A high priority topic within the Department for Environment, Food and Rural Affairs (DEFRA) water quality programme is the mitigation of diffuse rural pollution from agriculture. This programme was set up to meet the requirements of the European Water Framework Directive (WFD ) . Nutrient loss from agricultural land has been suggested as a major cause of elevated nutrient concentrations in surface waters in the UK . Nitrogen (N) and phosphorus (P) are of particular concern as an excess of either nutrient can lead to eutrophication of freshwater systems and coastal waters. Agriculture has also been identified as a significant source of suspended sediment concentrations in UK rivers . Suspended sediment (SS) can lead to loss of ecological integrity  and agriculturally derived sediment has been identified as a source of increased bed-sediment P concentrations in rivers .
Wetlands are often cited as being effective at reducing nutrient and sediment loadings to receiving waters. However, the research in this area is inconsistent, and whilst most studies have shown that both natural and constructed wetlands retain nutrients and sediments, others have shown that they have little effect, or even increase nutrient and sediment loads to receiving water bodies [6–8]. Many factors may have contributed to these disparate results, including the length of time the wetland has been established for, seasonality, the hydrogeomorphic landscape setting, type, size, level and type of management, and the input concentrations/loads and historic loading of the wetland. For example, a constructed wetland system in South-west England switched from a net annual sink to a source of phosphorus over a 10 year period, and from being a sink to a source of soluble reactive phosphorus (SRP) and ammonium between spring and summer [9, 10]. Phosphorus removal has been shown to correlate positively with an increase in wetland area, and a minimum wetland to farmyard area ratio of 1.3 has been proposed for effective removal of molybdate reactive phosphorus (MRP) . When deciding on wetland size, other factors such as ecosystem services [12, 13] should also be considered as discussed below.
In order to inform policy on whether to promote the conservation, management, restoration or construction of wetlands to mitigate the impacts of N, P and SS from agriculture, it is imperative that policy makers are provided with accurate, robust and independently reviewed information. Whilst there is a great deal of published material on this subject, a rigorous, independent systematic review has not been conducted. The current UK guidelines do not stipulate a target percent reduction of nutrient or SS concentrations required from wetlands and simply state that any reduction is sufficient. However, the Environment Agency (EA) wish to adopt the approach that some other European countries now have in place (e.g. Denmark), where a target percentage reduction is specified for different catchments. Hence DEFRA has commissioned a systematic review on how effective and what influences the effectiveness of wetlands at mitigating N, P and SS inputs from agriculture to receiving freshwater in England. It is also important to consider the potential trade-off between other ecosystem services perceived to be provided by wetlands, such as carbon sequestration and habitat provision [12, 13] during the development of any policy. This will be included as a secondary outcome within the review. The review will be impartial and transparent and will follow the guidance set out by the Collaboration for Environmental Evidence (CEE). To exclude bias, the review will encompass a comprehensive literature search on all available material on the subject, both published and unpublished within the British Isles. Specific inclusion criteria will be adhered to and a formal assessment of the quality and reliability of the studies will be undertaken. The data will then be extracted and a data synthesis undertaken. The systematic review will be used as a basis from which to review current, and guide future, policy. Stakeholders include DEFRA and the EA.