Evaluating nutrient source regulations at different scales in five agricultural catchments

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The European Union (EU) Nitrates Directive constrains nitrogen (N) and phosphorus (P) use and management on agricultural land with the expectation that better nutrient source management will improve water quality. Linking the effects of agricultural practices to impacts on water quality is a challenging task in terms of deciding on appropriate measurement scales. At national scale in Ireland, P fertiliser use and the numbers of soils tested with excessive P levels have declined since the introduction of the Nitrates Directive policies. However, in a detailed study of five benchmark agricultural catchments, between 6 and 26% of the soil still had excessive soil test P levels, showing the legacy effect of over application of P fertilisers and manures in the past. At farm and field scale large spatial variability in soil P fertility levels due to poor manure and fertiliser distribution was evident. The range of soil test P levels on most farms shows scope to correct these imbalances with better nutrient management planning. The application of critical source area management also needs to be considered; significant differences were shown between soil P attenuation and loss for different soil types across these catchments, for which the regulations do not discriminate. The multi-scale approach employed in this study allows a full spatial and temporal realisation of source and an appreciation of the management constraints which underlie nutrient management decisions on farms. A better understanding of nutrient source management in relation to regulatory compliance standards is needed in order to establish whether current policy instruments will bring about reductions in nutrient losses that are expected to result in improved water quality. For this a better farm-scale nutrient auditing tool could be used to improve the spatial distribution of nutrients, accounting for the influence of soil type and landscape hydrology factors on nutrient source mobilisation and loss.