## 2 Edge-of-Field Technologies
### Restored Wetlands
Restored wetlands have been mostly used as floodplains in riparian areas in order to reduce the load of nutrients in surface waters during flood events [116]. Thus, most of the literature in relation to RWs addressed these issues. Assessments on the potential of RWs in the retention of P from agricultural drainage discharge are, therefore, limited by the small number of studies. It has been suggested, however, that RWs may be a potential source of P in the initial period of operation due to previous agricultural practices and the associated effects described above [115; 116; 117; 77]. The release of considerable amounts of P from the soil over an initial period and/or enrichment with P sorbents from the drainage discharge may, however, turn RWs into P sinks. On the other hand, gradual loss of P \begin{table}
\begin{tabular}{l l c c c c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{**Name**} & \multirow{2}{*}{\(\mathbf{A_{RW}}\)} & \multicolumn{2}{c}{**Ratio of**} & \multirow{2}{*}{**Monitoring**} & \multirow{2}{*}{**Hydraulic**} & \multicolumn{2}{c}{**P**} & \multirow{2}{*}{**Load**} & \multirow{2}{*}{**P Retention**} & \multirow{2}{*}{**Study**} \\  & & & & & & & & & & & \\  & & & & & & & & & & \\ \hline  & & **m\({}^{2}\)** & **\%** & **yr** & **m yr\({}^{-1}\)** & **mg L\({}^{-1}\)** & **g m\({}^{-2}\) yr\({}^{-1}\)** & **\%** & \\ \hline Denmark & Ulleruplund & 130,000 & 21.7 & 1 & 0.4 & 0.03 & 0.01 & \(-\)0.04 & \(-\)88 & [119] \\ Denmark & Snaernose & 340,000 & 6.6 & 2 & - & - & 1.43 & 0.26 & 18 & [119] \\ Denmark & Lindkker & 840,000 & 9.2 & 1.4 & - & 0.07-0.93 * & 0.45 & \(-\)0.05 & \(-\)11 & [119] \\ Denmark & Geddeshecken & 410,000 & 16.7 & 1.8 & - & 0.08-0.51 * & 0.25 & 0.05 & 21 & [119] \\ Denmark & Egeskov & 6200 & 13.7 & 2 & 2.7 & 0.04 * & 0.09 & \(-\)0.004 & \(-\)4 & [120] \\ Denmark & Stor A & 5870 & 2.4 & 2 & 4.8 & 0.02 b & 0.11 & \(-\)0.06 & \(-\)56 & [120] \\ USA & Embarras River & 6000–8000 & 33–4.0 & - & - & - & - & - & 20 & [122] \\ \hline \hline \end{tabular}  
* Range; b average from annual means.  
\end{table} 
Table 2: Average annual hydraulic and phosphorus (P) load as well as mass P retention of restored wetlands (RWs) receiving agricultural drainage discharge investigated in different studies. Included are the RW areas (\(\mathrm{A_{RW}}\)) and their ratios to the agricultural catchment areas (\(\mathrm{A_{AC}}\)) as well as the average P concentration and percentage P retention for the entire monitoring period. sorbents from RW soils upon re-flooding, thus decreasing the P sorption capacity, is also feasible [121]. Therefore, it is fundamental to investigate the operating performance of RWs in the long-term to find out common patterns under different conditions. The systems included in this review were monitored for up to two years only (Table 2). Management methods to increase the efficiency of RWs may include the removal of the enriched topsoil with P prior to implementation, as tested in a wetland constructed on former agricultural soil receiving agricultural runoff [123]. [PERSON] et al. [120], in turn, suggested a periodic harvest of the vegetation in order to maintain or improve the retention capacity of the soil when the pool of organic P becomes large in relation to the annual P load. As for CWs, the main constraint in the implementation of a RW refers to the need of land which would otherwise be used for agriculture [77]. Strategic restoration plans, therefore, may focus on marginal agricultural areas less profitable for farming, optimal sizing and design [124]. [PERSON] [122] pointed out that P retention in RWs is highly sensitive to variations in HRT, which could be related to the effects associated to previous agricultural practices and likelihood of P release. This could be verified in [PERSON] et al. [125], where large variations in water flow throughout the monitoring period resulted in marked differences in P retention in RWs receiving agricultural runoff.