*Review* **Nitrogen Removal from Agricultural Subsurface Drainage by Surface-Flow Wetlands: Variability**

**Lipe Renato Dantas Mendes**

Independent Researcher, Natal 59064-740, Brazil; liperenato@hotmail.com; Tel.: +55-84-99891-8045

**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.

**Keywords:** surface-flow constructed wetland; nitrogen load; nitrate; ammonium; organic nitrogen; hydraulic load; hydraulic residence time; temperature; denitrification; biological uptake

**Citation:** -Mendes, L.R.D. Nitrogen Removal from Agricultural Subsurface Drainage by Surface-Flow Wetlands: Variability. *Processes* **2021**, *9*, 156. https://doi.org/10.3390/ pr9010156

Received: 7 December 2020 Accepted: 10 January 2021 Published: 15 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
