## 1 Introduction
In line with the above, some studies demonstrated a relationship between the losses of suspended particles and P in agricultural catchments [26; 27]. Finally, the varying loads and dominant forms of P from agricultural catchments are also attributed to differences in the P sorption capacity of soils, which often correlates positively and negatively to the retention of DRP and PP, respectively [15]--finer-textured soils have larger surface area for sorption reactions with P [28], but are likely to lose more particles and associated P to tile drains [29; 30]. Measures to mitigate P loss in drainage discharge may focus on strategies at the farm-scale or at the edge of agricultural catchments. Measures at the farm-scale mostly focus on controlling the application and management of mineral fertilizers and manure through farm policies [1] and agricultural practices [4]. The latter reflects an attempt to optimize the use of P by balancing the P inputs with crop uptake so as to approximate to the critical value, i.e., the P content in the soil where no additional P application is needed to promote crop growth [31]. This approach tends to minimize P accumulation in the soil and subsequent loss [32]. Other practices that mitigate P loss at the farm-scale include tillage, which decreases preferential flow in the soil profile [33; 34; 35]; catch crops, which support P uptake in the root zone [36]; liming, i.e., the addition of Ca compounds to the soil to enhance P retention, and incorporation of manure into the soil by mixing as opposed to broadcast application [32; 35]; and controlled drainage, which can be used to reduce the drainage discharge and subsequent transport of P [37]. However, attempts to minimize the P loss at the farm-scale may be limited by the lack of short-term results in soils with large P contents [38]. A limited effect of the practices described above may occur in an annual basis in response to soil desiccation and freezing-thaw cycles, which recreate preferential flow in the soil profile [39; 40]; no net uptake of P by catch crops due to the release of previously assimilated P during freezing-thaw cycles [41; 42]; intense precipitation events, which result in diffuse P losses [43]; and intensive livestock farming, with excess manure being produced [44; 45]. Finally, controlled drainage may otherwise increase the loss of DRP by elevating the water table and promoting the emergence of reducing conditions with concomitant release of P [46]. Strategies at the level of tile drains to reduce P loss may include attempts to increase the P sorption capacity of the subsoils around the drains by backfilling, e.g., with gypsum filters [47], Fe-containing material [48] or industrial by-products [49]. The maintenance requirements and low P retention efficiencies under intense precipitation events, however, may make these strategies impractical at the farm-scale. The P index is a process-based model which helps identifying the potential of agricultural catchments for P loss [4]. In catchments with high P index or high risk of P loss, i.e., critical source areas [50], there may be a need for additional measures to mitigate the P load such as the implementation of edge-of-field technologies, which are located at the edge of agricultural catchments. These may include constructed wetlands (CWs), restored wetlands (RWs), vegetated buffer strips (VBSs) and filter materials (FMs), which are recognized as non-point and economically feasible solutions. However, these are generally more costly than measures at the farm-scale, highlighting thus the importance to identify critical source areas as a first step [51]. Despite the higher costs, these technologies may offer other environmental benefits such as nitrogen removal, increases in biodiversity and flood control [52; 53; 54; 55]. Edge-of-field technologies function as drainage filters and are installed at the outlet of the main drainage pipe of a particular tile-drained agricultural catchment.