Comparison of soil organic carbon and total nitrogen contents in inundated and non-inundated zones in southern Québec, Canada

•This research shows the effects that cause frequent floods in soil depletion.•The amount and composition of soil biomass differ greatly in different flood zones.•A depletion of soil (low SOC% and STN%) was observed in frequent flood zones.•The soil depletion could have a long-term impact on the vitality of riverine forests.

Soil organic carbon (SOC) and soil total nitrogen (STN) play a key role in pedogenic processes and contribute to soil fertility. In flood zones, little is known about the processes associated with the development of alluvial soils subject to frequent flooding, in particular with respect to the accumulation of biomass on the ground, concentrations of SOC and soil nitrogen. In this case study, a paired-site approach (inundated and non-inundated zones) was used to determine the concentration of SOC and STN, and other soil properties (pH, CEC, texture) in alluvial soils in riparian areas located in the middle section of the Saint-François River (southern Québec, Canada). The results show that alluvial soils subject to frequent flooding (FFz: 0–20 year recurrence) are less rich in SOC and STN than soils outside the flood zones (NFz). The average rates obtained for surface horizons (0–20 cm depth) range from 2.0 ± 1.1% to 4.0 ± 4.1% (SOC) and from 0.2 ± 0.1% to 0.3 ± 0.2% (STN) for soils in flood zones (FFz) and those in non-flood zones (NFz). The comparison of the mean soil data from the various flood zones obtained with the Mann–Whitney U test largely confirms the differences observed between SOC and STN concentrations. The absence or virtual absence of litter in frequently flooded areas contributes to decreasing the input of organic matter in the surface horizons and progressively reduces SOC concentrations. Over time, this may alter pedogenic processes and reduce soil fertility, and, as a result, impact forest regeneration, among other aspects.