## 2 Edge-of-Field Technologies
### Constructed Wetlands
(2017) demonstrated that the percentage P retention depended on the stability of sediment-bound P. Therefore, it is also important to understand the effects of biogeochemical factors on internal P cycling so as to comprehend further the variability of P retention between SFCWs. The stability of the retained P is crucial to determine the potential for long-term retention. The largest pools of P in a SFCW are commonly the sediments and plants and the capacity of these to store P largely influences the retention. Therefore, it is important to understand the factors that regulate the stability of P in these pools. Geochemical characteristics largely control the stability of P in the sediments and soil of SFCWs, including transformation reactions between DRP and PP ([PERSON], 2008; [PERSON] et al., 2017). This is of particular importance as a considerable fraction of the incoming PP, especially P bound to clay-sized particles, can be labile and potentially bioavailable ([PERSON] et al., 2017). This fraction, however, can increase or decrease when passing through a SFCW, which is likely affected by the P load rate--higher load rates tend to increase the export of clay-sized particles and labile PP at the outlet ([PERSON] et al., 2017). As a result of this effect, the stability of P in the sediments may vary across the system. [PERSON] et al. (2017) found that only 22% on average of the P retained in the sediments was stable (i.e., Ca-bound P or residual P), while the P fraction bound to Fe and Al--thus potentially labile--was almost 40% (Figure 3). [PERSON] et al. (2017), in turn, found higher fractions of P bound to Fe and Al in the sediments, accounting from 58 to 100%, which is considered unstable depending on the pH and redox conditions (Figure 3). Dissolved reactive P normally binds to sorbents such as oxides of Fe and Al, and Ca ([PERSON] et al., 2017) (Figure 2). Calcium is the major P sorbent in SFCWs with mineral alkaline soils. Iron and Al oxides, on the other hand, are the most common P sorbents in SFCWs with mineral acidic soils, and the concentrations of these largely affect the stability of P. The P sorption capacity (Equation (8), where Fe\({}_{\text{ox}}\) and Al\({}_{\text{ox}}\) are oxalate extractable Fe and Al) is a pedotransfer function which is commonly used to estimate the quantity of amorphous Fe and Al oxides (i.e., the fraction of oxides with higher sorption capacity due to larger reactive surface area) in mineral acidic soils ([PERSON] et al., 2017; [PERSON] et al., 2017). Therefore, SFCWs receiving large loads of Fe and Al oxides may benefit by increasing the P sorption capacity in the sediments and ensure long-term retention. The increased P retention as a result of Fe inputs is in fact documented by some studies ([PERSON] et al., 2017; [PERSON] et al., 2017; [PERSON] et al., 2017). This can be particularly the case in systems presenting high sedimentation rates of suspended particles with reactive surfaces ([PERSON] et al., 2017; [PERSON] et al., 2017). This process can prevent the saturation of P sorption sites and reduce the release rate of P to the water column, which would consequently minimize the P export rate at the outlet. [PERSON] et al. (2017), for example, reported higher P sorption capacity in the sediment layer in comparison to the soil below (Figure 3), which suggested that the SFCWs were being continuously supplied with P sorbents. The SFCWs investigated in this study presented percentage P retentions between 41 and 51% as reported in [PERSON] et al. [73].