Low sulfide concentrations affect nitrate transformations in freshwater and saline coastal retention pond sediments

Coastal areas in the southeastern USA are prone to hurricanes and strong storms that may cause saltwater influx to freshwater aquatic sediments. These changes in environmental conditions may impact sediment processes including nitrogen (N) cycling. The relative abilities of sediment microbial communities from two freshwater golf course retention ponds that drain into the adjacent wetlands, and two proximal saline wetland ponds, to remove nitrate (NO(3)(-)) were compared to assess whether low concentrations of sulfide changed N-transformation processes. Microcosms were incubated with NO(3)-N (300 mu g g dw(-1)) alone, and with NO(3)-N and sulfide (H(2)S) (100 and 200 mu g g dw-1). Nitrous oxide (N(2)O), nitrite (NO(2)(-), NO(3)(-), ammonium (NH(4)(+)), SO(4)(2-) and acid volatile sulfides were analyzed over time. The acetylene block technique was used to measure denitrification in sediment microcosms with no added H(2)S. Denitrification was measured without acetylene (C(2)H(2)) addition in microcosms with added H(2)S. With no added H(2)S, denitrification was greater in the freshwater retention ponds than in the wetland ponds. Although low H(2)S concentrations generally increased NO(3)-N removal rates at all sites, lag periods were increased and denitrification was inhibited by low sulfide in the freshwater sediments, as evidenced by the greater concentrations of N(2)O that accumulated compared to those in the wetland sediments. In addition to the inability of the freshwater sediments to convert N(2)O to N(2) in the absence of C(2)H(2), anomalously high transient NO(2)-N concentrations accumulated in the retention pond samples. NH(4)-N formation generally decreased due to H(2)S addition at the freshwater sites; NH(4)-N formation increased initially at the wetland sites, but was greater when no H(2)S was added. Storm events that: allow influx of SO(4)(2-)-containing seawater into freshwater systems may change the dominant N species produced from nitrate reduction. Even low concentrations of sulfide produced incomplete denitrification and decreased formation of NH(4)(+) in these coastal freshwater sediments. (C) 2009 Elsevier Ltd. All rights reserved.