In addition to surface runoff, subsurface P loss through tile lines in agricultural lands has received attention over the last decade. It is not clearly understood how this strong ligand, phosphate, is distributed in subsoils (≤180 cm), contributing to subsurface P loss. We hypothesized that subsurface accumulation of labile P as a result of long‐term intensive agricultural management contributes to subsurface P loss. The depth sequence distribution of P and its speciation and reactivity were investigated in tile‐line‐dominated Midwestern agricultural fields via chemical extraction and digestion, batch desorption experiments, and X‐ray absorption near‐edge structure spectroscopy. The results suggest that labile P in subsoils exceeds the eutrophication‐inducing level (>0.01 mg L^–1). Because of intensive agricultural management, total P was ∼400 mg kg^–1 in surface soils and ∼200‐300 mg kg^–1 in subsoils. The depth sequence distribution of P indicated the translocation of P to subsoils. Although phosphate and phytic acid adsorbed by Al or Fe (oxyhydro)oxides and calcite were found in surface soils, phosphate adsorbed to calcite, hydroxyapatite, or both were the dominant P species in subsoils. Phosphate was highest in surface soils and decreased sharply with increasing depth. Slightly alkaline soil in subsoils suppressed the release of P but the process was continuous after 30 d. Subsurface P loss was influenced by the dissolution and desorption process of inorganic P phases and preferentially transported P from surface soils. Subsurface P loss should be considered in strategies to reduce agricultural P loss in the Gulf of Mexico.

中文翻译：

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集约化农业管理导致中西部农田土壤中不稳定磷的地下积累

除了地表径流，过去十年来，通过农田的瓷砖线造成的地下P损失受到关注。目前尚不清楚这种强配体磷酸盐如何分布在地下土壤（≤180cm）中，从而导致地下磷的流失。我们假设长期集约化农业管理导致不稳定磷的地下积累导致地下磷的流失。通过化学提取和消解，分批解吸实验和X射线吸收近边缘结构光谱学，在以瓷砖为主导的中西部农田中研究了磷的深度序列分布及其形态和反应性。结果表明，土壤中的不稳定磷超过了富营养化诱导水平（> 0.01 mg L ^–1）。因为集约农业管理的，全P为〜400毫克千克^-1在表面土壤和~200-300毫克千克^-1在地下。P的深度序列分布表明P向地下土壤的易位。尽管在表层土壤中发现了被Al或Fe（氢氧）氧化物和方解石吸附的磷酸盐和植酸，但吸附在方解石，羟基磷灰石或两者上的磷酸盐是底土中的主要P物质。磷酸盐在表层土壤中含量最高，并且随着深度的增加而急剧下降。底土中的弱碱性土壤抑制了P的释放，但此过程在30 d后连续进行。地下P的损失受无机P相的溶解和解吸过程的影响，并且优先从表层土壤转运P。在减少墨西哥湾农业磷损失的策略中应考虑地下磷的损失。