Abstract
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.
Highlights
This review has no focus on measures that mitigate N losses at the farm-scale, nor on approaches that intensify denitrification at the edge of agricultural catchments, but rather on the use of surface-flow constructed wetlands (SFWs) as a strategy to increase the hydraulic residence time (HRT) of tile drain discharge and promote N removal
Similar to the model for total N, the effect of hydraulic load was negative, much stronger. These results suggest that removal efficiency of organic N is the most susceptible to vary as a function of hydraulic load among the N forms
This could be verified in Drake et al [37], who reported in a three-year study that the highest NO3− removal rate and efficiency occurred in the year with moderate HRT and the least variation in hydraulic load
Summary
Nitrogen (N) is an essential element for crop systems, and has been progressively used in agriculture as fertilizers, which made agricultural catchments a major source of N with detrimental effects on the quality of inland and coastal surface waters, including an increased incidence of eutrophication [1,2,3] In this context, precipitation and mineralization of the soil organic matter are non-controllable factors which highly regulate the level of N leaching and subsequent loss [1]. Tile-drained agriculture can be highly diverse in space and time in relation to agricultural practices, geology, soil type, topography, hydrology and climate, which all contribute to determine the level of N loss between catchments, as well as within the same due to seasonality and annual differences [7,15]. N transport consists mainly of NO3−, as it is highly mobile and may be generated in situ
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