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)

There is increasing scrutiny of the impact of grazing systems on the wider environment in New Zealand. 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. Here we report on a series of three field trials where we measured 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). We measured EF3 over two winter trials and one summer trial, using both sheep and cattle urine. All three trials were conducted on a poorly drained soil on different areas of the same farm. We found a trend for a higher EF3 of urine from animals on a forage rape diet, both when applied to both the forage rape soil and the pasture soil, although these 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 from pasture-fed animals (1.54 vs. 1.20%). This was despite background emissions from the soil under forage rape being much higher than from the soil under pasture (7.1 vs. 1.7 g N ha-1 day-1). Our results suggest that there may be scope to influence N2O emissions (farm, catchment and national scale) via forage type on offer.