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 x10¹⁰ 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. In experiment two (chapter 5) the objectives were to concurrently examine the effects of varying the soil temperature and the water-filled pore space (WFPS) on NOx emissions from urine-N. In this experiment increasing the soil temperature enhanced both the rate of nitrification and the rate of decrease in soil pH. The relationship between the net NO-N flux, expressed as a percentage of the net NH4⁺-N depletion rate, and the level of soil acidity was again demonstrated at the warmest soil temperature (22°C) where soil acidification had progressed sufficiently to enable abiotic NO formation. The NO-N fluxes increased with decreasing soil moisture and increasing soil acidity indicating abiotic factors were responsible for NO production. The Q10 response of the NO flux between 5 to 15°C decreased from 4.3 to 1.5 as WFPS increased from 11% to 87% respectively. Fluxes of N₂O increased with increasing WFPS and temperature indicating that denitrification was the dominant process. Results from experiments 2 and 3 indicated that the rate of nitrification had a direct bearing on the ensuing soil acidity and that it was this in conjunction with the available inorganic-N pools that affected NOx production. Therefore the third experiment examined the effect of urine-N rate on NOx emissions, with urine-N rate varied over 5 levels from 0 to 1000 kg N ha⁻¹, the highest rate being that found under maximal urine-N inputs to pasture. Rates of nitrification were diminished at the highest rates of urine-N applied and decreases in soil acidity were not as rapid due to this. Again significant but separate linear relationships were developed, for each urine-N rate used, between the NO-N flux, expressed as a percentage of the net NH4⁺-N depletion rate, and the level of soil acidity. The slope of these relationships increased with increasing urine-N rate. The NO-N flux, expressed as a percentage of the net NH4⁺-N depletion rate, versus soil acidity was higher under 1000 kg N ha⁻¹, despite the lower soil acidity in this treatment. This indicated that the enhanced inorganic-N pool was also playing a role in increasing the NO flux. The N₂O fluxes were of limited duration in this experiment possibly due to conditions being disadvantageous for denitrification. In the field experiments two urine-N rates were examined under both summer and winter conditions at two urine-N rates. The emission factors after 71 days for NO-N in the summer were 0.15 and 0.20% of the urine-N applied for the 500 and 1000 kg N ha⁻¹ rates respectively while the respective N₂O-N fluxes were 0.14 and 0.16%. Under winter conditions the emission factors after 42 days for NO-N were <0.001% of the urine-N applied regardless of urine-N rate while the N₂O-N fluxes were 0.05 and 0.09% for the 500 and 1000 kg N ha⁻¹ urine-N rates respectively. The relationships and predictors of NO-N flux determined in the laboratory studies did not serve as strong indicators of the NO-N flux under summer conditions. Low emissions from urine-N over winter were due to the low soil temperatures and high WFPS. These studies have demonstrated that soil chemical and environmental variables influence the production of NOx and N₂O emissions from urine-N applied to soil and that seasonal effects have a significant impact on the relative amounts of NO-N and N₂O-N emitted from urine patches. Suggestions for future work are also made.
| Identifer | oai:union.ndltd.org:ADTP/270097 |
| Date | January 2009 |
| Creators | Khan, Shabana |
| Publisher | Lincoln University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://purl.org/net/lulib/thesisrights |
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