Articles | Volume 12, issue 15
Research article
06 Aug 2015
Research article |  | 06 Aug 2015

Simultaneous high C fixation and high C emissions in Sphagnum mires

S. F. Harpenslager, G. van Dijk, S. Kosten, J. G. M. Roelofs, A. J. P. Smolders, and L. P. M. Lamers

Abstract. 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 species- and 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.

Short summary
While pristine, growing peatlands are often considered to be net sinks of carbon dioxide (CO2), fluxes vary considerably and these systems can be net sinks or sources of CO2. To explain part of this huge variation, here we present a phenomenon of peat moss (Sphagnum)-driven CO2 production. Due to the acid excreted by Sphagnum, bicarbonate in the surface water is transformed into CO2. Thus, while these systems have high CO2 fixation rates due to growing Sphagnum, they show a net emission of CO2.
Final-revised paper