Through conducting spatial sampling campaigns, we characterised the soil and vegetation characteristics as a function of distance from a farm-scale feature with high cattle occupancy. We established a gradient of impact by livestock from low (standard pasture), medium (gateway soil) and high (sacrifice paddock) impact by livestock. This resulted in soils increasing in bulk density, dissolved organic C and decreasing in degree of vegetative cover. We conducted an incubation experiment with intact soil cores taken from across this transect and monitored N2O, CO2 and CH4 emissions and soil properties following cattle urine application. Cumulative N2O emissions increased alongside the gradient of impact by livestock, and the temporal dynamics of N2O emission and mineral N were affected by degree of impact by livestock. Results from sequencing analysis provided evidence that the microbial communities in the contrasting areas do respond differently to perturbation by livestock urine (Obj. 2). Specifically, the microbial diversity decreases upon urine application, likely due to the persistence of taxa that are able to withstand high salt and nutrient loads. This effect was much more pronounced in the areas highly impacted by livestock, suggesting the microbial community is potentially primed in these areas. The rapid response of the microbial community may also contribute to the observation of higher N2O emissions from these areas.
A field trial was conducted on an intensive dairy farm in sub-tropical NSW, Australia (Fig. 1), to determine whether the nitrification inhibitor, DMPP, would be effective in reducing nitrification and subsequent N2O emissions from an area of the farm receiving greater stocking densities (a gateway). Under the conditions of our study DMPP (1.5 kg ha-1) was ineffective in reducing nitrification rates or N2O emissions. We tested increasing rates of DMPP application in the laboratory but found no effect on DMPP performance when increasing from 1 to 10 % of the urine-N applied. In the return year two field trials were conducted, one on a temperate sheep farm and the other on an intensive dairy farm. On the sheep farm an area was created to simulate sheep congregation by placing large feed troughs in the field and rotating them periodically to achieve an area which was disturbed by livestock. Emissions of N2O were greater from urine applied to the area around the feeding troughs compared to the standard area of pasture. While DMPP reduced emissions slightly in the standard pasture, the inhibitor was not effective in reducing emissions of N2O from the livestock impacted area even though applied at a rate of 10 kg ha-1, which is ten times the rate that would normally be applied alongside fertilizers. On the intensive dairy farm in North Wales, N2O emissions from cattle urine applied to a gateway area or an area of standard pasture, with and without DMPP applied at 10 kg ha-1. As was expected emissions were higher from the gateway area, and here the DMPP was effective in reducing N2O emissions by 49% in the standard pasture, but only by 24% in the gateway. Taken together the results suggest that a higher application rate of DMPP is required for urine than would typically be applied with fertilizers. Additionally, nitrification inhibitors are concluded to be not as effective in livestock congregation areas. This is because there is limited vegetation in these areas to mop up any urine-N held in the ammonium form for longer via the inhibitors.
