Cattle excreta deposited on grazed grasslands are a major source of the greenhouse gas (GHG) nitrous oxide (N2O). Currently, many countries use the IPCC default emission factor (EF) of 2% to estimate excreta-derived N2O emissions. However, emissions can vary greatly depending on the type of excreta (dung or urine), soil type and timing of application. Therefore three experiments were conducted to quantify excreta-derived N2O emissions and their associated EFs, and to assess the effect of soil type, season of application and type of excreta on the magnitude of losses. Cattle dung, urine and artificial urine treatments were applied in spring, summer and autumn to three temperate grassland sites with varying soil and weather conditions. Nitrous oxide emissions were measured from the three experiments over 12months to generate annual N2O emission factors. The EFs from urine treated soil was greater (0.30-4.81% for real urine and 0.13-3.82% for synthetic urine) when compared with dung (-0.02-1.48%) treatments. Nitrous oxide emissions were driven by environmental conditions and could be predicted by rainfall and temperature before, and soil moisture deficit after application; highlighting the potential for a decision support tool to reduce N2O emissions by modifying grazing management based on these parameters. Emission factors varied seasonally with the highest EFs in autumn and were also dependent on soil type, with the lowest EFs observed from well-drained and the highest from imperfectly drained soil. The EFs averaged 0.31 and 1.18% for cattle dung and urine, respectively, both of which were considerably lower than the IPCC default value of 2%. These results support both lowering and disaggregating EFs by excreta type.

*Corresponding authors karl.richards@teagasc.ieHighlights  The N 2 O emission factor for CAN was substantially higher than the IPCC default and highly variable between sites and across years. Urea products decreased direct N 2 O emissions from CAN on average by 80%  Switching from CAN to urea products reduces both N 2 O emissions and fertiliser costs. 3 AbstractThe accelerating use of synthetic nitrogen (N) fertilisers, to meet the world's growing food demand, is the primary driver for increased atmospheric concentrations of nitrous oxide (N 2 O). The IPCC default emission factor (EF) for N 2 O from soils is 1% of the N applied, irrespective of its form. However, N 2 O emissions tend to be higher from nitrate-containing fertilisers e.g. calcium ammonium nitrate (CAN) compared to urea, particularly in regions, which have mild, wet climates and high organic matter soils. Urea can be an inefficient N source due to NH 3 volatilisation, but nitrogen stabilisers (urease and nitrification inhibitors) can improve its efficacy. This study evaluated the impact of switching fertiliser formulation from calcium ammonium nitrate (CAN) to urea-based products, as a potential mitigation strategy to reduce N 2 O emissions at six temperate grassland sites on the island of Ireland. The surface applied formulations included CAN, urea and urea with the urease inhibitor N-(nbutyl) thiophosphoric triamide (NBPT) and/or the nitrification inhibitor dicyandiamide (DCD). Results showed that N 2 O emissions were significantly affected by fertiliser formulation, soil type and climatic conditions. The direct N 2 O emission factor (EF) from CAN averaged 1.49% overall sites, but was highly variable, ranging from 0.58% to 3.81.Amending urea with NBPT, to reduce ammonia volatilisation, resulted in an average EF of 0.40% (ranging from 0.21 to 0.69%)-compared to an average EF of 0.25% for urea (ranging from 0.1 to 0.49%), with both fertilisers significantly lower and less variable than CAN.Cumulative N 2 O emissions from urea amended with both NBPT and DCD were not significantly different from background levels. Switching from CAN to stabilised urea formulations was found to be an effective strategy to reduce N 2 O emissions, particularly in wet, temperate grassland. 4Key words nitrous oxide mitigation; emission factor; calcium ammonium nitrate; stabilised urea; nitrification inhibitor Dicyandiamide (DCD); urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT).5

Emissions of greenhouse gas (GHG) and ammonia (NH 3 ) from cattle and cow slurry storage can be minimised by acidification. A knowledge gap exists to find chemical amendments that are suitable and cost effective to mitigate both GHG and NH 3 gases simultaneously. This study showed that ferric chloride, sulphuric acid, alum and acetic acid reduced NH 3 by 96%, 85%, 82% and 73%, respectively. In terms of methane (CH 4 ), ferric chloride, alum, sulphuric acid and acetic acid reduced emissions by 98%, 96%, 95% and 94%, respectively. Previous studies have found that the reduction of pH to below 6 can inhibit the release of these gases; however, the effectiveness can vary depending on each amendment's composition.Considering the cost per tonne of amendment, N savings (based on fertiliser replacement value) and the reduction in GHG, the best performing amendment was alum. Currently, the cost of implementing these amendments is, at best, cost neutral. Therefore, incentivising chemical amendments for the abatement of GHG and NH 3 gases from slurry storage is needed. This incubation experiment is an effective means of pre-screening amendments before they are explored at pilot-or full-scale with subsequent field application. Future research should consider assessment of cheaper on-and off-farm alternative waste streams as slurry amendment.

24Ruminant urine patches deposited onto pasture are a significant source of greenhouse gas

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