Dual controls on carbon loss during drought in peatlands

Peatlands store one-third of global soil carbon(1). Drought/drainage coupled with climate warming present the main threat to these stores(1-4). Hence, understanding drought effects and inherent feedbacks related to peat decomposition has been a primary global challenge(5,6). However, widely divergent results concerning drought in recent studies(3,7-11) challenge the accepted paradigm that waterlogging and associated anoxia are the overarching controls locking up carbon stored in peat. Here, by linking field and microcosm experiments, we show how previously unrecognized mechanisms regulate the build-up of phenolics, which protects stored carbon directly by reducing phenol oxidase activity during short-term drought and, indirectly, through a shift from low-phenolic Sphagnum/herbs to high-phenolic shrubs after long-term moderate drought. We demonstrate that shrub expansion induced by drought/warming(2,6,10,12,13) in boreal peatlands might be a long-term self-adaptive mechanism not only increasing carbon sequestration but also potentially protecting historic soil carbon. We therefore propose that the projected 'positive feedback loop' between carbon emission and drought in peatlands(2,3,14,15) may not occur in the long term.