Variability of soil properties in different flood-risk zones and link with hydroclimatic changes (Southern Québec, Canada)

We have analyzed and compared concentrations of soil organic carbon (SOC), soil total nitrogen (STN) and other soil properties in different inundated areas based on governmental flood risk maps (recurrence intervals of 0-20 years and 20-100 years). In this case study, a paired-site approach was used to determine the impact of periodic floods on soil properties (organic carbon, total nitrogen, pH, cation exchange capacity (CEC), phosphorus, potassium and grain size particles) in riverine areas located in the mid section of the Saint-François River (southern Québec). Two specific flood-risk zones, i.e. Frequent flood zones (FFz) and Moderate flood zones (MFz), in connection with the flood recurrence intervals (20-year vs 100-year flood) were considered to evaluate the variability of different soil properties. Sampling sites located outside (NFz: No-flood zones) of the two flood-risk zones were also selected for a cross-comparison. In this study area of the Saint-François river catchment, an increase in the flood rate was noted over the last few decades, especially from 1970 to 2000, which has the effect of modifying the hydrogeomorphic conditions and pedogenetic processes. The results show that alluvial soils subject to frequent flooding (FFz) have a lower concentration in soil organic carbon (SOC%) and soil total nitrogen (STN%) than soils in non-inundated zones. The average values obtained for surface horizons (0-20 cm depth) range from 1.79 ± 0.83 to 2.16 ± 1.28 (SOC%) and from 0.14 ± 0.05 to 0.17 ± 0.07 (STN%) for soils in frequent flood zones (FFz), while for soils in no-flood zones (NFz), the values range from 5.17 ± 2.99 and 3.82 ± 1.88 (SOC%), and from 0.30 ± 0.28 to 0.24 ± 0.10 (STN%). The t-student tests conducted among the various zones confirm this trend of depletion in soil organic carbon and soil total nitrogen for the soils found in the Frequent Flood Zones (FFz). A lower concentration of nutrients was also noted for the soils affected by floods. Soil acidity is higher in the no-flood zones, which could be attributed to the acidifying input of the litter, which is more abundant and better formed (LFH layers) in the no-flood forest zones. The soil textural matrices are relatively similar among the three zones under study, although the textures are generally coarser for the soils outside the flood zones. Lastly, if the current hydrological conditions are maintained (i.e. an increase in the flood rate), one can expect a nutrient storage depletion that could restrict the regeneration of forest stands and the vitality of riverside ecosystems.