Pyrogenic Carbon Contributes Substantially to Carbon Storage in Intact and Degraded Northern Peatlands

Pyrogenic carbon (PyC) derives from incomplete combustion of organic matter and is ubiquitous in terrestrial and aquatic systems. Most PyC is inherently more stable against decomposition than plant residues, and PyC therefore forms an important component of the global carbon (C) cycle. During the Holocene, about 436 Pg organic C accumulated in northern peatlands, and we hypothesize that PyC may contribute substantially to that C stock. We studied 70 samples from 19 intact and degraded European peatland sites and analyzed their PyC content by 13C nuclear magnetic resonance spectroscopy and molecular modeling and peat age and accumulation by radiocarbon dating. Classification of a peatland as either intact or degraded was based on the comparison between apparent and expected long-term C accumulation rates. On average, PyC amounted for 13·5% of soil C across sites, and accounted for up to 50% at single sites. The amount of PyC increased significantly with peat age. Degraded peatlands had lost approximately 56 kg C m−2, half of their former C stock. However, degraded peat had higher PyC contents than intact one. Selective enrichment of PyC during both peat build-up and decomposition seems to be an important factor fostering PyC accumulation. Assignment of our results to peatlands of the northern hemisphere, stratified by age, revealed an estimated PyC stock of 62 (±22) Pg. Our estimate indicates a substantial and hitherto unquantified contribution of northern peatlands to global PyC storage.