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Grazing animals' GHG emissions may be lower than thought

Article-Grazing animals' GHG emissions may be lower than thought

TEAGASC TEAGASC nitrous oxide testing.jpg
Technique used for measuring nitrous oxide emissions.
Based on better quantifying nitrous oxide from animal excreta, next step is to develop targeted mitigation strategies for emissions at pasture.

A recent project in Ireland measured nitrous oxide from urine and dung deposited by grazing animals and found lower greenhouse gas emissions than previously thought.

"Our pasture-based livestock system, which dominates Irish agriculture, is ‘leaky’ in terms of their nitrogen use efficiency. In fact, less than 30% of applied nitrogen 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 nitrogen 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)," explained Dominika Krol with Teagasc, Ireland's agriculture and food development authority.

Krol said nitrous oxide emissions on a national level are calculated by multiplying agricultural activity, in this case nitrogen excreted by animals, by an emission factor (EF). "Up to recently, Ireland used the IPCC [Intergovernmental Panel on Climate Change] default EF of 2% to estimate excreta-derived nitrous oxide, meaning that 2% of nitrogen in the dung and urine patches was believed to be lost as nitrous oxide," Krol said. "However, nitrous oxide can vary greatly depending on whether it is dung or urine, soil type and time of deposition. Hence, we need accurate accounting of nitrous oxide emissions from ruminant dung and urine."

Based on better understanding and quantifying nitrous oxide from animal excreta, Krol said the next step is to develop targeted mitigation strategies 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.

During a recent project, Teagasc team members measured nitrous oxide from urine and dung in spring, summer and autumn on well-drained, moderately drained and imperfectly drained pasture soils. They found that the average nitrous oxide emission factors were substantially lower than the IPCC default, at 0.31% and 1.18% for cattle dung and urine, respectively, Krol said.

These losses were driven by rainfall, temperature and soil moisture, with the highest emissions in autumn and from the imperfectly drained soil, she added. Overall, the original GHG inventory showed that as much as 41% of nitrous oxide produced from Irish agriculture comes from urine and dung deposited by grazing animals; however, based on this research, this fraction was reduced to approximately 23%, clearly showing that the grass-based system has lower emissions than originally expected, Krol said.

Moreover, large differences among excreta, soils and weather can all help mitigate emissions, Krol said, adding, "We can now further explore changes in animal diet to help reduce nitrogen 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."

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