A Cross-Comparison of Sediment Incubation Methods to Elucidate Denitrification Rates with Natural Waters from Middle Mississippi River Floodplain Wetlands of Southern Illinois

Flood regimes in large river systems such, as the Mississippi River are inherently stochastic, meaning that floodplain wetlands experience varying hydrostatic pressures of groundwater upwelling (exfiltration) and infiltration from overland flooding. Distinctions in water delivery to floodplains can drastically alter the oxygen levels and the penetration of groundwaters into wetland sediments where anaerobic microbial communities can remove nitrates as nitrogen gas through denitrification. In experiments using upwelling (exfiltration) of anoxic Mississippi River water (0.76 + 0.25 mg NO3-N /L) through a volume of sediment cores, average dentification rates from four wetland sites were 19.84 + 1.49 (SE) mg N/m3/day. Using the same data, the rate calculated per sediment surface area as done in most incubation studies, denitrification averaged 485.33 + 28.47 (SE) mg N/m2/day. This estimation of denitrification with surface flooding is, however, only a calculation.

Our research goal is to inform modeling for denitrification rates associated with oxic flood waters versus sediments receiving anoxic groundwaters. We conducted a four-by-four factorial set of laboratory incubations that included oxic, anoxic waters (degassed with helium), waters introduced by exfiltration (Fig. 1A) or infiltration (Fig. 1B).

In this ongoing study, sediments were collected in triplicate from four wetlands across the Dogtooth Bend floodplain of the middle Mississippi River of southern Illinois and incubated with surface water from the Mississippi River. Incubations were conducted at 29 oC for 96 h. Nitrogen gas production was measured using a membrane inlet mass spectrometer (MIMS). Inflow and outflow waters were analyzed for nitrate, ammonia, phosphate, and dissolved organic carbon.

Measuring denitrification in several ways will better inform models of denitrification rates across floodplain wetlands depending on the magnitude and duration of flood events associated with the climate-change driven increase in extreme precipitation events and subsequent flooding across the Mississippi River Basin. Estimates from surface area incubations address conditions of surface flooding. Exfiltration experiments normalized by volumes of sediment provide insights into denitrification rates from groundwater interactions.