The contemporary role of mires in carbon exchange with the atmosphere is intensely debated. Thus, understanding the variation in Holocene peat accumulation is particularly important. We investigated carbon (C) and nitrogen (N) accumulation rates and their potential controls during the Holocene at the oligotrophic fen Degerö Stormyr (64°11′N, 19°33′E, 270 m a.s.l.), Sweden. The peat stratigraphy was dominated by remains of Eriophorum spp. and oligotrophic–mesotrophic Sphagnum spp. The long-term rate of C accumulation (LORCA) was 13.7 ± 5.5 (SD) g C m−2 yr−1, while the long-term rate of N accumulation (LORNA) was 0.28 ± 0.14 (SD) g N m−2 yr−1. Carbon and N accumulation rates exhibited similar variations that coincided with major changes in botanical composition. The botanical composition of the peat also had a major impact on the percentage of amorphous peat (a proxy for degree of decomposition). Bulk δ13C values increased with decreasing C content, which probably reflected the relative increase of 13C depleted compounds in the peat during decomposition. Different plant groups exhibited different relationships, likely due to different δ13C signatures of initial litters from Eriophorum spp. and Sphagnum spp. The δ15N values increased significantly with decreasing C:N ratio in bulk peat, likely reflecting preferential uptake of 14N by plants concomitant with nitrogen mineralization. Here, we demonstrate the importance of botanical composition in affecting C and N accumulation rates under a changing climate and suggest that primary production drives the variation in rates of accumulation. Furthermore, we point out the importance of including 13C and 15N signatures in the analysis of peat stratigraphies to advance interpretation of Holocene peat growth and decay.
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