## 1 Introduction  
Agriculture is generally the main source of phosphorus (P) to inland and coastal waters [1, 2]. Application of mineral fertilizers and manure in agricultural fields and the presence of excess manure in intensive livestock farming commonly result in the surplus of P in the soil followed by loss [1, 3]. This represents a problem in the long-term as P losses tend to increase as a result of P accumulation in the soil [4]. The P surplus can be transported during precipitation events through surface runoff, subsurface flow and/or erosion, eventually reaching downstream surface waters [4], where P loads exceeding a certain threshold may cause eutrophication [5, 6]. This is particularly the case in freshwater ecosystems, where P is generally the limiting nutrient [7, 8]. The European Water Framework Directive 2000/60/EC plays an important role in this context, as it regulates the use of P in agriculture, helping to preserve the good ecological quality of surface waters, i.e., only a slight change in the biological community is allowed. Tile-draining is a widespread method used to improve agricultural production in certain areas. It can increase the crop yield and reduce annual variability, thus ensuring consistent production [9]. In addition, it decreases the P losses through surface runoff and erosion [10]. Tile drainage is particularly advantageous in catchments with fine-textured soils such as clayey soils, where subsurface flow is severely reduced and soil waterlogging is recurrent, by increasing the hydrological connectivity between agricultural fields and surface waters. On the other hand, it decreases the contact time of P to soil particles when water infiltrates and flows down the soil profile to the tile drains, thus minimizing P sorption [11]. Therefore, tile drainage accelerates the transport of P and largely contributes to the overall P load [12; 13; 14], where P concentrations in the drains are often highly variable and exceeding the threshold for eutrophication in surface waters [15]. Tile-drained agricultural catchments vary in geology, climate, topography, soil type, soil test P concentration and agricultural practices, which all influence the amount of P surplus in the soil and subsequent loss, i.e., the P load and dominant P form. Variations in the P load and form can be rather large not only between agricultural catchments [1], but also within the same [16]. The latter occurs as a result of seasonal and inter-annual variations in precipitation, which cause base and peak-flow events, in addition to changes that may occur in the agricultural practices (e.g., inputs of P). Thus, large P losses are generally observed during periods of intense precipitation [15]. These observations highlight the large spatiotemporal variation in P loss from agricultural fields [1]. Phosphorus is commonly found in agricultural soils bound to particles and oxides of iron (Fe), aluminum (Al) and manganese as well as to calcium (Ca) and magnesium (Mg) [10]. However, pH and redox conditions regulate the sorption reactions of P to these elements, and consequently the proportion of soluble and insoluble inorganic P compounds [17; 18]. Therefore, P loss in drainage discharge mostly occurs as inorganic compounds comprised by dissolved reactive P (DRP) and particulate or particle-bound P (PP). The load of these forms in tile drains can be similarly high [19] and is largely controlled by preferential macro-pore flow, which connects the soil surface to the drains. The loss of DRP is favored by the application of mineral fertilizers or manure followed by precipitation or snowmelt events, which cause P desorption and leaching [20; 21; 22] as a result of soil waterlogging and emergence of reducing conditions [18; 23]. The loss of PP, in turn, depends on the loss of particles and associated P, which occurs more frequently in fine-textured soils [24] during peak-flow events [25]. Thus, a considerable fraction of PP in tile drains may be colloidal or bound to fine size particles with low sedimentation rates [24].