Nitrous oxide emissions from grazed grasslands: novel approaches to assessing spatial heterogeneity
Item statusRestricted Access
Embargo end date30/11/2021
Maire, Juliette Lucie Marie
Nitrous oxide (N2O) is a potent greenhouse gas mainly produced by microbial processes in the soil. Anthropogenic N2O is principally emitted from soils after nitrogen fertiliser and manure applications on agricultural land. This thesis focuses on emissions from grazing systems, which are known to be the largest source of uncertainty in global and national N2O emission inventories. Nitrogen-rich excreta deposits from grazing livestock are recognised as hotspots of N losses (N2O emissions in particular). The non-uniform distribution of these emissions hotspots within a typical field contributes significantly to the spatial heterogeneity of emissions often observed in addition to the natural variability of soil properties within the field such as pH, moisture and nutrient availability. However, it is extremely difficult to characterise the spatial and temporal pattern of these grazing inputs other than through the use of demanding and costly approaches such as manual observation or animal based-sensors. Two separate experiments were conducted during this study, in Scotland on sheep grazed grasslands and in Ireland on a dairy cow grazed grassland. Both sites were commercially used and were intensively managed with a nitrogen fertiliser application rate of 225 kg ha-1 yr-1 and 261 kg ha-1 yr-1, respectively. In Scotland, at Easter Bush fields the experiment was conducted during a 9 month campaign of gas, soil and grass sampling over the grazed field to study the spatial and temporal variability of the fluxes and soil properties to improve up-scaling of the fluxes from the plot scale to the field scale. In Ireland, at the Johnstown Castle farm, the experiment was conducted during an 11 month campaign on an experimental plot excluded from grazing. At the Scottish site, gas, soil and grass samples were collected regularly on soil which received different treatments within a randomised block design (e.g. urine deposition, fertiliser application, urine and fertiliser application or no N addition as a control). At both sites, Remotely Piloted Aerial System (RPAS) imagery was collected to study the spatial variability of the grass growth with the aim to map excreta depositions over the whole field. The Scottish site was used as a proof of concept of the method and the method was then used weekly on the Irish site over the entire grazing season. More generally, this thesis details the novel use of remote sensing techniques using high-resolution cameras linked to RPAS to improve our understanding of the spatial and temporal patterns of excreta deposition. This method proved to be repeatable for future studies as it can be automated, is easily deployable in the field, low-cost and the measurements are non-destructive (i.e. has no influence on the soil, vegetation or livestock). Excreta depositions contribute to very high emissions of N2O from relatively small areas of soil and can vary throughout the growing season in response to climatic conditions. Therefore, mapping of the excreta nitrogen inputs to the field facilitated a more accurate estimation of the annual field-scale N2O emission from grazing grasslands. Both experiments conducted in this study showed a high spatial and temporal N2O emissions variability due to the nature of N2O production within the soil and high variability of the soil properties (soil pH, soil moisture content, soil temperature) which influence the microbiological processes. Interaction on N2O emissions between fertiliser application and urine deposition was proved to be statistically significant and the magnitude of the interactions depended on the time of application within the year. The results showed a link between the variability of the emission factors of excreta deposition and fertiliser application and to the variation in weather conditions. This technique can be employed to up-scale emissions to a national level. This study plays a part in the on-going development of precision agricultural tools, based on image analysis of the grass sward to mitigate emissions from grazed grassland. Possible mitigation approaches, based on the methods presented in this thesis, include the use of RPAS technology to deliver nitrification inhibitors to newly deposited excreta within the field to reduce the potential nitrogen losses to the environment. This research indicates the future potential to better adjust fertiliser application using variable-rate fertiliser applications matching the vegetation nitrogen needs and limit nitrogen losses. This thesis identifies opportunities to develop innovative approaches to N2O mitigation by better evaluating emission estimations from agricultural practices, which could then be implemented in the national and global greenhouse gas inventories established by the Intergovernmental Panel on Climate Change.