## 2 Edge-of-Field Technologies
### Constructed Wetlands  
Constructed wetlands are systems designed to copy the nutrient reduction processes of natural wetlands. They have a broad application, extending to the treatment of wastewater of different sources [66]. In tile-drained agricultural catchments, they are commonly constructed at the outlet of the main discharge pipe. The retention of P occurs through physical, chemical and biological processes mediated by the interaction of different P pools, i.e., plants, macro and microorganisms, sediments, soil and water. A net P retention consequently results in the reduction of the water P pool while flowing through the system. Constructed wetlands are classified as surface-flow or subsurface-flow systems depending on whether the water flows over or under the soil surface. The performance of CWs is dependent on the operational parameters regulated by the catchment characteristics, e.g., P load and dominant P form, and the CW design, which influences the hydraulic residence time (HRT) and hydraulic efficiency of the CW [67]. These parameters normally show a large degree of variation seasonally and annually, which leads to the large variability in P retention between CWs. This consequently makes the retention performance difficult to predict. Generally, the studies demonstrate that surface-flow constructed wetlands (SFCWs) work as P sinks [68; 69; 70; 71; 72; 73; 74], while some studies indicate that these can also be P sources [52; 54; 75; 76] when receiving event-driven agricultural drainage discharge. Moreover, surface-flow wetlands constructed in former agricultural lands may initially be P sources due to the accumulation of P from previous agricultural practices [77]. When assessing the compilation of these studies, however, it is clear that the inter-annual variation in mass and percentage P retention between SFCWs can be considerable (Figure 1). The variability in P retention between CWs is caused by hydrochemical, hydrological and biogeochemical factors which are controlled by the catchment characteristics and CW design. The P load (Equation (1)) correlates positively to the mass P retention (Equation (2)) and is normally a large explanatory variable [68,71,73]. This suggests that higher mass P retention is likely found under increasing P loads, as the P retention mechanisms are dependent on the inputs of P. [PERSON] et al. [78] demonstrated that P load can be rather variable between SFCWs (as also observed in Table 1) and indeed a strong factor regulating the mass P retention. \[\text{P load}=\frac{\text{water flow }(\text{m}^{3}\text{ yr}^{-1})\times\text{P concentration }(\text{g}\,\text{m}^{-3})}{\text{ wetland area }(\text{m}^{2})} \tag{1}\]  
\[\text{mass P retention}=\text{P load }(\text{g}\,\text{m}^{-2}\text{ yr}^{-1})-\text{P load at the outlet }(\text{g}\,\text{m}^{-2}\text{ yr}^{-1}) \tag{2}\]  
The retention of P in SFCWs occurs through a number of processes and mainly includes sedimentation of PP forming a layer of deposited material on the soil surface; sorption of DRP to reactive sites in the soil; precipitation of DRP with oxides of Fe and Al, and Ca in the water column to the top sediments; and biological uptake by plants, phytoplankton and bacteria of bioavailable forms [79] (Figure 2). As the different retention mechanisms carry out the retention of different P forms, the overall mass retention varies depending on the dominant P form at the incoming P load. Therefore,  
Figure 1: Inter-annual variation in mass (**a**) and percentage (**b**) phosphorus (P) retention in surface-flow constructed wetlands receiving agricultural drainage discharge from 10 reference studies. Different colors in the same study indicate different wetlands. Negative values indicate net P export. * Average values. it is important to assess the efficiency of different P retention mechanisms when estimating the mass retention of P in SFCWs. Surface-flow constructed wetlands are widely known as sinks of PP due to sedimentation of suspended particles and organic material [76; 79].