Simultaneous high C fixation and high C emissions in Sphagnum mires

Peatlands play an important role in the global carbon (C) cycle due to their large C storage potential. Their C sequestration rates, however, highly vary depending on climatic and geohydrological conditions. Transitional mires are often characterised by floating peat with infiltration of buffered groundwater or surface water. Sphagnum mosses grow on top, producing recalcitrant organic matter and fuelling large C stocks. As Sphagnum species strongly differ in their tolerance to the higher pH in these mires, their species composition can be expected to influence C dynamics in transitional mires. We therefore experimentally determined growth and net C sequestration rates for four different Sphagnum species (Sphagnum squarrosum, S. palustre, S. fallax and S. magellanicum) in aquaria, with floating peat influenced by the infiltration of buffered water. Surprisingly, even though the first three species increased their biomass, the moss-covered peat still showed a net efflux of CO2 that was up to 3 times higher than that of bare peat. This species-dependent C release could be explained by Sphagnum’s active lowering of the pH, which triggers the chemical release of CO2 from bicarbonate. Our results clearly illustrate that high Sphagnum biomass production may still coincide with high C emission. These counterintuitive C dynamics in mire succession seem to be the result of both speciesand biomass-dependent acidification and buffered water infiltration. Together, these processes can explain part of the large variation in C fluxes (ranging from C sequestration to C release) reported for pristine mires in the literature.