This study examines the spatial distribution of organic carbon in alluvial soils subjected to frequent flooding according to different flood risk zones, that is, interval recurrences of 0-20 years (FFz) and 20-100 years (MFz). Sites located outside of flood zones (NFz) were also selected to compare the soil organic carbon (SOC) in different zones. The selected sites are located in floodplains covered by forest dominated by silver maple (Acer saccharinum L.) and green ash (Fraxinus pennsylvanica Marsh.) in southern Québec. These floodplains are affected by frequent flooding, especially in the last decades, which has a direct impact on pedogenic processes, particularly in terms of in situ soil biomass and organic matter. The soil samples (0-20 cm depth) collected in a frequent flood zone (FFz), generally show a lower content of soil organic carbon (SOC%) ranging from 1.74 to 2.59% (median values), and mean values between 1.79 and 2.83%, respectively. In areas not affected by the floods, levels of SOC (%) are generally higher, with values ranging between 2.86 and 3.73% (mean), and mean values between 3.18 and 5.17%. Loss of biomass (litter) during the flood recession causes a net loss of organic matter to the subsurface soils. Successive flooding leads to an impoverishment of alluvial soils and undermining of the pedogenic processes and soil development. This confirms the trends observed in our previous work on soil depletion in active floodplains in the study area.
Key words: Alluvial soils, soil organic carbon (SOC), floods, spatial variability, climate change.
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