Seasonal dynamics in the allocation of newly assimilated carbon in a northern peatland

Peatlands play an essential role for the global climate, providing a large storage of carbon (C) and the largest natural source of methane (CH₄). Previous studies revealed that carbon dioxide (CO₂) flux varies at the seasonal scale depending on plant phenology and species compositions. In addition to the C sink-strength, allocation of newly assimilated C is another important process to estimate C pool size and turnover, and also a key to understand the connection between plant-assimilated CO₂ and CH₄ emissions. To date, seasonal variations in the link between plant CO₂ uptake and CH₄ emissions in response to phenology have not been investigated in detail.

To reveal diurnal and seasonal dynamics of CO₂ and CH₄ flux with a high temporal resolution, we used data from an automated chamber system established in an oligotrophic minerogenic mire complex in northern Sweden (Degerö Stormyr). To identify the role of plant species composition, experimental plots without vascular plants (moss plots) and without any vegetation (bare peat plots for assessing heterotrophic respiration) were established next to the natural control plots. We conducted two in-situ ¹³C pulse labelling experiments at two distinct phenology stages (green-up and senescence) during 2023. The fate of the newly assimilated ¹³C was tracked through the entire C flow from plants (vascular plants and mosses), to dissolved organic and inorganic ¹³C in pore water, to ¹³CO₂ and ¹³CH₄ flux. The main objectives were to investigate 1) seasonal variations in the C allocation pattern and turnover time, and 2) the separate roles of vascular plants and mosses in regulating C allocation dynamics.

Our results indicate that in the green-up stage, both natural and moss plots released around 20% of the total newly assimilated ¹³C as CO₂ flux during the 30 days after the labelling. In the senescence stage, the amount in the moss plots increased to 26 ± 3%, while that of natural plots remained at 20%. In comparison, release of newly assimilated C as CH₄ did not show any seasonal variations in neither natural nor moss plots, highlighting a close link between plant C uptake and CH₄ emission. However, existence of vascular plants increased the proportional release as CH₄ emission tenfold from 0-0.02% of total ¹³C uptake in moss plots to 0.1-0.2% in natural plots.

These results highlight the importance of plant species composition and phenology in regulating the allocation of assimilated carbon in northern peatlands.