Grazing related nitrous oxide emissions: from patch scale to fieldscale

Grazed pastures are strong sources of the greenhouse gas nitrous oxide (N2O). The quantification of the emissions is challenging due to the strong spatial and temporal variability of the emission sources and therefore emission estimates are very uncertain. This study presents N2O emission measurements of two grazing systems in western Switzerland over the grazing season 2016. Two herds of dairy cows were kept in an intensive rotational grazing management. The diet for the cows consisted of different protein to energy ratios resulting in different N excretion rates. The N in the excretion was estimated by an animal budget model taking into account the measurements of feed intake, milk yield and body weight of the cow herds. Excreta patches and background surfaces on the pasture were identified manually after different grazing rotations and the magnitude and temporal pattern of the single emission sources were measured with a Fast-box (FB) chamber. The field scale fluxes were quantified using two eddy covariance (EC) systems. The FB measurements were finally up-scaled to the field and compared to the EC measurements for quality control by using EC footprint estimates of a backward Lagrangian stochastic dispersion model. Neglecting emission periods influenced by fertilizer applications resulted in significant higher system emissions (960 ± 219 g N2O-N, or 25 %) for the full grazing regime (system G) compared to the system with the N balanced diet (system M). Relating the found emissions to the excreta N resulted in grazing related EFs of 1.24 ± 0.20 % for system M and 1.36 ± 0.26 % for system G. The found grazing related EFs were thus significantly smaller compared to the EF of 2 % of the IPCC guidelines. Disaggregating the up-scaled fluxes into single contributors showed that urine patch emission dominated the field scale fluxes (57 %), followed by significant background emissions (38 %) and only a small contribution of dung patch emission (5 %). The resulting EFs of 1.13 ± 0.3 % and 0.17 ± 0.04 % for urine and dung indicates the need to disaggregate the grazing related EFs by excreta type. The study also highlights the advantage of an N optimised diet which resulted in reduced N2O emissions on the system level.