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
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Date
2012
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Massey University
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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.
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Keywords
Greenhouse gas emissions, Nitrous oxide, Nitrous oxide emissions, Agricultural pollution, Pastoral hill country, Nitrogen fertiliser, Nitrogen in soil, Sheep-grazed pasture