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Agriculture has long been considered a great source of nitrogen (N) to surface waters and a major cause of eutrophication. Thus, management practices at the farm-scale have since attempted to mitigate the N losses, although often limited in tile-drained agricultural catchments, which speed up the N transport, while minimizing natural removal in the landscape. In this context, surface-flow constructed wetlands (SFWs) have been particularly implemented as an edge-of-field strategy to intercept tile drains and reduce the N loads by re-establishing ecosystems services of previously drained water ponded areas. These systems collect the incoming water volumes in basins sufficiently large to prolong the hydraulic residence time to a degree where biogeochemical processes between the water, soil, sediments, plants, macro and microorganisms can mediate the removal of N. Despite their documented suitability, great intra and inter-variability in N treatment is still observed to date. Therefore, it is essential to thoroughly investigate the driving factors behind performance of SFWs, in order to support their successful implementation according to local catchment characteristics, and ensure compliance with N removal goals. This review contextualizes the aforementioned issue, and critically evaluates the influence of hydrochemistry, hydrology and biogeochemistry in the treatment of N by SFWs.

Agriculture is often responsible for the eutrophication of surface waters due to the loss of phosphorus—a normally limiting nutrient in freshwater ecosystems. Tile-drained agricultural catchments tend to increase this problem by accelerating the transport of phosphorus through subsurface drains both in dissolved (reactive and organic phosphorus) and particulate (particle-bound phosphorus) forms. The reduction of excess phosphorus loads from agricultural catchments prior to reaching downstream surface waters is therefore necessary. Edge-of-field technologies have been investigated, developed and implemented in areas with excess phosphorus losses to receive and treat the drainage discharge, when measures at the farm-scale are not able to sufficiently reduce the loads. The implementation of these technologies shall base on the phosphorus dynamics of specific catchments (e.g., phosphorus load and dominant phosphorus form) in order to ensure that local retention goals are met. Widely accepted technologies include constructed wetlands, restored wetlands, vegetated buffer strips and filter materials. These have demonstrated a large variability in the retention of phosphorus, and results from the literature can help targeting specific catchment conditions with suitable technologies. This review provides a comprehensive analysis of the currently used edge-of-field technologies for phosphorus retention in tile-drained catchments, with great focus on performance, application and limitations.

Nutrient losses from agricultural fields have long been a matter of concern worldwide due to the ecological disturbance this can cause to surface waters downstream. In this paper a new design concept, which pairs a surface-flow constructed wetland (SFW) with a woodchip bioreactor (WB), was tested in relation to its capacity to reduce both nitrogen (N) and phosphorus (P) loads from agricultural tile drainage water. A nutrient mass balance and a comparative analysis were carried out together with statistical regressions in order to evaluate the performance of four SFW+WBs under different catchment conditions. We found marked variations between the systems in regard to hydraulic loading rate (0.0 to 5.0 m/day) and hydraulic retention time (1 to 87 days). The paired system worked as nutrient sinks throughout the study period. Total N and total P removal efficiencies varied from 8% to 51% and from 0% to 80%, respectively. The results support the use of the new design concept for nutrient removal from tile-drained agricultural catchments in Denmark as part of national management plans, with the added advantage that smaller areas are needed for construction (0.1% to 0.2% of the catchment area) in comparison to standalone and currently used SCWs (~1%).