Assessing the effectiveness of actions to mitigate nutrient loss from agriculture: a review of methods.

Diffuse nutrient loss from agriculture is degrading surface and groundwater quality throughout Europe, leaving water bodies at risk of not reaching targets set by the Water Framework Directive (WFD). Mitigation methods to reduce diffuse agricultural nutrient loss need to be implemented where water bodies have been identified as at risk of not reaching good status by 2015. Though the effectiveness of individual mitigation methods has usually been assessed in controlled experiments, it is necessary to quantify impact under a wider range of environmental and agricultural conditions and at the catchment scale to ensure that action taken now will be sufficient to meet WFD targets. Due to catchment buffering and long transit times (>50 years), it is unlikely that responses to intervention will be observed by 2015 in many water bodies. In this review, we compare the attributes and usefulness of different approaches (direct measurement, nutrient budgeting, risk assessment and modelling) to assess the efficacy of actions to mitigate sources and transport of nitrogen (N) and phosphorus (P) from agricultural land to water. Modelling and 'measured changes in farm activity' through budgeting avoid the time lags associated with direct measurement and enable rapid evaluations of different options before implementation. Budgeting approaches using on-farm data also provide a simpler, more communicable means of assessment but currently fail to consider the timing and transport aspects of mitigation and assume a direct causal relationship between potential and actual nutrient loss. Risk assessment and modelling applications are potentially more comprehensive and able to better reflect choice of mitigation at a range of scales, but assessments demand increased availability of data, and there is a large degree of uncertainty associated with their spatial and temporal dimensions which is difficult to validate adequately. The failings of individual approaches suggest that these assessment methods should be integrated to maximise their potential usefulness and positive attributes. This will enable nutrient inputs to be utilised most efficiently at broad scales and site specific actions to reduce nutrient transport and delivery can be targeted most cost-effectively at smaller scales. Such an integrated approach will also more effectively engage and involve the farmer in what must be an iterative process.