## 2 Edge-of-Field Technologies
### Constructed Wetlands
[80], for example, reported that most of the P sedimentation in three SFCWs occurred in a sedimentation pond prior to the second wetland basin. The absence of a sedimentation basin, however, may result in high accumulation of sediments at the area near the inlet of the system [69,95]. The removal of excess sediments may be necessary to prevent resuspension during high hydraulic load events or P release from other processes. An experiment demonstrated that the removal of the organic sediment layer in mesocosms resulted in lower P concentrations in the overlying water, attesting the effect of sediments removal on improving the P retention performance [78]. The application of this method in SFCWs, however, can be expensive and the results may be limited [78]. Alternatively, the amendment of SFCW soils with P sorbents may be more promising. [PERSON] et al. [110] showed that this method decreased the release of P from the soil to the overlying water in a SFCW. Moreover, [PERSON] and [PERSON] [111] reviewed a range of materials potentially useful in the amendment of SFCW soils. The review found that allophane, Papakai tephra, limestone and alum demonstrated the highest potential, considering the P retention characteristics, availability in New Zealand and cost. This study also indicated the potential for removing the saturated topsoil with P or mixing it with the subsoil as a measure to prevent P release. The effect of maintenance operations on increasing the lifetime of SFCWs as P sinks when receiving agricultural drainage discharge, however, is still poorly reported in the literature. Despite the advantages of SFCWs as a measure to reduce the P loads, the common requirement of large areas to allow sufficient HRT and achieve acceptable P retention (Table 1) is a major limiting factor for their application, especially in lands highly used for agriculture. Systems receiving high P loads may benefit from smaller areas in order to increase the area specific mass retention of P. However, lower P concentrations at the outlet are normally achieved at the expense of larger SFCW areas, and consequently higher ratios of SFCW area to the agricultural catchment area [78]. Systems receiving large loads of PP, however, may require smaller areas to achieve acceptable P retention than systems predominantly receiving loads of DRP (as suggested by Table 1), owing to the different requirements in terms of HRT as discussed above. Therefore, the area requirement may vary depending on the catchment characteristics, e.g., P load and dominant P form, and the P retention goal. Horizontal or vertical subsurface-flow constructed wetlands can be an advantage in relation to SFCWs in regards to smaller area requirements. [PERSON] et al. [112] demonstrated that five newly constructed experimental horizontal-flow wetlands effectively retained P from agricultural drainage discharge (35%, 38%, 38%, 57% and 31%), which was likely owing to the unsaturated medium with P. The study also demonstrated that the percentage P retention increased after the addition of a carbon source, which probably favored microbial uptake of P, and under longer HRTs. The use of subsurface-flow constructed wetlands, however, is largely restricted to the treatment of wastewater (e.g., sewage) and is less used in agricultural catchments than SFCWs due to high maintenance costs, risk of clogging and limited P sorption capacity of the medium [113]. Other limitations of SFCWs include the low retention potential for agricultural drainage discharge dominated by DRP as discussed above, unless the inlet water carries considerable amounts of P sorbents or the P sorption capacity of the SFCW soil is markedly high to allow acceptable and long-term P retention. Therefore, cost-efficiency assessments are necessary to evaluate the feasibility of SFCWs as an appropriate measure to reduce P loads. The costs of SFCWs are related to land acquisition, construction, energy consumption (e.g., pumping in flat lands) and maintenance. [PERSON] et al.