Elevated CO2 effects on peatland plant community carbon dynamics and DOC production

Northern peatlands are important stores of carbon and reservoirs of biodiversity that are vulnerable to global change. However, the carbon dynamics of individual peatland plant species is poorly understood, despite the potential for rising atmospheric CO2 to affect the vegetation's contribution to overall ecosystem carbon function. Here, we examined the effects of 3 years exposure to elevated CO2 (eCO(2)) on (a) peatland plant community composition and biomass, and (b) plant carbon dynamics and the production of dissolved organic carbon (DOC) using a (CO2)-C-13 pulse-chase approach. Results showed that under eCO(2), Sphagnum spp. cover declined by 39% (P < 0.05) and Juncus effusus L. cover increased by 40% (P < 0.001). There was a concurrent increase in above- and belowground plant biomass of 115% (P < 0.01) and 96% (P < 0.01), respectively. Vascular species assimilated and turned over more (CO2)-C-13-derived carbon than Sphagnum spp. (49% greater turnover of assimilated C-13 in J. effusus and F. ovina L. leaf tissues compared with Sphagnum, P < 0.01). Elevated CO2 also produced a 66% rise in DOC concentrations (P < 0.001) and an order of magnitude more 'new' exudate (DOC)-D-13 than control samples (24 h after (CO2)-C-13 pulse-labelling 2.5 +/- 0.5 and 0.2 +/- 0.1% in eCO(2) and control leachate, respectively, P < 0.05). We attribute the observed increase in DOC concentrations under eCO(2) to the switch from predominantly Sphagnum spp. to vascular species (namely J. effusus), leading to enhanced exudation and decomposition (litter and peat). The potential for reduced peatland carbon accretion, increased DOC exports and positive feedback to climate change are discussed.