Relative importance of soil fertility and microtopography as CO and CH exchange drivers in a northern boreal fen ecosystem.

While peatland C cycling is generally well covered, understanding of the role of soil fertility in driving the spatial variation of C fluxes within peatlands remains scattered. Our aim was to examine the relative effects of fertility and microtopography on CO and CH exchange within a boreal fen and to link these effects to the spatial variation in plant and soil attributes. Fertility zones (eutrophic, mesotrophic, oligotrophic) were judged by moss species appearances, and the growing season CO and CH exchange was measured by static chambers for microforms (string, Sphagnum lawn, flark) and fertility zones and by eddy covariance technique for the entire ecosystem in three years. Plant leaf area index, plant functional type biomasses, soil C and N concentrations and litter decomposition were measured at study plots placed on the microforms and fertility zones. We found that higher fertility led to greater fluxes in both gases: the eutrophic zone had 111% higher net ecosystem CO exchange, 102% higher gross primary production, 83% higher ecosystem respiration and 93% higher CH emissions than the oligotrophic zone. Peat N concentration was lowest in the eutrophic zone, indicating fast N cycling. The relative importance of microtopography and fertility differed between the two gases: while microform explained 31-39% and fertility 10-15% of total variation in CO exchange, microform explained 14% and fertility 36% of variation in CH exchange. These results show that growing season CO and CH fluxes can be significantly affected by within-fen variation of fertility and that CH emissions can be more closely associated with fertility than microtopography. It seems that understanding of within-site variation in soil nutrient availability is highly relevant for predicting current and future C exchange in peatlands.