Nitrate applications to control phosphorus release from sediments of a shallow eutrophic estuary: an experimental evaluation

Abstract

Effects of nitrate on phosphate release from sediments of a eutrophic estuary were investigated under laboratory conditions, using reconstituted sediment–water cores. Application of nitrate (5–100 mg L^-1 NO₃-N) increased redox potential near the sediment–water interface from –200 mV to about 200 mV during 25 days of incubation. The effective concentration of nitrate differed between sediments, reflecting differences in sediment properties, particularly bioavailable carbon. Reduced phosphate after nitrate application is attributed mainly to: (1) increased iron (III) binding near the sediment–water interface; and (2) increased dissolved oxygen in the water column due to lowered oxygen demand, with increased oxidation of ferrous iron and substances binding soluble reactive phosphate. High nitrate concentrations (50 and 100 mg L^-1 NO₃-N) did not persist through a 155-day incubation, suggesting that without carbon limitation added nitrate will eventually be consumed by microbial activity, and increased phosphate release may occur. Nitrate application directly into the surface sediment increased nitrate consumption, and so was less effective than application to the water column. Frequent resuspension increased dissolved oxygen concentration, so reduced nitrate consumption and lowered concentration of soluble reactive phosphate.