Accumulation of soil phosphorus within closed depressions of a drained agricultural watershed

Abstract

Closed depressions are common landscape features across glaciated landscapes. Erosion and runoff from depression hillslopes may result in phosphorus (P) accumulation near the bottom of the depression, with this “legacy P” potentially at risk of loss to surface waters when drained via tile drainage. We assessed spatial patterns of soil P within a tile-drained watershed in northeastern Indiana as a function of landscape position and agricultural management practices. Paired soil samples (depression bottom vs. hillslope contributing area) were collected from agricultural (n = 14) depressions at four depths (0–60 cm). Water-extractable phosphorus (WEP), Mehlich-3 extracted phosphorus (M3-P), total phosphorus (TP), Hedley P fractions, and other physical and chemical characteristics were determined. To assess the risk of P loss, P desorption from surface soils (0–5 cm) was quantified using flow-through experiments. Results showed that WEP, M3-P, and TP were 2–10 times greater in the depression bottom compared to hillslopes across all depths. Long-term management practices such as P application history and tillage influenced the magnitude of soil P concentration, degree of P saturation, and vertical stratification of soil P. Flow-through experiments highlighted that the risk of P loss was highly dependent on M3-P concentration for both hillslope and depression soils. Findings therefore indicate that closed depressions may act as hotspots for P cycling and loss in tile-drained watersheds. Including low-lying depressional areas as part of a routine soil sampling strategy combined with variable rate P application could lessen P accumulation in depressions and reduce P loading to surface waters.