Greenhouse gas emissions from Scottish arable agriculture and the potential for biochar to be used as an agricultural greenhouse gas mitigation option

Abstract

Nitrous oxide (N2O) is a powerful greenhouse gas (GHG) which has a global warming potential 296 times greater than that of carbon dioxide (CO2). Agriculture is a major source of N2O and in the UK approximately 71 % of N2O emissions are produced by agricultural soils, mainly as a result of the application of nitrogenous fertilisers. Despite previous research into agricultural N2O emissions which has demonstrated that N2O emissions have high spatial and temporal variability, there is still a lack of knowledge surrounding the factors that influence the magnitude of emissions from agricultural soils. Agricultural N2O emissions for the UK’s annual GHG inventory are currently estimated using a 1.25 % emission factor (EF) (to be decreased to 1 % in 2015) which assumes that 1.25 % of applied nitrogen (N) fertiliser is emitted as N2O. The EF does not take into account influencing factors such as location or fertiliser type. Mitigation of N2O emissions is vital if future climate change is to be prevented, yet this must also be combined with the need to intensify agricultural production to feed the increasing global population. Biochar which is a carbon rich material produced during the pyrolysis of biomass has been identified as a potentially useful soil amendment with the ability to mitigate N2O emissions. However, most previous research has focused on laboratory scale experiments and there is a need to investigate the use of biochar in a field environment. Other N2O mitigation options such as nitrification inhibitors, or altering fertiliser management practices, require testing under different conditions to assess their suitability for use. This thesis aims to investigate a). The factors affecting N2O emissions from synthetically and organically fertilised arable soils, and b). To explore the potential of various N2O mitigation options for arable systems, including biochar. This thesis firstly investigates N2O emissions from synthetically fertilised arable soil. Varying application rates of ammonium nitrate fertiliser were applied to a Scottish arable soil during a year long field experiment and the effects of mitigation options such as a nitrification inhibitor (DCD) were assessed. N2O emissions were shown to be significantly affected by soil water filled pore space and the 1.25 % EF was demonstrated to be generally greater than those calculated in this experiment. The use of DCD significantly decreased N2O emissions and crop yields. A second year long field experiment was carried out to investigate N2O and NH3 emissions from an organically fertilised arable soil and to explore the effect of the timing, form and method of organic fertiliser application on emissions and EFs. Slurry, poultry litter, layer manure and farmyard manure were applied in the autumn and the spring. Cumulative N2O emissions were generally greater from the autumn applications and NH3 emissions were greater from the spring applications, due to wetter soil conditions and incorporation of fertiliser during the autumn. The type of fertiliser applied affected the magnitude of emissions with the greatest cumulative N2O and NH3 emissions from the layer manure. The method of fertiliser application had no effect on emissions. The following experiment investigated the ability of different biochars to retain N from a solution and the effect of biochar particle size on retention. A batch sorption experiment was used to test the affinity and capacity of six biochars for ammonium (NH4+) and nitrate (NO3-) from different concentrations of NH4NO3 solution. All of the biochars studied demonstrated the ability to retain NH4+ and NO3- from solution although greater NH4+ retention was observed. Differences in biochar affinity for N could be explained by pyrolysis temperature, but there was no effect of particle size or pH. Oil seed rape straw biochar was demonstrated to have the greatest NH4+ and NO3- retention capacity and as such was chosen for use in the next experiment. This work investigated the potential for oil seed rape straw biochar to decrease emissions of N2O, CH4 and CO2 from stored slurry and whether any GHG mitigation effects would continue following application of the slurry to arable soil. The effect on emissions of amending the biochar and slurry mixture with DCD after application to the soil was also explored. There was no significant effect of the biochar on GHG emissions from the stored slurry although the slurry initially acted as a sink for N2O and CO2. There were no significant differences between emissions from any treatments following application to the soil. The overall results of these studies indicate that N2O emissions are highly dependent on weather conditions, and hence location, in addition to fertiliser type and application timing. It was concluded that the use of a standard 1.25 % EF for synthetic and organic N fertiliser applications for the whole of the UK is inappropriate. Mitigation options such as the use of DCD, altering fertiliser application season or fertiliser type have been shown to possess the potential to mitigate N2O emissions but tradeoffs between N2O and NH3 emissions, and impacts on crop yields must be considered. Biochar was demonstrated to retain NH4+ and NO3- ions and this property may account for biochar’s N2O mitigation capabilities as observed by previous researchers. However, if N retention is taking place, the N appears to still be available for production of N2O and crop uptake.