The Effect of Closed Depressions on Dissolved Reactive Phosphorus Losses in Tile-Drained Fields.

Phosphorus (P) losses from non-point sources contribute to eutrophication of water bodies and the proliferation of algal blooms, especially in freshwater ecosystems. Although surface runoff is the predominant transport pathway of P to streams, several studies suggest that artificial subsurface drainage (tiles) can enhance P losses from agricultural fields. However, the transport mechanisms of P through tile drainage systems in flat agricultural fields are not well understood. We analyzed dissolved reactive phosphorus (DRP) concentrations from 36 parallel tile lines (30 m apart) across an 80 ha field to determine how topographic features affect DRP loss in tile-drained fields under a corn (Zea mays L.) and soybean (Glycine max L.) rotation. We estimated the depth of closed depressions and computed a depression index (DI) to quantify the probability of ponding at the field scale. We also conducted Bray P1 soil tests in 2015 and 2016 across the depression gradient. Log transformed annual tile DRP loads and DI were positively correlated in both 2015 (R2=0.22) and 2016 (R2=0.32). One area of the field with extremely high soil P was later determined to be the location of an old farm (>50 years ago). Piecewise regression revealed a critical threshold in the depth of depressions at 0.38 m at which soil P content abruptly shifted. Above this value, there was a strong positive association with soil P (R2=0.91). This area of high soil P caused elevated DRP values in three tile lines and these tiles were not used in the regression. These results suggest that DRP leaching is enhanced in areas where repeated ponding occurs due to the accumulation of P and the increased connectivity between the soil surface and tile drains via preferential flow. We also highlight the importance of addressing legacy P sources that can contribute to increased DRP losses.