Organic carbon from graminoid roots as a driver of fermentation in a fen

Fen Schlöppnerbrunnen is a moderately acidic methane-emitting peatland overgrown by Molinia caerulea and other wetland graminoids (e.g. Carex rostrata). Recently, the accumulation of H2, an indicator for fermentation, was observed with anoxically incubated C. rostrata roots but not with root-free fen soil. Based on this finding, we hypothesized that root-derived organic carbon has a higher capacity to promote fermentation processes than peat organic carbon from root-free fen soil. To address this hypothesis, C. rostrata and M. caerulea roots were anoxically incubated with or without fen soil and the product profiles of root treatments were compared with those of root-free soil treatments. Ethanol, acetate, propionate, butyrate, H2 and CO2 accumulated in root treatments and collective amounts of carbon in accumulating products were 20-200 times higher than those in root-free soil treatments, in which mainly CO2 accumulated. Analyses of 16S rRNA and 16S rRNA gene sequences revealed that Clostridium, Propionispira and Rahnella, representatives of butyrate, propionate and mixed acid fermenters, respectively, were relatively enriched in root treatments. In contrast, differences of the microbial community before and after incubation were marginal in root-free soil treatments. Collectively, these findings supported the assumed stimulatory effect of root-derived organic carbon on fen fermenters.