## 2 Edge-of-Field Technologies
### Constructed Wetlands
\[\text{P sorption capacity}=\text{constant}\;\alpha\times(\text{Fe}_{\text{ox }}\;(\text{mmol kg}^{-1})+\text{Al}_{\text{ox}}\;(\text{mmol kg}^{-1})) \tag{8}\]  
Iron oxides are normally the major P sorbent in SFCW soils and the saturation of this fraction is estimated by molar ratios of Fe to P, including oxalate (Equation (9), where Fe\({}_{\text{ox}}\) and P\({}_{\text{ox}}\) are oxalate extractable Fe and P) and bicarbonate-dithionite (Equation (10), where Fe\({}_{\text{BD}}\) and P\({}_{\text{BD}}\) are bicarbonate-dithionite extractable Fe and P) extractable fractions [80]. These can be a good measure of the stability of P in sediments and soils where Fe oxides are the major P sorbent [88]. [PERSON] et al. [104] reported that a molar ratio of Fe to P equal to 10 in the water column increased the P retention in comparison to a molar ratio of five. Moreover, [PERSON] et al. [73] demonstrated that high molar ratios at the inlet water increased the retention of P, particularly the percentage P retention. In line with this, [PERSON] et al. [80] reported molar ratios of bicarbonate-dithionite extractable Fe to P in the sediments ranging from 21 to 49, which ensured high stability and consequently high retention of Fe-bound P.  
\[\text{molar ratio of Fe}_{\text{ox}}\;\text{to P}_{\text{ox}}=\frac{\text{Fe}_{ \text{ox}}\;(\text{mmol kg}^{-1})}{\text{P}_{\text{ox}}\;(\text{mmol kg}^{-1})} \tag{9}\]  
\[\text{molar ratio of Fe}_{\text{BD}}\;\text{to P}_{\text{BD}}=\frac{\text{Fe}_{ \text{BD}}\;(\text{mmol kg}^{-1})}{\text{P}_{\text{BD}}\;(\text{mmol kg}^{-1})} \tag{10}\]  
The stability of Fe-bound P is particularly regulated by redox conditions [79] (Figure 3). Reducing conditions are common under the sediment or soil-water interface due to the consumption of terminal electron acceptors (e.g., oxygen, nitrate and sulfate by microorganisms [80]). Under these conditions, ferric Fe is reduced to ferrous Fe and concomitantly releases P into the soil pore water (Figure 3). The DRP may diffuse upwards the soil profile to layers where oxidizing conditions are found, which cause the oxidation of ferrous Fe to ferric Fe with concomitant precipitation with P [79]. The lack of oxidizing conditions at the top soil layers, however, may result in the release of DRP from the soil pore water to the water column [80,88,104] (Figure 3). This may be particularly the case during periods with higher temperatures due to increased consumption of terminal electron acceptors [80]. Therefore, the presence of an aerobic sediment or soil-water interface is essential to prevent the release of Fe-bound P to the water column [80]. Moreover, concentration gradients between the soil pore water and water column also control the release rate of P, where lower P concentrations in the water column enhance the release [87,88]. Plants can play a role on regulating the redox conditions and thus the P retention. This is because wetland plants can release oxygen to the water column through photosynthesis and support an aerobic sediment or soil-water interface. [PERSON] et al. [104], for example, demonstrated that the presence of plants in a lacustrine wetland receiving agricultural drainage discharge increased the dissolved oxygen concentration in the water column, which contributed to the retention of Fe-bound P. Moreover, wetland vascular plants can also release oxygen to the rhizosphere though the roots, supporting the precipitation of P with Fe [105, 106, 107]. The effect on the sediments or soil redox conditions, however, is normally restricted to the root surface area (few millimeters apart) [105], which limits the influence of plants on the dynamics of P with Fe. The presence of plants in SFCWs induces the transformation of inorganic P into organic P compounds through biological uptake and storage (Figure 2). This may involve the transformation of relatively stable P forms in the sediments or soil into unstable organic P forms susceptible to mineralization [108].