Redox-Active Organic Matter in a Boreal Peatland Demonstrates Resistance to Global Climate Change

Redox-active organic matter (RAOM) reduction is an important control on carbon cycling in boreal peatlands, suppressing methane production via its energetic favorability. However, the effects of global climate change drivers-notably, warming and elevated atmospheric carbon dioxide (CO₂)-on the relationship between RAOM and production of greenhouse gases remains unknown, constituting an important knowledge gap. Here, we leveraged an experimental boreal peatland in northern Minnesota (USA) that has been subjected to a gradient of warming (+0 to +9°C) and elevated CO₂ (+500 ppm) for almost 10 years. To understand in situ effects of field treatments on RAOM, we equilibrated a homogenized peat substrate and peat from the bog along the depth profile for 1 month in the field. Elevated CO₂ did not have a significant effect on RAOM reduction (p > 0.05) in either peat type. Increased experimental temperatures stimulated RAOM reduction in the homogenized peat substrate (p < 0.05), but there were no effects of warming on RAOM reduction in peat from the bog (p > 0.05). To better understand indirect effects of the treatments, we also measured the potential for RAOM reduction in peat from each treatment under standardized laboratory conditions. The amount of reduced RAOM was variable at 10-20 cm (~15-70 μmoles e^-/g dw peat) and there were no clear patterns of warming or elevated CO₂ effects on RAOM reduction. We compared these findings to measurements conducted in 2016 and found similar microbial processing of the RAOM pool among treatments and a slight decrease in potential RAOM pools over time at three depths (10-20 cm p = 0.60; 75-100 cm and 175-200 cm p < 0.05). Collectively, our findings suggest an unexpected conclusion: peatland RAOM reduction may be resistant to warming and elevated atmospheric CO₂.