Evaluation of the N2 flux approach for measuring sediment denitrification

Direct gas chromatographic measurement of denitrification rates via N2 fluxes from aquatic sediments can avoid some of the artifacts and complexities associated with indirect approaches and tracer techniques. However, measurement protocols have typically been determined based upon initial results or previous studies. We present a process-level study and simulation model for evaluating and optimizing N2 gas flux approaches in closed chamber incubations. Experimental manipulations and simulations of both artificial and natural sediments were used to conduct sensitivity analyses of key design parameters in N2 flux measurements. Experimental results indicated that depletion of labile organic matter during the long incubations required by common protocols (for diffusive off-gassing of porewater N2) may result in underestimates of denitrification rates in some systems. Simulations showed that the required incubation time was primarily a function of sediment thickness. The best approach found to minimize incubation time and reduce errors was to select the minimum sediment thickness necessary to include the entire depth distribution of nitrification-denitrification for a particular sediment system. Attempts to increase measurement sensitivity and shorten incubation times by reducing the headspace thickness to 1-2 cm generally cause denitrification to be underestimated by 3-13% for gas headspaces, and up to 80% for water headspaces. However, errors were negligible with gas and water headspace thicknesses of 10 cm and 15 cm, respectively. Anaerobic cores to control for non-denitrification N2 fluxes shortened incubation time, but introduced artifacts in sediments with extensive macrofaunal irrigation.