Phosphorus Transformations in the Sediments of Delaware's Agricultural Drainageways: I. Phosphorus Forms and Sorption

Abstract

Abstract Many soils in Delaware's Inland Bays' watershed are high in phosphorus (P) from long‐ term manuring and fertilization and require artificial drainage (open ditches) for successful crop production. We characterized the properties, forms of P, and P sorption capacity of sediments from 17 drainage ditches as part of an ongoing effort to develop best management practices for agricultural drainage. Upper sediment layers (0–5 cm depth) were enriched in organic matter, Fe/Al oxides, soil test (Mehlich 1‐P; 0.05 N HCl + 0.025 N H 2 SO 4 ) P and biologically available P (BAP) relative to lower sediment depths (5–15 cm) and to agricultural subsoils at the same depth as the ditch bottoms. Sediment BAP was comparable to values for topsoils in adjacent fields and >90% of sediment P was NH 4 F‐P or NaOH‐P. The P sorption capacity for top sediments ranged from 95 to 1671 mg P kg −1 and equilibrium P concentrations at zero sorption (EPC 0 ) from 0.02 to 0.28 mg P L −1 , consistently above P concentrations associated with fresh water eutrophication (0.02 mg P L −1 ). Soluble P concentrations in drainage ditch waters averaged 0.05, 0.06, and 0.15 mg P L −1 in the winter, spring, and summer. Sediment BAP (0–5 cm) could be predicted from Mehlich 1‐P and %OM ( R 2 = 0.80***) and P sorption capacity from a single point P sorption index (PSI) and %OM ( R 2 = 0.81”**) suggesting routine soil tests could help identify ditches with significant potential for nonpoint source pollution of surface waters.