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.

中文翻译：

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排水农业流域封闭洼地内土壤磷的积累

闭合洼地是冰川景观中常见的景观特征。洼地山坡的侵蚀和径流可能会导致洼地底部附近磷 (P) 的积累，这种“遗留磷”在通过瓷砖排水系统排出时可能面临地表水流失的风险。我们评估了印第安纳州东北部瓦片排水流域内土壤 P 的空间格局，作为景观位置和农业管理实践的函数。成对的土壤样本（洼地底部与山坡贡献区）是从农业（n= 14) 四个深度（0-60 厘米）的凹陷。测定了水提取磷 (WEP)、Mehlich-3 提取磷 (M3-P)、总磷 (TP)、Hedley P 分数以及其他物理和化学特性。为了评估磷损失的风险，使用流通实验对表层土壤（0-5 cm）的磷解吸进行了量化。结果显示，与所有深度的山坡相比，洼地底部的 WEP、M3-P 和 TP 高出 2-10 倍。施磷历史和耕作等长期管理实践会影响土壤磷浓度的大小、磷饱和度和土壤磷的垂直分层。流通实验强调，磷损失的风险高度依赖于 M3-P山坡和洼地土壤的浓度。因此，研究结果表明，封闭洼地可能是瓦片排水流域磷循环和损失的热点。将低洼洼地地区作为常规土壤采样策略的一部分，结合可变施磷量，可以减少洼地的磷积累，并减少地表水的磷负荷。