An investigation of the spatial distribution of N2O emissions from sheep grazed hill country pastures in New Zealand :|ba thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Environmental Science at Massey University, Palmerston North, New Zealand

Abstract

New Zealand’s (NZ) greenhouse gas (GHG) profile is unique amongst developed countries as almost 50% of GHG emissions are derived from agriculture. In contrast, agricultural sectors of other developed countries typically contribute <10% to the national total GHG profile. In NZ, agricultural GHG emissions are dominated by methane (CH4) from enteric fermentation and nitrous oxide (N2O) from excreta deposition and nitrogen (N) fertiliser application. Nitrous oxide emissions from agricultural soils are largely affected by N inputs and soil moisture conditions, and contribute 33% of agricultural GHG emissions. In pastoral hill country these factors are inherently more variable than in flat land pastures due to topographydriven differences in excretal N returns and in soil moisture. This limits the application of N2O emission data collected from trials conducted on flat land to hill country situations. The objective of this thesis was to determine the influence of topography and fertiliser N inputs to soil on N2O emissions in hill country. Small scale trials were conducted to measure these aspects of N cycling. Three trials were conducted to measure the effect of slope and fertiliser N input on nitrification potential (NP) and N2O emissions. The results of these short term trials suggested that slope class and fertiliser N rates significantly affected nitrification rates and N2O emissions in hill country due to differences in N inputs and moisture status, as affected by slope. Both NP and N2O emissions were highly spatially variable during the measurement periods and the results presented in this thesis suggest that the majority of N2O emissions in sheep grazed hill country are produced from low slope/stock camping areas. Based on our findings it is recommended that mitigation options to reduce the risk of N loss from sheep grazed hill country should be targeted at low slope/stock campsite areas. Due to the significant relationship between slope class and N2O emissions, slope class may be a suitable parameter for up-scaling estimates of N2O emissions from sheep grazed hill country.