The use of flow-through sediment reactors in biogeochemical kinetics: Methodology and examples of applications

Flow-through reactors (FTR) provide a means to measure reaction rates on undisturbed sediment slices. Because in this approach the porous structure and the spatial arrangements of particle-bound constituents, including microorganisms, are preserved, kinetic parameters are obtained whose values are representative of the initial in situ conditions. The theory and applications of FTRs are reviewed here using data on sulfate and nitrate reduction in a number of nearshore sediments. In particular, we focus on the determination of maximum potential reduction rates (Rmax) and half-saturation constants (Km) of the terminal electron acceptors. Alternative methods for extracting these kinetic parameters from time-series outflow concentration measurements are compared. To deal with the uncertainties associated with temporal and spatial variations in solute concentrations within the FTR, a novel method is presented, based on a continuous reactive transport model representation of the FTR system. Potential biases related to the loss of dissolved organic substrates via the outflow are addressed by analyzing the results of variable flow nitrate reduction experiments. For sediment intervals on which both nitrate and sulfate reduction rates have been measured, Rmax values (in units of C equivalents) are systematically higher for nitrate reduction. In addition, the relative difference in Rmax between the two terminal electron accepting processes increases with decreasing mineralization rates.