Data for plant phenolic trait regimes govern contrasting peatland carbon responses to drying

Northern peatlands store one-third of global soil carbon, yet drainage effects on decomposition remain paradoxical, sometimes accelerating and sometimes stabilizing carbon loss. Here we show that phenolics of differing molecular weight, organised into distinct plant phenolic trait regimes and shifted by woody plant encroachment, control these divergent responses. Laboratory incubations reveal high-molecular-weight phenolics suppress aerobic CO2 emissions by 50%, while low- and medium-weight compounds stimulate microbial activity and carbon release; all phenolics inhibit decomposition under waterlogged conditions. Along a 36-year drainage gradient in northeast China, drainage-driven succession to woody dominance boosts high-molecular-weight phenolics in the solid peat matrix, stabilizing stocks despite oxygenation. A global meta-analysis of 41 studies confirms woody peatlands gain 35% phenolics under drainage or drought with minimal carbon loss, unlike non-woody systems that emit 47% more CO2. By linking molecular mechanisms, long‑term field change and a global synthesis, we show that vegetation‑driven shifts in phenolic molecular weight trait regimes determine whether drainage unlocks or preserves peat carbon stocks, resolving a central paradox in the ‘enzymic latch’ framework. Our findings provide a mechanistic basis for prioritising vulnerable non‑woody peatlands and designing restoration strategies that couple rewetting with vegetation and phenolic trait management.