## 2 Edge-of-Field Technologies
### Constructed Wetlands
Thus, the accretion of deposited material at the bottom forming a sediment layer constitutes the main retention mechanism, which is generally the case for most of the P retained [69; 70; 80]. Thereby, the pool of P in the sediments is generally the largest and represents a long-term retention. The uptake rate of bioavailable forms of P by plants increases during the growing season [81]. It is, however, normally low compared to other retention mechanisms and the contribution to the overall retention can be limited [81], unless the P load to the SFCW is low [82]. Emergent plant species (e.g., _Typha latifolia_, _Phragmites australis_ and _Juncus effusus_), however, may be recommended in the retention of dissolved reactive and organic P in SFCWs under higher P loads [82; 83; 84]. Nevertheless, the retention of P by plants and microorganisms represents a small fraction due to seasonal turnover, which includes the accumulation of organic particles at the bottom during the senescence period followed by mineralization and release of dissolved reactive and organic P [85; 86] (Figure 3). Therefore, the P pool in the plants and other organisms is considered short-term. A small fraction, however, is retained in the long-term as refractory organic P, which contributes to new soil material [79] (Figure 3). Figure 3: Schematic of the release processes of phosphorus (P) from the sediments to the water column in a surface-flow constructed wetland receiving agricultural drainage discharge. The numbers indicate the fraction of total P (%) and are followed by the reference study (within square brackets). * Average values for individual wetlands. Figure 2: Schematic of the retention mechanisms of phosphorus (P) in a surface-flow constructed wetland receiving agricultural drainage discharge. The numbers indicate the percentage and mass (g m\({}^{-2}\) yr\({}^{-1}\) within brackets) retention of particulate and dissolved reactive P loads (red and blue box, respectively) in different systems and are followed by the reference study (within square brackets). * Values in g m\({}^{-2}\) season\({}^{-1}\). The stability of DRP retained by chemical interactions with P sorption sites at the soil is variable and depends on the geochemical characteristics, pH and redox conditions as well as on the concentration gradient across the soil-water interface [80,87,88]. This way, DRP can be released under certain conditions, including low sorption capacity of the soil, non-neutral pH, reducing conditions (Figure 3) and low P concentration in the overlying water. The observations above suggest that PP is more easily retained and stored in SFCWs than DRP. Therefore, the overall mass retention of P may increase under higher fractions of PP from the total P in the incoming P load due to higher efficiency of the related retention mechanisms (as suggested by the data in Figure 2). [PERSON] et al. [73], for example, demonstrated that mass P retention increased more promptly under higher P loads in SFCWs receiving PP as the dominant fraction. Moreover, [PERSON] et al. [69] found that 78% of the P load, comprised mostly by PP (69%, Table 1), had been retained in the inlet zone by sedimentation forming a layer of accumulated material. Finally, [PERSON] et al. [89] reported from annual data of 16 SFCWs receiving either agricultural runoff or drainage discharge that the ratio of DRP to total P correlated negatively not only with mass P retention (r = \(-\)0.63) but also with percentage P retention (r = \(-\)0.39). However, factors such as the particle size and density as well as the vegetation cover, which mitigates sediments resuspension, influence the sedimentation velocity and retention of particles and associated P, as seen for SFCWs receiving agricultural runoff [90,91]. [PERSON] [24], for example, indicated that retention of P associated to colloids and fine size particles in SFCWs can be difficult due to low sedimentation rates (<1 cm d\({}^{-1}\)). The above highlights the importance of dominant P forms in the P load to estimate the retention potential.