Developing revised emission factors for nitrous oxide emissions from agricultural pasture treated with nitrification inhibitors
Executive Summary
We conduct a literature review and develop three methods to describe how anthropogenic nitrous oxide (N2O) emissions from pastoral agriculture soils can be reduced using nitrification inhibitors. Most nitrification inhibitors have not been assessed for their effectiveness in reducing N2O emissions from grazed pasture systems. Further, it is essential that application of the nitrification inhibitor to New Zealand soils is sustainable with no deleterious environmental consequences. Dicyandiamide (DCD) (chemically written as C2H4N4) has been studied for more than 80 years (for example, McGuinn 1924) and subject to many tests with no reported environmental side effects. Suter et al. (2006) determined that DCD and DMPP (3, 4-dimethylpirazol phosphate) were the available nitrification inhibitors most suited for use in pastoral systems. In New Zealand, Suter et al. reported DMPP is only available as a coated ammonium nitrate fertiliser. They concluded that this form of DMPP delivery would greatly limit its efficacy with respect to urine excreta patches in soils beneath grazed pasture and also make the inhibitor’s use cost prohibitive. We thus focus on DCD.
The rate of DCD degradation in soils depended strongly on temperature (i.e., slower degradation rate in cooler soils). For DCD decomposition in soils, based on the peer reviewed literature including a New Zealand study, we fitted an exponential function with soil temperature as the independent variable and accounted for 91 % of the variance. We expressed DCD decomposition as the time taken for its concentration in soils to decline to half the initial (application) value, called the half life, t½. Based on average soil temperature during New Zealand field trials involving DCD application and longterm average soil temperatures throughout New Zealand, DCD application should be most effective if restricted to May – September when soil temperature < 12 °C (for example, when soil temperature was 4, 8 and 12 °C, our function predicted t½ was 109, 73 and 49 days, respectively). A suitable DCD application rate was 10 kg/ha and two applications each year were one following grazing in autumn and another following grazing in late winter.
Based on the New Zealand peer-reviewed literature, in conjunction with dairy cattle urine application during autumn to Lismore and Templeton soils located at Lincoln, DCD application corresponded with a 74 ± 4% (average ± standard deviation, n = 5) reduction in nitrate leaching (FracLEACH). Based on the peer reviewed literature and dairy cattle urine application, we recommend that DCD application be considered to correspond with a 74% reduction in FracLEACH.
Based on the New Zealand peer-reviewed literature, in conjunction with dairy cattle urine application during autumn to the Lismore, Templeton and Horotiu soils as well as a pumice soil located at Taupo, DCD application corresponded with a 67 ± 6% reduction in the direct N2O emissions factor (EF3(PRP)). For a dairy cattle grazing trial on Pukemutu soil, DCD application corresponded with a percentage reduction in EF3(PRP) that was statistically indistinguishable from the peer-reviewed literature average. Based on these data, we recommend that DCD application be considered to correspond with a 67% reduction in EF3(PRP).
To put these DCD application responses into perspective, sensitivity calculations may be done according to the New Zealand’s N2O emissions inventory. For New Zealand, this inventory is largely determined by the direct emissions factor for excreta nitrogen (N) deposited onto soils during grazing, EF3(PRP). Iindirect emissions are included in the inventory and these depend on indirect emission factors (a composite value of 0.025 will be used here for illustration) and the fraction of deposited N that leaches beyond the soil, FracLEACH. The New Zealand specific value for FracLEACH is 0.07. Consequently, 93 % of N deposited onto soils determine the direct emissions according to the fractional value of EF3(PRP). The New Zealand specific value for EF3(PRP) is 0.01. Hence, for excreta deposited onto soils during grazing, a 67% reduction in EF3(PRP) and no change in FracLEACH corresponded with a 56 % reduction in (total) N2O emissions follows DCD application. In contrast, no change in EF3(PRP) and a 74% reduction in FracLEACH corresponded with only a 7 % reduction in N2O emissions following DCD application. For no change in EF3(PRP) and a 48% reduction in FracLEACH, a 5 % reduction in N2O emissions corresponded with DCD application. The N2O inventory’s response to DCD application was thus much more sensitive to changes in EF3(PRP) than FracLEACH. Computationally, the relatively consistent response of EF3(PRP) to DCD application was more important than the more variable response of FracLEACH.
There has been one field trial, conducted at Lincoln, quantifying the effects of repeated applications of DCD to grazed pasture. The recently-published, peer-reviewed paper about this trial (Moir et al. 2007) focussed herbage production. Based on seasonal measurements over four years, on average, application of DCD corresponded with a 21 % increase of dry matter production on whole paddock and annual bases. The increase ranged from 17 % in an inter-urine area during year two to 36 % for an area that received urine in year 3. For the N2O emissions inventory, dry matter intake of grazing animals is determined by calculating the energy requirements for maintenance and production of milk, meat and wool. Intake is based on annually-updated information (weight, determining the maintenance requirement, and production data). Because animal production rate is effectively determined in real time by the inventory, dry matter intake could correctly capture a positve effect of DCD on pasture herbage production.
Our first emissions calculation method, called method 1, is an aggregated N2O emissions inventory comparable to current calculations reported by government. The effects of nitrification inhibitors are calculated using ‘annualised’ revisions of emissions factors EF3(PRP) and EF1 and term FracLEACH. For the other two methods, separate calculations are done for October – April when nitrification inhibitors should not be used and May – September when nitrification inhibitors should be used because they will then be more effective. For method 2, the nitrogen applied to soils as excreta from grazing animals remains an aggregation of urine and dung, so calculations for October – April are the same as for method 1. For May – September, method 2 uses a second set of revised values for EF3(PRP) and EF1 and FracLEACH. Method 3 includes this disaggregation plus the excreta are also disaggregated into urine and dung, so a third set of revised values for EF3(PRP), EF1, and FracLEACH are used for each of the two periods. There is sparse date for these emission factors. However, disaggregation of excreta into urine and dung is strongly supported by a scientific argument. For cattle urine, on average, based on New Zealand field trials, EF3(PRP) was 5 and 100 times larger than that of cattle and sheep dung, respectively. This comparison mostly reflects the lower N content of dung. Further, sheep dung is also relatively dry and N2O emissions increase significantly under anaerobic (wet) conditions.
As case studies, we determined changes in emissions between 1990 and 2004 and 2010 with DCD applied to all land grazed by dairy cattle. Using method 1, with DCD, the increase of emissions was 3.0 Gg by 2004 and 5.3 Gg by 2010. Compared to the emissions increases in the absence of DCD, DCD mitigation (the reduced change of emissions) was 4.7 Gg in 2004 and 5.3 Gg in 2010. According to method 2, the corresponding DCD mitigation was significantly greater at 7.7 Gg in 2004 and 8.8 Gg in 2010. Finally, according to method 3, all emissions were reduced each year by around 50 % (for example, from 31.2 to 18.5 Gg in 1990) and the corresponding DCD mitigation was 6.5 Gg in 2004 and 7.4 Gg in 2010. We conclude that method 2 is most strongly supported by research that has been conducted in New Zealand, recognising only sparse data are available for emissions from excreta disaggregated into urine and dung components. However, the research gaps deserves attention because Method 3 provides the most realistic portrayal of N2O emissions from pastoral agriculture soils.
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Sustainable Land Management and Climate Change
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