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. Although wetland soils represent a relatively smallportion of the terrestrial landscape, they account for an estimated20 %–30 % of the global soil carbon (C) reservoir. C stored in wetland soilsthat experience seasonal flooding is likely the most vulnerable to increasedseverity and duration of droughts in response to climate change. Redoxconditions, plant root dynamics, and the abundance of protective mineralphases are well-established controls on soil C persistence, but theirrelative influence in seasonally flooded mineral soils is largely unknown.To address this knowledge gap, we assessed the relative importance ofenvironmental (temperature, soil moisture, and redox potential) andbiogeochemical (mineral composition and root biomass) factors in controlling CO2 efflux, C quantity, and organic matter composition along replicatedupland–lowland transitions in seasonally flooded mineral soils.Specifically, we contrasted mineral soils under temperature deciduousforests in lowland positions that undergo seasonal flooding with adjacentupland soils that do not, considering both surface (A) and subsurface (B and C)horizons. We found the lowland soils had lower total annual CO2 effluxthan the upland soils, with monthly CO2 efflux in lowlands moststrongly correlated with redox potential (E h ). Lower CO2 efflux ascompared to the uplands corresponded to greater C content and abundance oflignin-rich, higher-molecular-weight, chemically reduced organic compoundsin the lowland surface soils (A horizons). In contrast, subsurface soils inthe lowland position (C g horizons) showed lower C content than the upland positions (C horizons), coinciding with lower abundance of rootbiomass and oxalate-extractable Fe (Fe o , a proxy for protective Fe phases). Our linear mixed-effects model showed that Fe o served as the strongest measured predictor of C content in upland soils, yet Fe o hadno predictive power in lowland soils. Instead, our model showed that E h and oxalate-extractable Al (Al o , a proxy of protective Al phases) became significantly stronger predictors in the lowland soils. Combined, our results suggest that low redox potentials are the primary cause for Caccumulation in seasonally flooded surface soils, likely due to selectivepreservation of organic compounds under anaerobic conditions. In seasonallyflooded subsurface soils, however, C accumulation is limited due to lower Cinputs through root biomass and the removal of reactive Fe phases underreducing conditions. Our findings demonstrate that C accrual in seasonallyflooded mineral soil is primarily due to low redox potential in the surfacesoil and that the lack of protective metal phases leaves these C stockshighly vulnerable to climate change.

Shifting mineral and redox controls on carbon cycling in seasonally flooded mineral soils