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Lower GHG emissions from urine and dung patches than previously thought

17 April 2020
Type Media Article

ENVIRONMENT: A recent project measured N2O from urine and dung in spring, summer and autumn on well-drained, moderately-drained and imperfectly-drained pasture soils - Dominika Krol tells us more.

Emissions from ruminant urine and dung patches: Our pasture-based livestock system, which dominates Irish agriculture is ‘leaky’ in terms of their nitrogen (N) use efficiency. In fact less than 30% of applied N is recovered in final products such as meat and dairy. As spring has finally arrived, animals are now out grazing, but this also means that ruminant livestock excrete 70–95% of their N intake onto pasture as dung and urine. These excreta patches, in particular urine, are an important source of the potent greenhouse gas (GHG) nitrous oxide (N2O).

Commitment to reduce GHG emissions: Ireland has committed to reducing national GHG emissions under the EU Energy and Climate Package and the Climate Action Plan to Tackle Climate Breakdown published in 2019 posed a sectoral GHG reduction target on agriculture. Achieving these reduction targets and realising the opportunities for growth identified in the Food Wise 2025 report poses a significant challenge for Irish agriculture, which accounts for 90% of national N2O emissions.

How research can help to meet our targets: Nitrous oxide emissions on a national level are calculated by multiplying agricultural activity, in this case N excreted by animals, by an emission factor (EF). Up to recently, Ireland used the IPCC default EF of 2% to estimate excreta-derived N2O, meaning that 2% of N in the dung and urine patches was believed to be lost as N2O. However, N2O can vary greatly depending on whether it is dung or urine, soil type and time of deposition. Hence, we need accurate accounting of N2O emissions from ruminant dung and urine. Based on better understanding and quantifying N2O from animal excreta, we need to develop targeted mitigation for these emissions at pasture. To address this knowledge gap, Teagasc carried out research to quantify nitrous oxide emissions from urine and dung deposited to pasture by grazing animals.

Technique used for measuring N2O emissions

Developing nitrous oxide emission factors for excreta deposited on pasture: During a recent project, we measured N2O from urine and dung in spring, summer and autumn on well-drained, moderately-drained and imperfectly-drained pasture soils. We found that the average N2O emission factors were substantially lower than the IPCC default, 0.31% and 1.18% for cattle dung and urine, respectively. These losses were driven by rainfall, temperature and soil moisture, with highest emissions in autumn and from the imperfectly-drained soil. Overall, the original GHG inventory showed that as much as 41 % of N2O produced from Irish agriculture comes from urine and dung deposited by grazing animals, however based on this research, this fraction reduced to approximately 23%, clearly showing that our grass-based system has lower emissions than originally expected. Moreover, as we noticed large differences between excreta, soils and weather, they can all help us mitigate emissions. We can now further explore changes in animal diet to help reduce N in urine or move it towards dung, and adjust grazing times to extend grazing on well-drained soils but restrict grazing on poorly-drained soils as they get very wet. This has an additional potential of avoiding poaching and retaining good soil structure.

Preparation of urine treatments for field application