Denitrification measurements of sediments using cores and chambers

Denitrification is commonly measured using in situ benthic chambers or laboratory incubations of sediment cores. These techniques are similar in principle but differ considerably in cost and practicality. Despite widespread use of both techniques, it is uncertain whether they give comparable results. We compared cores and chambers for measuring fluxes (dissolved oxygen [DO], N2, NH4, NO3 and NO2) and denitrification efficiency at 2 sites in Port Phillip Bay, Victoria, Australia. Overall, denitrification efficiency was not significantly different between cores and chambers, but fluxes of DO, NO3 and NO2 differed. Chambers demonstrated higher levels of oxygen consumption and net fluxes of NO3 and NO2 out of the sediment, suggesting that denitrification and nitrification were closely coupled. In contrast, there was a greater relative importance for uncoupled denitrification in cores as indicated by reduced oxygen consumption and net fluxes of NO3 into the sediment. We conclude that cores and chambers give different flux results and therefore are not comparable techniques for measuring denitrification. To ascertain the cause of this, we tested the hypothesis that cores failed to adequately incorporate the impacts of macrofauna on fluxes, due to the small size of cores relative to chambers. However, densities of macrofauna were not significantly different in cores and chambers. We then hypothesised that disturbance during core collection, transportation, and handling may account for differences, but cores deployed in situ and in the laboratory gave similar results. We suggest that compression of sediment during insertion of core cylinders into the sediment may account for differences between core and chamber fluxes.