Sheep urine patch nitrous oxide emissions : measurement and mitigation

Demand for livestock products are expected to rise due to increasing global population, urbanisation and affluence. Sustainably managing livestock excreta will be central to achieving an expansion in production whilst minimising environmental damage. The deposition of excreta to soil by livestock accounts for ca. 21% of the UK agricultural N2O emissions. Accurate quantification of N2O and assessments of the efficacy of mitigation technologies are key research areas for progressing toward enhanced sustainability and productivity in grazed grasslands. The overall thesis aims are to enhance understanding of N cycling and losses in sheep urine patches, as ‘hot-spots’ and ‘hot-moments’ for rapid nutrient cycling. Objectives were (i) to determine how sheep urine patch and environmental parameters influence N2O emissions, (ii) to determine the optimal way to accurately measure N2O emissions from sheep urine patches via the static chamber technique, and (iii) to assess the efficacy of synthetic nitrification inhibitors as a mitigation strategy for urine patch N2O emissions. Sheep-grazed grasslands were selected for study, based on the lack of current available evidence for these agroecosystems. N2O emissions were monitored from sheep urine-influenced soil in small incubation vessels, or by the static chamber technique in the field (manual and automated campaigns). The use of 14C-labelled inhibitors were also employed in laboratory studies, to trace the fate of nitrification inhibitors in the plant-soil-microbe system and provide a better understanding of the factors that affect the efficacy of inhibitors to reduce N2O emissions. Urine patch size and N concentration were found to be important parameters influencing emissions of N2O from sheep urine patches. Emissions of N2O were generally lower than the IPCC default of 1% of the N applied in sheep excreta, where peaks in emissions occurred alongside rainfall events. Total extractable N, oxidation reduction potential and soil water-filled pore space were determined to be key drivers of N2O emissions from sheep urine under controlled conditions. The urine patch diffusional area was shown to be important for accurate quantification of N2O emissions when using the chamber technique; the importance of daily sampling of emissions, an assessment of the diurnal nature of N2O emissions and having a high number of replicate chambers to adequately represent the large spatial variability in N2O emissions was also confirmed. The nitrification inhibitors DCD and DMPP had contrasting behaviours in differing soil types. DCD had a greater sorption in comparison to DMPP and microbial uptake and degradation were concluded to be important parameters influencing their effective period in the soil. A graminaceous plant was shown to be able to acquire DCD intact through its roots and translocate the compound to shoots which raises concerns about contamination of food products. A liquid application of DMPP was not effective in reducing cumulative N2O emissions from sheep urine patches in the field. The efficacy of nitrification inhibitors to reduce N2O appears to vary widely, nevertheless they are a mitigation strategy that could be implemented in the short term. Achieving enhanced sustainability and productivity in grazed grasslands is a complex problem, requiring an interdisciplinary approach and the involvement of policy-makers and farmers to resolve. There are several mitigation strategies available or being developed, and some which require more research before being practicable. Advances in technologies to measure and mitigate N2O emissions will greatly enhance our knowledge of N cycling and losses, and the potential to alleviate such losses from the urine patch environment in the near future.