Nitrous oxide emissions from dairy cattle urine in a mixed temperate forest - pasture system : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Applied Science at Lincoln University

Abstract

Agriculture is the largest anthropogenic global emitter of nitrous oxide (N₂O), a potent and long-lived greenhouse gas. The single largest source of N₂O emissions in New Zealand are dairy cattle urine patches. The integration of trees into pastures (agroforestry), has been suggested as a strategy to reduce the climate impact of agriculture by increasing carbon storage. However, the impact of integrating trees into pastures on N₂O emissions from cattle urine patches remains poorly understood, with even less known about urine induced emissions at the forest edge, where distinct microclimatic and biogeochemical conditions may influence nitrogen (N) cycling. This study investigated the effect of integrating small forest patches (<1 ha) into dairy pastures on N₂O emissions from cattle urine using the static chamber method. Following the application of a synthetic urine patch (599 kg N ha⁻¹) N₂O fluxes were measured at five points along a 56 m transect extending from the interior of a deciduous forest patch into an adjacent conventional dairy pasture in Canterbury, New Zealand. Unexpectedly, total urine-induced N₂O emissions were highest from the forest edge soil (249.9 ± 76.7 mg N2O-N m⁻²), exceeding those from the pasture soil (29.4 ± 19.9 mg N2O-N m⁻²) by a factor of 7.5. A distinct difference in the timing of N₂O emissions after urine application was also observed, with a 50-day delay in emissions at the edge, compared to the pasture, where emissions rose after 24 hours. Soil temperature and nitrate (NO₃⁻) concentrations were positively correlated with N₂O fluxes, whereas there was no association between soil moisture or ammonium levels and N2O fluxes. The NO₃⁻ to N₂O ratio at the edge was more than 12 times lower than the pasture. These findings provide evidence that forest edges can act as hotspots for N₂O emissions in response to cattle urine and highlight the need for further research into how forest edges influence N cycling and N₂O emissions in agroforestry systems.