Carbon accumulation in recently deposited peat is reduced by increased nutrient supply.

High-latitude mires store a considerable part of the global soil carbon. Current understanding suggests that wetter conditions promote carbon accumulation. This paradigm is based primarily on temperate ombrogenic bogs and overlooks the influence of minerogenic water from the catchment area, despite most northern mires being minerogenic fens. Here we show that minerogenic water is the main negative influence on past century carbon accumulation in boreal fens. This effect is most pronounced in mires formed during the last millennia. Rather than enhancing productivity, minerogenic water stimulates organic matter decay, apart from in elevated hummocks where both decay and productivity were stimulated. These findings reshape our understanding of carbon cycling at high-latitudes, highlighting how shifts in precipitation-evapotranspiration may impact carbon sequestration in fens, which are widespread in the circum-arctic. Contrary to expectations for temperate regions, we argue that increased catchment water input in sub-arctic peatlands is unlikely to enhance mire carbon accumulation.