Evaluating the evidence-base for the Nitrate Directive regulations controlling the storage of manure in field heaps

Due to its high nitrogen, phosphorus content and biological oxygen demand, organic manures stored in field heaps, pose a significant threat to water quality, if mobilised and transferred to waterbodies during rainfall events. The Nitrate Directive regulations aim to minimise these risks by preventing the mobilisation of contaminants during rainfall events and by providing a buffer zone to waterways, if mobilisation does occur. However, there is concern that the current regulations do not sufficiently mitigate the risk posed to water quality, with a number of European Union (EU) member states being asked to provide the EU Commission with an improved evidence-base to justify the continuation of this practice. The aim of this review is to consider if the evidence-base supports the continuation of this practice, or if the cessation of the storage of manure in field heaps is warranted as per the precautionary principle. Research indicates: (1) manure heaps established on impermeable surfaces are a potential threat to water quality; (2) current regulations on the annual relocation and covering of manure heaps are effective mitigation measures to decrease contaminant loss; (3) the risk to water quality is significantly lowered where field heaps have been established on bare soil; and (4) are located at a distance from waterbodies. However, a significant research gap revealed in the literature review is the contribution of manure heaps to edge-of-field contaminant losses and whether the magnitude of these edge-of-field losses poses a threat to water quality. It could be argued that this knowledge gap warrants the application of the precautionary principle banning this farm practice until such time as this evidence can be provided. However, given the evidence that best practice manure heap management is effective and the significant financial implications of such a decision on farmers, a more measured approach is to fund further research in this area. Finding a balance between use of the precautionary principle and evidence-based policy-making requires a commitment to carry out periodic reviews, such as presented in this paper, and a willingness to revise regulations where necessary.

► Manure field heaps can pose a significant risk to water quality, if managed incorrectly.
► However, there is a lack of evidence of their contribution to edge-of-field losses of contaminates.
► The current regulations in Northern Ireland are adequate to protection water quality.
► There is need for periodic review of the scientific evidence-base for environmental regulations.