Holocene carbon and nitrogen accumulation rates and contemporary carbon export in discharge : a study from a boreal fen catchment

Abstract: The contemporary role of mires in land-atmosphere exchange of carbon dioxide (CO₂) is the subject of intense scientific debate. Investigating patterns and mechanisms of variations in past carbon (C) accumulation rates as well as contemporary C fluxes of peatlands facilitates predictions of future mire C sink strength under a changing climate. Nitrogen (N) is an important macro-nutrient and thereby strongly coupled to the C accumulation rate. Thus, I have investigated Holocene C and N accumulation rates and their potential controls, and the annual and seasonal stream C export during the Holocene at the oligotrophic fen Degerö Stormyr in northern Sweden. The mean (±SD) long-term apparent rates of C and N accumulation (LORCA and LORNA) were estimated to 13.7±5.5 g C m⁻² yr⁻¹ and 0.28±0.14 g N m⁻² yr⁻¹, respectively. These rates were relatively low compared to those of other northern peatlands. Variations in C and N accumulation rates exhibited similar temporal patterns largely coinciding with major changes in botanical remains, and mainly primary production controlled the accumulation rates. The botanical composition had a major impact on the degree of decomposition, with indications of enrichment of recalcitrant compounds in highly degraded peat, as well as differences in ¹³C-¹²C composition of litter at the onset of decomposition. There were also indications of preferential plant uptake of ¹⁴N at higher N availability. Stream total C export was primarily driven by discharge, with average annual C export estimated to 12.2 g m⁻² yr⁻¹ over twelve years. This flux typically corresponded to 25-37% of the mean annual net uptake of CO₂ (NEE). However, during a year with a dry summer and wet autumn the discharge C export corresponded to as much as 90%, highlighting the importance of seasonality. The highest TOC export was exhibited during summer when accounting for differences in discharge, underlining the importance of timing of precipitation for the seasonal C export. Seasonal variations in C quality (SUVA₂₅₄) indicate different within mire sources of TOC during the year. In this thesis I conclude the importance of primary production for C and N accumulation rates, and highlight the importance of seasonality for stream C export from mires.