Grazing-related nitrous oxide emissions: from patch scale to field scale

Abstract

Abstract. Grazed pastures are strong sources of the greenhouse gas nitrous oxide (N2O). The quantification of N2O emissions is challenging due to the strong spatial and temporal variabilities of the emission sources and so N2O emission estimates are very uncertain. This study presents N2O emission measurements from two grazing systems in western Switzerland over the grazing season of 2016. The 12 dairy cows of each herd were kept in an intensive rotational grazing management. The diet for the two herds of cows consisted of different protein-to-energy ratios (system G: grass only diet; system M: grass with additional maize silage) resulting in different nitrogen (N) excretion rates. The N in the excretion was estimated by calculating the animal nitrogen budget taking into account the measurements of feed intake, milk yield, and body weight of the cow herds. Directly after the rotational grazing phases, background and urine patches were identified based on soil electric conductivity measurements while fresh dung patches were identified visually. The magnitude and temporal pattern of these different emission sources were measured with a fast-box (FB) chamber and the field-scale fluxes were quantified using two eddy covariance (EC) systems. The FB measurements were finally upscaled to the field level and compared to the EC measurements for quality control by using EC footprint estimates of a backward Lagrangian stochastic dispersion model. The comparison between the two grazing systems was performed during emission periods that were not influenced by fertilizer applications. This allowed the calculation of the excreta-related N2O emissions per cow and grazing hour and resulted in considerably higher emissions for system G compared to system M. Relating the found emissions to the excreta N resulted in excreta-related emission factors (EFs) of 0.74±0.26 % for system M and 0.83±0.29 % for system G. These EF values were thus significantly smaller compared to the default EF of 2 % provided by the IPCC guidelines for cattle excreta deposited on pasture. The measurements 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 source-specific EFs exhibited a clear difference between urine (1.12±0.43 %) and dung (0.16±0.06 %), supporting a disaggregation of the grazing-related EFs by excreta type in emission inventories. The study also highlights the advantage of a N-optimized diet, which resulted in reduced N2O emissions from animal excreta.

Nitrous Oxide Emission from Grazing Is Low across a Gradient of Plant Functional Diversity and Soil Conditions

“…The N 2 O EFs observed for urine and dung in the current study might be related to the soil, climate, and management practices. Previous studies conducted in similar climates [25,[27][28][29][30] reported low EFs for urine and dung in line with our findings. De Klein et al [53] and Marsden et al [26] reported low (0.03-0.3%) N 2 O-N EF for animal excreta deposited on free-draining soils.…”

“…In accordance, recent studies [25][26][27][28][29][30] have reported low N 2 O-N emission factors (EFs) for urine and dung patches in improved grazing management systems, compelling the Intergovernmental Panel on Climate Change (IPCC) to revise the IPCC [31] default emission factor from 2 to 0.77% for wet temperate climates [32]. In this study, we assessed the applicability of the revised default N 2 O EF [32] to an organic dairy farm located on a sandy loam soil in northern Germany.…”

“…N 2 O emissions from grazed pastures are associated with substantial heterogeneity due to the random nature of emission hotspots from urine and dung depositions, making the eddy covariance method preferred to the chamber method [29] for non-simulated grazing experimental designs. However, the chamber method can capture the magnitude and temporal patterns from emission hotspots (i.e., urine and dung patches), and it makes the comparison of different treatments in parallel possible.…”

Elucidating three-way interactions between soil, pasture and animals that regulate nitrous oxide emissions from temperate grazing systems

“…While techniques exist to directly measure total N 2 O emissions at a field scale, e.g. the eddy covariance method, data acquired in this manner can neither be used to quantify source-specific EFs nor identify mechanistic pathways linking grazing management strategies and resultant emissions ( Voglmeier et al, 2019 ).…”

Effects of grazing management on spatio-temporal heterogeneity of soil carbon and greenhouse gas emissions of grasslands and rangelands: Monitoring, assessment and scaling-up

“…However, the key component required for monitoring GHGs, nutrient cycle, soil health, and grass regrowth is the patch excretions and preferences of animals within a paddock, which ultimately determines grass and nutrient heterogeneity. Failure to integrate plant and animal processes at appropriate temporal and spatial scales can result in an incorrect representation of field scale for grazing land management (Voglmeier et al, 2019). Therefore, causal relationships between animal activity, grazing intensity and frequency are challenging to establish but remain fundamental to building the grazing model and understanding these causal links.…”