Can nitrogen input mapping from aerial imagery improve nitrous oxide emissions estimates from grazed grassland?

Abstract

Most nitrogen (N) lost to the environment from grazed grassland is produced as a result of N excreted by livestock, released in the form of nitrous oxide (N 2 O) emissions, nitrate leaching and ammonia volatilisation. In addition to the N fertiliser applied, excreta deposited by grazing livestock constitute a heterogeneous excess of N, creating spatial hotspots of N losses. This study presents a yearlong N 2 O emissions map from a typical intensively managed temperate grassland, grazed periodically by a dairy herd. The excreta deposition mapping was undertaken using high-resolution RGB images captured with a remotely piloted aircraft system combined with N 2 O emissions measurements using closed statics chambers. The annual N 2 O emissions were estimated to be 3.36 ± 0.30 kg N 2 O–N ha −1 after a total N applied from fertiliser and excreta of 608 ± 40 kg N ha −1 yr −1 . Emissions of N 2 O were 1.9, 3.6 and 4.4 times lower than that estimated using the default IPCC 2019, 2006 or country-specific emission factors, respectively. The spatial distribution and size of excreta deposits was non-uniform, and in each grazing period, an average of 15.1% of the field was covered by urine patches and 1.0% by dung deposits. Some areas of the field repeatedly received urine deposits, accounting for an estimated total of 2410 kg N ha −1 . The method reported in this study can provide better estimates of how management practices can mitigate N 2 O emissions, to develop more efficient selective approaches to fertiliser application, targeted nitrification inhibitor application and improvements in the current N 2 O inventory estimation.

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