## 3 Conclusions and Perspectives  
A series of edge-of-field technologies have been investigated and implemented in tile-drained agricultural catchments in order to reduce the excess loads of P to downstream surface waters and prevent eutrophication. The review observed that the catchment characteristics and system design regulate the hydrochemical, hydrological and biogeochemical factors, which all affect the P retention performance. Moreover, it became clear that HRT is a key factor regulating P retention, especially the percentage P retention. Therefore, systems receiving event-driven drainage discharge, which reflect on marked variations in HRT, normally show large variability in P retention--potentially shifting from P sinks to P sources. These systems are generally less effective (%) than systems with more stable hydrological regime. The review found that P load correlates positively to mass P retention due to cumulative P retention in an area/volume basis and time period. However, increasing P loads may lead to lower percentage P retention in case these are accompanied by shorter HRTs. The inflow P concentration, on the other hand, normally contributes positively to both mass and percentage P retention. Sedimentation of PP proved to be the main P retention mechanism in CWs and RWs, resulting in the formation of a sediment layer, which ensures long-term P retention and generally represents the largest P pool. Retention of DRP in these systems, however, is limited by the availability of P sorbents, especially in RWs, whose soils are normally enriched with P from previous agricultural practices. Moreover, non-neutral pH, reducing conditions and low P concentrations in the water column can inhibit the retention of DRP. Finally, biological uptake of DRP and storage proved to be limited owing to low uptake rates and mineralization of previously assimilated P during the senescence period. The review also found that retention of DRP in CWs and RWs requires longer HRTs and tends to be more variable in comparison to the retention of PP. Therefore, P retention in these systems is largely affected by the dominant P form (e.g., DRP or PP), where PP is more easily retained and displays a larger contribution to the overall P retention. Based on the above, the review recommended determining the P sorption potential of the soil prior to wetland restoration to assess its capacity for DRP retention. Filter materials are porous and rich in P sorbents, which promote P sorption reactions, and proved to be potential alternatives in catchments dominated by DRP loads, either as single systems or paired with woodchip bioreactors. The review found that Fe and Al-based filters are generally superior to those based on Mg and Ca, as the former allows faster sorption reactions and ensures higher P stability, achieving higher P retention at shorter HRTs. The review observed, however, a dearth of field studies testing the performance of FMs for P retention, which are essential to ascertain their successful application. Differently from the edge-of-field technologies described above, the review found that VBSs are rarely used in the treatment of agricultural drainage discharge, as the P loads normally bypass these systems in tile-drained catchments and skip the P retention mechanisms. However, these may be used as a complement to design alternatives, where the bypass of P loads is avoided and retention mechanisms for DRP are enhanced. The review reported that hydraulic load affects P retention by regulating the HRT and hydraulic efficiency. Thus, percentage P retention is favored under low to moderate hydraulic loads so as to allow sufficient HRT for a significant fraction of the P load to be retained. Hydraulic loads exceeding a critical threshold, on the other hand, favor preferential flow and low level of P mixing across the system, which compromise P retention. The stability of the P retained and long-term P retention, in turn, are largely controlled by the amount of P sorbents in the system and redox conditions.