Anoxic incubation of sediment in gas-tight plastic bags: a method for biogeochemical process studies

Incubation of sediment in gas-tight plastic bags is described as a method for experimental studies of biogeochemical processes. Sediment incubation in these bags allows time-course experiments to be conducted on homogenised sediment without dilution, continuous stirring, or gaseous head-space. Consequently, bag incubations of sediment combine the advantage of low heterogeneity in slurry incubations with the more natural conditions in jar and whole-core incubations. The bag material is a transparent laminated plastic comprised of Nylon, ethylenevinyl alcohol, and polyethylene with a low permeability for the studied gases: O-2, CO2, H2S, CH4, N-2, H-2, and He. Estimated fluxes of biologically active gases through the plastic bag during sediment incubation were insignificant compared to rates of microbial processes and to gas concentrations in coastal sediments. An exception was CH4, for which process calculations should include a correction for the exchange of CH4 during incubation. Sulphate reduction rates measured in intact sediment cores and in sediment sectioned and incubated in the bags showed similar profiles in 3 coastal sediments with oxygen penetrations from a few millimetres to similar to1 cm. In the most reduced sediment, whole-core and bag-based depth-integrated rates were the same while bag-rates exceeded whole-core rates by 1.4- and 3.2-fold in the intermediate and the most oxidised sediment, respectively. The differences may be related to the interruption of the biomediated transport of oxidants and the decay of fauna in the bag incubations.