Tile Drainage Phosphorus Loss with Long‐Term Consistent Cropping Systems and Fertilization

Phosphorus (P) loss in tile drainage water may vary with agricultural practices, and the impacts are often hard to detect with short‐term studies. We evaluated the effects of long‐term (≥43 yr) cropping systems (continuous corn [CC], corn–oats–alfalfa–alfalfa rotation [CR], and continuous grass [CS]) and fertilization (fertilization [F] vs. no‐fertilization [NF]) on P loss in tile drainage water from a clay loam soil over a 4‐yr period. Compared with NF, long‐term fertilization increased concentrations and losses of dissolved reactive P (DRP), dissolved unreactive P (DURP), and total P (TP) in tile drainage water, with the increments following the order: CS > CR > CC. Dissolved P (dissolved reactive P [DRP] and dissolved unreactive P [DURP]) was the dominant P form in drainage outflow, accounting for 72% of TP loss under F‐CS, whereas particulate P (PP) was the major form of TP loss under F‐CC (72%), F‐CR (62%), NF‐CS (66%), NF‐CC (74%), and NF‐CR (72%). Dissolved unreactive P played nearly equal roles as DRP in P losses in tile drainage water. Stepwise regression analysis showed that the concentration of P (DRP, DURP, and PP) in tile drainage flow, rather than event flow volume, was the most important factor contributing to P loss in tile drainage water, although event flow volume was more important in PP loss than in dissolved P loss. Continuous grass significantly increased P loss by increasing P concentration and flow volume of tile drainage water, especially under the fertilization treatment. Long‐term grasslands may become a significant P source in tile‐drained systems when they receive regular P addition.