Factors affecting nitric oxide and nitrous oxide emissions from ...

Abstract of a thesis submitted in partial fulfilment of the
requirements for the Degree of Doctor of Philosophy
Factors affecting nitric oxide and nitrous oxide
emissions from grazed pasture urine patches
under New Zealand conditions
Abstract
by Shabana khan
New Zealand is dominated by its agricultural industry with one of the most intensive farming
practices being that of intensive dairying. New Zealand currently has approximately 5.3
million dairy cows that excrete up to 2.2 L of urine, per urination event, up to 12 times per
day. This equates to 5.1 x10 10 L per year or enough urine to fill over 1.2 million milk tankers.
This sheer volume of urine and its associated N content has implications for the cycling of N
within the pasture soils utilised, and New Zealand’s greenhouse gas budget due to the
emission of N2O from urine affected areas. The emission of nitric oxide (NO) from
agricultural systems is also receiving increasing attention due to concerns about alterations in
the balance of atmospheric trace gases and sinks. Worldwide there is a dearth of information
with respect to the emissions of NO from urine-N deposition onto soils with only two in situ
studies and no studies on the effects of soil pH, environmental variables or urine-N rate on
NO fluxes. This present study has provided some fundamental information on the factors and
processes affecting the emission of NO from bovine urine applied to pasture soils. Five
experiments were performed in total; three laboratory experiments and two field experiments.
The first laboratory experiment (chapter 4) examined the effect of the initial soil pH on
NOx emissions from urine-N applied at 500 kg N ha -1 . Soil was treated to alter the initial soil
pH over the range of 4.4 to 7.6. Initial soil pH affected rates of nitrification which in turn
affected the decline in soil pH. Emissions of NO increased with increasing soil pH. However,
a strong positive linear relationship was established between the NO-N flux, expressed as a
percentage of the net NH4 + -N depletion rate, and the level of soil acidity. The NO-N fluxes
were higher under the more acidic soil conditions where N turnover was lower. The fluxes of
N2O did not follow the same pattern and were attributed to biological mechanisms.
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