 and 400 mg NH₄⁺-N L-1 treatments contained 15N to monitor the fate of FDE NH₄⁺-N. N₂O gas samples were taken daily for the first week, then every 2-3 days for the remainder of the trial. Soil inorganic-N pools were monitored every 7 days. Pasture production was measured on days 19 and 35. Peak N₂O emissions occurred within 24 hrs of applying the FDE. The highest N₂O emisions were produced in the 400 mg NH4⁺-N L⁻¹ treatment averaging 65 kg N₂O ha⁻¹ day⁻¹. FDE treatments produced significantly more than the control until day 7. Emission factors ranged from 0.18 to 0.32 percent of total N applied, significantly less than the 1% currently used to calculate New Zealands GHG inventory. The emission period was relatively short due to low soil nitrate concentrations, and/or relatively dry soil conditions. Ammonium concentration is a key driver of N₂O emissions. Cumulative soil N₂O emissions increased linearly with FDE NH₄⁺-N concentration, however, it is likely this relationship may change in soils with a higher moisture content. Therefore, the use of a single relationship between the two variables for all environmental situations may not be possible. Further studies that analyse the influence of NH₄⁺-N concentrations on soil N₂O emissions, in a range of typical environments, are required to fully understand this relationship.