## 2 Edge-of-Field Technologies
### Restored Wetlands  
Differently from CWs, which are built in sites where no natural wetland previously existed, RWs are implemented with the aim to reestablish a wetland which was drained for the expansion of agriculture [79]. Thus, the restoration of wetlands generally occurs in areas previously used for agriculture, where the original physical and chemical composition of the soil had been changed as a result of agricultural practices [115]. The process of restoration in tile-drained agricultural catchments occurs by disconnecting the main drainage pipe at the border of a lowland field so as to allow the discharge water to flood over a certain area before reaching downstream surface waters. This consequently increases the HRT of the drainage discharge and allows interaction between the water and soil as well as biogeochemical processes. Wetlands are restored with the main goal of reducing the load of nutrients from upstream agricultural catchments to surface waters, as for CWs. This can be advantageous for the removal of nitrogen if the restoration is performed in soils previously enriched with carbon, which would support denitrification. In relation to P, RWs are normally effective in the retention of PP through sedimentation, whereas the effect on DRP may be uncertain [116]. This is owing to previous agricultural practicessuch as P inputs through mineral fertilizers or manure, which likely enriched the soil with P and consequently reduced the availability of P sorption sites. This effect can result in a P concentration gradient across the soil-water interface upon re-flooding, where desorption of P followed by diffusion to the water column would be potentially favored. This release process can be intensified if the inlet water carries low P concentrations. Moreover, the emergence of reducing conditions at the topsoil as soon as it is re-flooded supports the release of Fe-bound P to the water column [117; 118]. Thus, it is essential that the topsoil layers contain sufficient amounts of redox-stable P sorbents (e.g., Al and Ca) to precipitate with P and prevent its release [118]. Finally, the accumulation of organic P from previous crop rotations may support mineralization and release of dissolved reactive and organic P under waterlogged conditions [117]. The effects described above create uncertainty on the potential of RWs as sinks for DRP. This can counteract the retention of PP and result in large variability in the net P retention between systems, from P sinks to P sources (Table 2). Despite the uncertain sorption capacity of RW soils, other retention mechanisms for DRP such as biological uptake by plants and microorganisms can support net P retention [117], although these are short-term P pools, as discussed in the previous section. Therefore, it is crucial to determine the P sorption capacity and level of P saturation of the soil prior to wetland restoration in order to ensure retention of DRP. [PERSON] et al. [119] demonstrated that two out of four RWs were able to retain P (Table 2). The net P release observed in the remaining RWs was likely caused by excess accumulation of P in the soils from previous agricultural practices and emergence of reducing conditions upon re-flooding. Net and modest P releases were also found in [PERSON] et al. [120] (Table 2), despite the large amounts of P in the topsoil and low P concentrations in the inlet water. This study suggested that the stability of P in the topsoil was controlled by sorption processes, where high molar ratios of Fe to P and oxidizing conditions likely supported the P stability in spite of the conditions described above. These observations were supported to some extent by [PERSON] et al. [121], who demonstrated that the P sorption capacity of RW soils receiving agricultural runoff may be markedly high, even exceeding those of natural wetlands. [PERSON] [122], in turn, reported contrasting results to [PERSON] et al. [120], with net P retention reaching approximately 20% in RWs (Table 2), despite the predominance of dissolved P in the inlet water.