Improving the accuracy of nitrous oxide emission factors estimated for hotspots within dairy-grazed farms

Abstract

Nitrous oxide (N₂O) emissions from dairy-grazing pastures can be dominated by large emissions from small areas ('hotspots') frequently used by grazing dairy cattle (i.e., water troughs and gateways). N₂O emissions from these hotspots are quantified by investigating whether N₂O emissions and emission factors (% of applied N emitted as N₂O, EF₃) from potential hotspots are different from non-hotspots. To better characterise N₂O emissions from hotspots and non-hotspots of farms to understand their contributions to national agricultural greenhouse gas inventory calculations, a series of measurements were conducted during winter and spring on two NZ typical dairy farms with contrasting soil drainage (poorly versus well drained). Before measurements were taken, the soils either received a cow urine application or remained untreated. The results showed that changes in water-filled pore space (WFPS) and mineral N around water troughs and gateways, due to additional stock movements and disproportionate excreta-N deposition during previous grazing events, affected both background and total N₂O emissions. But there was little impact on EF₃ values (calculated using IPCC guidelines) from deposited urine between hotspot and pasture areas. These results suggest the same EF₃ values can be used for both to calculate emissions from urine deposited on grazed pastures. However, these results raise concerns about higher background emission in hotspots subtracted from measured emissions from urine-N deposition in calculating EF₃ values and discounting the effects of disproportionate N inputs in intensive agriculture on increased background emissions (legacy effect). This IPCC inventory method does not account for the legacy effect of N loading prior to the measurements which may underestimate the emissions. Thus, an allowance for higher hotspot background emissions could be included in the Inventory to accurately estimate total emissions from agriculture.

Keywords: Greenhouse Gas Inventory methodology; Legacy effect; Urine-N deposition; Water-filled pore space.