Nitrous oxide emission factors for urine from sheep and cattle fed forage rape (Brassica napus L.) or perennial ryegrass/white clover pasture (Lolium perenne L./Trifolium repens)

Abstract

There is increasing scrutiny of the impact of grazing systems on the wider environment. Urine deposition during grazing has a dominant influence on inorganic nitrogen (N) loss to air and water, and in particular on emissions of nitrous oxide (N2O), a potent greenhouse gas. A series of three field trials were conducted to determine N2O emission factors of urine (EF3: N2O-N emitted as % of urine N applied) from animals on two different forage diets: forage rape (Brassica napus L.) and a conventional ryegrass/white clover (Lolium perenne L./Trifolium repens). Emission factors were measured over two winter trials and one summer trial, using both sheep and cattle urine. All three trials were conducted on a poorly drained soil. It was found that there was a tendency for a higher EF3 for urine from animals on a forage rape diet, both when applied to the forage rape soil and when applied to the pasture soil, although differences were not significant on an individual trial basis. When the data for all three trials was combined in a meta-analysis, urine from forage rape-fed animals had a significantly higher EF3 than urine from pasture-fed animals (1.54 vs. 1.20%). This was despite background emissions from the soil under forage rape being higher than from the soil under pasture. Our results suggest that there may be scope to influence N2O emissions via forage type on offer.