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 f or experimental studies of biogeochemical processes. Sediment incubation in these bags allows time-course experiments to be conducted on homogenised s ediment without dilution, continuous stirring, or gaseous head-space. Conse quently, bag incubations of sediment combine the advantage of low heterogen eity in slurry incubations with the more natural conditions in jar and whol e-core incubations. The bag material is a transparent laminated plastic com prised of Nylon, ethylenevinyl alcohol, and polyethylene with a low permeab ility for the studied gases: O-2, CO2, H2S, CH4, N-2, H-2, and He. Estimate d fluxes of biologically active gases through the plastic bag during sedime nt incubation were insignificant compared to rates of microbial processes a nd to gas concentrations in coastal sediments. An exception was CH4, for wh ich process calculations should include a correction for the exchange of CH 4 during incubation. Sulphate reduction rates measured in intact sediment c ores and in sediment sectioned and incubated in the bags showed similar pro files in 3 coastal sediments with oxygen penetrations from a few millimetre s 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 r ates by 1.4- and 3.2-fold in the intermediate and the most oxidised sedimen t, respectively. The differences may be related to the interruption of the biomediated transport of oxidants and the decay of fauna in the bag incubat ions.