Phosphorus transformations in the sediments of Delaware's agricultural drainageways: II. Effect of reducing conditions on phosphorus release

Most agricultural fields in Delaware's Inland Bays' watershed are high or excessive in P from long-term applications of poultry litter (PL) and fertilizers and many are arable only because they have been drained by large open ditches. The possible role of sediments in these ditches in P loading to the Inland Bays was assessed in laboratory studies, Sediments collected from 17 ditches ranged in biologically available P (BAP) from 5 to 1218 mg P kg(-1) and in soil test P from 3 to 62 mg P kg(-1). Sequential fractionation indicated that Al-P (0.05 N NH4F) and Fe-P (1.0 N NaOH) were the predominant forms of sediment inorganic P. The degree of phosphorus saturation (DPS), defined as BAP divided by PSI (P sorption index, a rapid measure of sediment P sorption capacity), averaged 46 and 37% in the top and bottom sediments, respectively. The potential for P release from sediments to drainage waters was determined by incubating all sediments under flooded, anoxic conditions for 21 d, The effects of time and temperature (7 and 35 degrees C) on P release were determined in a separate study with six sediments incubated for 42 d, Initial P in solution was well correlated with BAP (r = 0.74***) and organic matter (r = 0.66***), while P release after 21 d was well correlated with DPS (r = 0.75***) and total Fe-oxide content (r = 0.66***), A critical sediment DPS value of 40% is proposed to target drainage ditches with higher potential to release P to overlying waters.