## 2 Edge-of-Field Technologies
### Constructed Wetlands
The HRT (Equation (3)), also depicted below as nominal HRT (Equation (4))--commonly used by the authors--quantifies the contact time of the drainage discharge with the SFCW and is a key factor regulating P retention [76,92]. The P retention mechanisms present different requirements in terms of HRT for proper retention. The retention of PP by sedimentation, for example, commonly requires shorter HRT than that for DRP, as suspended particles transported by drainage discharge tend to  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{**Name**} & \multirow{2}{*}{** \begin{tabular}{c} **Ratio of** \\ **ArxivC** \\ **to Axc** \\ **to Axc** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Monitoring** \\ **Time** \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} **Hydraulic** \\ **Load** \\ \end{tabular}  } & \multirow{2}{*}{**P**} & \multirow{2}{*}{**Load**} & \multirow{2}{*}{**D/PP**} & \multirow{2}{*}{ \begin{tabular}{c} **Hydraulic** \\ **Reidence** \\ **Time** \\ \end{tabular}  } & \multirow{2}{*}{**P Retention**} & \multirow{2}{*}{**Study**} \\  & & & & & & & & & & & & \\ \hline  & & **m\({}^{2}\)** & **\%** & **yr** & **m yr\({}^{-1}\)** & **mg L\({}^{-1}\)** & **g m\({}^{-2}\)** & **\%** & **d** & **g m\({}^{-2}\)** &  
sediment when the water velocity decreases at the inlet of a SFCW [69; 76] (Figure 2). The retention of DRP, on the other hand, depends on diffusion processes regulated by concentration gradients until this P form is retained, e.g., by soil sorption or plant uptake [87; 88] (Figure 2). Therefore, the retention of P load consisting mainly of DRP can be more variable under different HRTs. The requirement of longer HRT for DRP can consequently limit its retention. In addition, DRP is more likely to be exported and produce a negative net retention than PP, in case the HRT is not sufficiently long [52; 75; 76]. This way, PP generally shows a major contribution to the overall mass retention of P. [PERSON] [68], for example, investigated SFCWs receiving predominantly PP from the P load (average 78%) and found that the retention of total P was considerably high (Figure 1) despite the short HRT. This occurred owing to an increase of particles sedimentation velocity under higher hydraulic loads (Table 1). The retention of DRP, on the other hand, was estimated at only 5% (Figure 2). [PERSON] and [PERSON] [75], on the other hand, reported SFCWs receiving large fractions of DRP from the P load (Table 1), which generally resulted in negative net retention of total P (Figure 1; Table 1). This study demonstrated that outflow loads of DRP and total P markedly increased with shorter HRTs. [PERSON] et al. [76] similarly reported a SFCW receiving mostly DRP from the P load (62%), and consequently a higher fraction of dissolved reactive and organic P than PP (Table 1). This study found that P retention was highly regulated by the HRT, where a minimum HRT was suggested to reach half the retention of bioavailable P forms. The retention of PP from the P load, however, was considerable (\(\geq\)80%) and contributed to the overall mass and percentage P retention. \[\text{hydraulic residence time}=\frac{\text{wetland volume with active water flow (m}^{3})}{\text{water flow (m}^{3}\text{ yr}^{-1})} \tag{3}\]  
\[\text{nominal hydraulic residence time}=\frac{\text{wetland volume (m}^{3})}{\text{water flow (m}^{3}\text{ yr}^{-1})} \tag{4}\]  
As demonstrated above, hydrochemical factors such as P load and dominant P form are normally able to estimate the mass retention of P in SFCWs receiving agricultural drainage discharge to a considerable degree. Other factors, however, are also responsible for the variation in mass P retention and can explain, for example, the difference in performance between SFCWs receiving similar P loads and dominant P form.