## 2 Edge-of-Field Technologies
### Constructed Wetlands
Thus, plants can also play a role on the biogeochemical processes which regulate the stability of P in the sediments by adding organic P to the top layers through rhizodeposition and litter fall [69,88]. The accumulation of organic P can potentially lead to P release in the long run due to mineralization processes (Figure 3). These processes, however, are normally retarded in SFCWs due to reducing conditions in the sediment layer. Further, the stability of organic P can vary depending on the plant species and biodegradability of the organic material [83]. The stability of this P fraction in the sediments is largely controlled by enzymatic activity, and thus by the availability of terminal electron acceptors. Moreover, organic P transformations can be particularly regulated by hydrological fluctuations. Drawdowns followed by re-flooding may decrease the release of P from organic plant detritus to the water column due to consolidation of the detrital material (i.e., microbial immobilization and humification of P) [86]. Relatively long periods of drawdowns, however, may favor soil air-drying followed by the death of microorganisms, which can increase the mineralization rate of unstable organic P forms upon re-flooding, and thus the release of P [86,109]. A fraction of the organic P, however, is refractory (Figure 3) and thus highly stable, contributing to long-term P retention [79]. Surface-flow constructed wetlands are widely used as a mitigation measure to reduce the P loads from agricultural drainage discharge not only for their retention potential, but also for their capacity to operate in the long-term (Table 1) with minimal need for maintenance. These characteristics lead them to be considered cost-effective alternatives. However, the accumulation of P in the sediments either by sedimentation of suspended particles, sorption to reactive sites or buildup of organic P can negatively affect the stability of P in the long-term, and thus the retention. The effects may include the saturation of P sorption sites and increase in the amount of unstable organic P forms, potentially leading to higher P release rates and lower net P retention. This scenario may be more likely in systems continuously receiving high loads of P. Old SFCWs with considerable levels of P saturation and receiving low P loads may also present marked P release rates due to large differences in P concentration between the soil pore water and water column [78]. [PERSON] et al. [80] suggested that long-term retention of P in SFCWs can be achieved if there is a continuous supply of excess P sorbents so as to ensure the availability of P sorption sites. The study also suggested that the supply of oxygen by the drainage discharge and photosynthesis is important to maintain an aerobic sediment or soil-water interface and prevent the release of Fe-bound P from the soil pore water to the water column, as the soil is normally under reducing conditions. The effects of these may not only extend the lifetime of SFCWs as P sinks but also minimize the need for maintenance. However, there is still need for more studies investigating the long-term and maturation effects of SFCWs on the retention of P. The studies reporting mass balance analyses included in this review had monitoring periods of up to seven years (Table 1). Maintenance operations, however, shall take place when the stability of the P retained has been significantly affected, e.g., when the conditions described above are not met. These can include periodic harvesting of plants to prevent buildup of organic P in the sediments, removal of the sediment layer and immobilization of P with the addition of P sorbents into the SFCW [78]. The harvest of plants, however, may be impractical due to the high costs involved and low market value of the harvested material [78]. Management operations in systems receiving high loads of PP may include a sedimentation basin constructed prior to the second wetland compartment. [PERSON] et al.