PHOSPHORUS TRANSFORMATIONS IN THE SEDIMENTS OF DELAWARE AGRICULTURAL DRAINAGEWAYS - I - PHOSPHORUS FORMS AND SORPTION

Many soils in Delaware's Inland Bags' watershed are high in phosphorus (P) from long-term manuring and fertilization and require artificial drainage (open ditches) for successful crop production, We characteriz ed the properties, forms of P, and P sorption capacity of sediments fr om 17 drainage ditches as part of an ongoing effort to develop best ma nagement 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 H2SO4) P and biologically available P (BAP) relative to lower sediment depths (5-15 cm) and to agricultur al subsoils at the same depth as the ditch bottoms,Sediment BAP was co mparable to values for topsoils in adjacent fields and >90% of sedimen t P was NH4F-P or NaOH-P. The P sorption capacity for top sediments ra nged from 95 to 1671 mg P kg(-1) and equilibrium P concentrations at z ero sorption (EPC0) from 0.02 to 0.28 mg P L-1, consistently above P c oncentrations 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 s urface waters.