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

• Published in: Biogeosciences Vol. 16; no. 13; pp. 2573 - 2589
• Main Authors: LaCroix, Rachelle E, Tfaily, Malak M, McCreight, Menli, Jones, Morris E, Spokas, Lesley, Keiluweit, Marco
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 04-07-2019; European Geosciences Union; Copernicus Publications
• Subjects: Abundance; Accumulation; Anaerobic conditions; Anoxic conditions; Biogeochemistry; Biomass; Carbon content; Carbon cycle; Carbon dioxide; Carbon dioxide control; Climate change; Composition; Deciduous forests; Drought; Duration; Efflux; Electrode potentials; Environmental Sciences & Ecology; Flooded soils; Flooding; Floods; Forests; Geology; GEOSCIENCES; Global temperature changes; Iron; Lignin; Lowlands; Metals; Mineral composition; Organic chemistry; Organic compounds; Organic matter; Oxalates; Oxalic acid; Oxidoreductions; Phases; Plant protection; Plant roots; Preservation; Proxy; Redox potential; Redox properties; Reservoirs (Water); Soil; Soil carbon; Soil moisture; Soil surfaces; Soil temperature; Soils; Stocks; Temperature; Weight reduction; Wetland soils; Wetlands
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-16-2573-2019

Although wetland soils represent a relatively small
portion of the terrestrial landscape, they account for an estimated
20 %–30 % of the global soil carbon (C) reservoir. C stored in wetland soils
that experience seasonal flooding is likely the most vulnerable to increased
severity and duration of droughts in response to climate change. Redox
conditions, plant root dynamics, and the abundance of protective mineral
phases are well-established controls on soil C persistence, but their
relative influence in seasonally flooded mineral soils is largely unknown.
To address this knowledge gap, we assessed the relative importance of
environmental (temperature, soil moisture, and redox potential) and
biogeochemical (mineral composition and root biomass) factors in controlling
CO2 efflux, C quantity, and organic matter composition along replicated
upland–lowland transitions in seasonally flooded mineral soils.
Specifically, we contrasted mineral soils under temperature deciduous
forests in lowland positions that undergo seasonal flooding with adjacent
upland soils that do not, considering both surface (A) and subsurface (B and C)
horizons. We found the lowland soils had lower total annual CO2 efflux
than the upland soils, with monthly CO2 efflux in lowlands most
strongly correlated with redox potential (Eh). Lower CO2 efflux as
compared to the uplands corresponded to greater C content and abundance of
lignin-rich, higher-molecular-weight, chemically reduced organic compounds
in the lowland surface soils (A horizons). In contrast, subsurface soils in
the lowland position (Cg horizons) showed lower C content than the upland positions (C horizons), coinciding with lower abundance of root
biomass and oxalate-extractable Fe (Feo, a proxy for protective Fe phases). Our linear mixed-effects model showed that Feo served as the strongest measured predictor of C content in upland soils, yet Feo had
no predictive power in lowland soils. Instead, our model showed that Eh and oxalate-extractable Al (Alo, 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 C
accumulation in seasonally flooded surface soils, likely due to selective
preservation of organic compounds under anaerobic conditions. In seasonally
flooded subsurface soils, however, C accumulation is limited due to lower C
inputs through root biomass and the removal of reactive Fe phases under
reducing conditions. Our findings demonstrate that C accrual in seasonally
flooded mineral soil is primarily due to low redox potential in the surface
soil and that the lack of protective metal phases leaves these C stocks
highly vulnerable to climate change.