The effect of fertiliser and grazing on nitrogen export in surface runoff from rain-fed and irrigated pastures in south-eastern Australia

Nutrients, including nitrogen (N), exported from agricultural systems contribute to eutrophication and the development of algal blooms. Understanding the relative effect of farm management on nutrient export will indicate the extent to which farmers can potentially mitigate this process. Six years of monitoring surface runoff from rain-fed and irrigated dairy pastures was carried out in south-eastern Australia. Over the monitoring period, the flow-weighted mean annual total N (TN) concentrations in runoff varied between 8.2 and 29 mg/l for rain-fed pasture and 8.7 and 58 mg/l for irrigated pasture. An additive component model describing N concentrations separated the management (grazing and fertiliser) and year effects from other processes. The model accounted for 40 and 47% of the variation in N concentrations for the rain-fed and irrigated pastures, respectively. While fertiliser application, grazing and year had a significant effect on concentrations, other variables that are not necessarily under management control significantly affected N export. With current knowledge, improved management of pasture-based systems such as improved timing of fertiliser application and grazing relative to runoff may only result in a small decrease in N export. The remainder of the variability was attributed to environmentally mediated changes of N concentrations in runoff water. The collection of more detailed information on environmental parameters including soil moisture and soil temperature is proposed, to enable a better prediction of N concentrations and therefore improved understanding of potential management strategies.