Intensive agricultural management-induced subsurface accumulation of labile phosphorus in Midwestern agricultural soils dominated by tile lines

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 similar to 400 mg kg(-1) in surface soils and similar to 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.