Radiocarbon and Stable Isotopic Signatures Reveal Accelerated Carbon Cycling in a Boreal Peatland Subjected to Warming and Elevated CO2

Natural wetlands account for approximately one-third of global methane (CH₄) emissions, while northern peatlands store more than 20% of terrestrial carbon. Environmental change has the potential to enhance microbial decomposition of peat, mobilizing long-stored carbon as CO₂ or CH₄. However, predicting future peatland trace gas fluxes remains challenging due to limited mechanistic understanding and a lack of long-term, ecosystem-scale experimental data for model evaluation. The Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment addresses this gap by providing a rare, whole-ecosystem manipulation of warming and elevated CO₂ in an ombrotrophic forested bog in northern Minnesota.
Here, we measured radiocarbon (¹⁴C) and stable carbon (¹³C) isotopic signatures of surface-emitted CH₄ and CO₂ at the onset of experimental treatments and after five and seven years of combined warming and elevated CO₂. Across treatments, CH₄ emissions were on average approximately a decade older than co-emitted CO₂, indicating differences in carbon source age and processing between the two gases. Despite this age offset, surface carbon fluxes were dominated by recently fixed photosynthates rather than older peat-derived carbon. This finding is consistent with previous work at SPRUCE demonstrating rapid incorporation of newly fixed carbon into dissolved organic carbon pools throughout the peat profile.
In plots exposed to elevated CO₂, isotopic signatures of both ¹⁴C and ¹³C in chamber air were depleted relative to ambient conditions. Correspondingly, surface-emitted CH₄ and CO₂ from elevated CO₂ plots exhibited depleted isotopic values compared to non-elevated plots, reflecting rapid transfer of newly assimilated carbon from vegetation to atmospheric fluxes. Peat sampled four years after the initiation of elevated CO₂ treatments also showed depletion in carbon isotopic values within shallow peat layers relative to ambient CO₂ plots, further supporting enhanced incorporation of recent photosynthates into near-surface peat carbon pools.
Unexpectedly, we found little evidence for increased decomposition or mobilization of older peat carbon, even under conditions that would typically favor peat degradation. Warming treatments, combined with episodically dry conditions, resulted in significant lowering of the water table and measurable loss of surface elevation over the course of the experiment. Despite these physical changes, isotopic evidence did not support substantial contributions of deep or old peat carbon to surface CO₂ or CH₄ emissions.
Together, our results indicate that elevated surface CH₄ and CO₂ fluxes observed under warming at SPRUCE are primarily fueled by rapidly cycling carbon recently fixed by bog vegetation, rather than by accelerated decomposition of long-stored peat carbon. These findings underscore the importance of hydrologic and biogeochemical interactions in regulating peatland carbon dynamics and have critical implications for interpreting experimental manipulations, improving process-based wetland models, and extrapolating peatland responses to climate change across boreal ecosystems.