Abstract
Pasture-based livestock farming contributes considerably to global emissions of nitrous oxide (N2O), a powerful greenhouse gas approximately 265 times more potent than carbon dioxide. Traditionally, the estimation of N2O emissions from grasslands is carried out by means of plot-scale experiments, where externally sourced animal excreta are applied to soils to simulate grazing conditions. This approach, however, fails to account for the impact of different sward types on the composition of excreta and thus the functionality of soil microbiomes, creating unrealistic situations that are seldom observed under commercial agriculture. Using three farming systems under contrasting pasture management strategies at the North Wyke Farm Platform, an instrumented ruminant grazing trial in Devon, UK, this study measured N2O emissions from soils treated with cattle urine and dung collected within each system as well as standard synthetic urine shared across all systems, and compared these values against those from two forms of controls with and without inorganic nitrogen fertiliser applications. Soil microbial activity was regularly monitored through gene abundance to evaluate interactions between sward types, soil amendments, soil microbiomes and, ultimately, N2O production. Across all systems, N2O emissions attributable to cattle urine and standard synthetic urine were found to be inconsistent with one another due to discrepancy in nitrogen content. Despite previous findings that grasses with elevated levels of water-soluble carbohydrates tend to generate lower levels of N2O, the soil under high sugar grass monoculture in this study recorded higher emissions when receiving excreta from cattle fed the same grass. Combined together, our results demonstrate the importance of evaluating environmental impacts of agriculture at a system scale, so that the feedback mechanisms linking soil, pasture, animals and microbiomes are appropriately considered.
Highlights
Agriculture is one of the greatest contributors to emissions of nitrous oxide (N2O) (Galloway et al, 2004; Reay et al, 2012), a key greenhouse gas (GHG) approximately 265 times more potent than carbon dioxide (IPCC, 2013) with an atmospheric residence time of around 116 years (Prather et al, 2015)
The lowest herbage N concentration was found on White clover mix (WC), which subsequently resulted in lower urine and dung N compared to the Permanent pasture (PP) diet
Given that less N2O was emitted from PP than High sugar grass (HS) (Section 4.2), the current findings suggest that the historic avoidance of ploughing on the PP farmlet may have caused a positive effect on soil N retention and resulting N2O emissions (Krol et al, 2016)
Summary
Agriculture is one of the greatest contributors to emissions of nitrous oxide (N2O) (Galloway et al, 2004; Reay et al, 2012), a key greenhouse gas (GHG) approximately 265 times more potent than carbon dioxide (IPCC, 2013) with an atmospheric residence time of around 116 years (Prather et al, 2015). As farmlands occupy 37 % of the Earth’s land surface and grasslands account for 67 % of that area (FAO, 2016b), there is a clear and urgent need to reduce N2O emissions that originate from grazing livestock production systems (McAuliffe et al, 2018a). This case is strong as current efforts to mitigate climate impacts of agriculture do not match up against that of many other industries.
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