Several hydromorphological and soil factors may be the cause of variations in the total organic carbon (TOC%) and total nitrogen (TN%) content of riparian soils. Despite the importance of these two essential components in soil-forming processes, few studies have focused on the variability of carbon and nitrogen content for soils subjected to frequent flooding. Successive floods may in fact result in soil depletion. Measurements of TOC% and TN% content as well as of other physico-chemical soil properties (e.g. litter thickness, texture, pH, Fe and Al concentrations, C/N ratio, bulk density, colour) were performed in various flood zones (recurrence intervals of 0-20 and 20-100 years) and in no-flood zones (outside of floodplains). To do so, soil samples were systematically collected along transects perpendicular to the riverbank which cross through the various flood zones. The results show that TOC% and TN% content varies significantly from one zone to another. The concentrations of these two components are significantly lower in the flood zone with a recurrence interval of 0-20 years (29% ±0.80 TOC and 0.17% ±0.05 TN) compared to the other two zones, name 3.45% ±1.56 TOC and 0.26% ±0.10 TN (recurrence interval of 20-100 years), and 3.52% ±1.57 TOC and 0.27% ±0.11 TN (no-flood zone). There is often no soil biomass (litter) in flood zones with a flood recurrence interval of 0-20 years (72% of sites without litter), whereas litter is almost always present in the flood zone with a recurrence interval of 20-100 years and in the no-flood zone, with average thicknesses of 2.84 and 3.65 cm, respectively. The absence or virtual absence of litter in the frequent-flood zones progressively results in soil depletion in terms of CO and N, which over time could adversely affect forest stand regeneration and deeply alter current river ecosystems.
Key words: Alluvial soils, total organic carbon (TOC), total nitrogen (TN), soil biomass, floodplain, flood, recurrence intervals.
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