Owens, Jennifer2016-12-072016-12-072016-06-15https://hdl.handle.net/10182/7637This thesis is a combination of field and laboratory studies aimed at understanding how irrigation influences nitrous oxide (N₂O) emissions from grazed pastures in New Zealand. The general goal was to understand how nitrous oxide reductase (N₂OR) was affected by irrigation practices, and other factors, and to assess the potential to minimize N₂O emissions by encouraging N₂O reduction to dinitrogen (N₂). EXPERIMENT 1 (Chapter 4) - A field monitoring campaign measured N₂O fluxes for 35 days from urine and non-urine treated grazed and irrigated dairy pasture, situated on a free-draining soil. Two irrigation frequencies - a 3 day irrigation frequency and a 6 day irrigation frequency - applied the same total amount of water by the end of the experiment. The original hypothesis was that a more frequent irrigation regime would keep soil moisture higher, thereby lowering soil oxygen (O₂), resulting in greater N₂OR activity, and reduced N₂O emissions. Soil O₂ measurements showed that soil O₂ was lower at 50 and 100 mm soil depth in the more frequently irrigated soil. Denitrification potential measurements taken over the course of the experiment, using the acetylene inhibition method, showed that potential N₂O/(N₂O+N₂) was lower under the more frequent irrigation regime, suggesting greater potential for N₂OR. Contrary to the original hypothesis, however, there was no difference in the N₂O fluxes from the different irrigation frequencies, despite the soil chemical and biological differences. Estimates of soil relative gas diffusivity (DP/DO) showed that DP/DO was too high for N₂O to be reduced to N₂, according to the thresholds identified by Balaine et al. (2013). The results from this experiment raised questions warranting exploration. EXPERIMENT 2 (Chapter 5) Can we further explore and compare how well soil O₂ measurements and DP/DO an expression of soil O₂ diffusion, explain N₂O fluxes under variable hydrological conditions on a heavy soil? EXPERIMENT 3 (Chapter 6) Temporal dynamics of N₂OR and denitrification potential after a wetting event need to be better understood. Can we interpret whether lower N₂O/(N₂O+N₂) is attributed to just increased soil moisture, or is it also related to priming of the microbial pathway for N₂OR? EXPERIMENT 4 (Chapter 7) Is the diel cycling of soil O₂ temperature and respiration driven? Is there also diel cycling of N₂O and N₂OR related to plant dynamics, such as expulsion of root exudates? Can we isolate and explore these factors by measuring N₂O and ¹⁵N-N₂O and ¹⁵N-N₂ recovery from soils with and without plants in the absence of temperature change? EXPERIMENT 5 (Chapter 8) The effects of plants on N₂OR should be assessed by comparing rhizosphere and bulk soils to enable laboratory results, many of which do not include plants, to be transferred to field scenarios, where plants are common. Along this same thread, spatially variability in the field should be explored, as this variability can inform sampling strategies and extrapolation of local results.enhttps://researcharchive.lincoln.ac.nz/pages/rightsirrigation frequencyurinerelative soil gas diffusivitysoil oxygenpasturesoil chemistrynitrous oxidegrazed pastureThesisANZSRC::079901 Agricultural Hydrology (Drainage, Flooding, Irrigation, Quality, etc.)ANZSRC::050304 Soil Chemistry (excl. Carbon Sequestration Science)Q112931457