Impacts of seasonal flooding on soil carbon and nutrients across an ecosystem disturbance gradient

Background/Question/Methods

Flooding due to climate change may increase ecosystem losses of soil carbon (C) and nutrients. Changes in soil water alter chemical and biological processes that control nitrogen (N), phosphorous (P) and C dynamics leading to shifts in their bioavailability. Soil microbial communities shape how C and nutrients are cycled and are sensitive to moisture conditions, resulting in unknown consequences for C, N and P cycling with flooding. Ecosystems under varying land uses and degrees of disturbance likely support different microbial communities that may modify how soil processes C, N, and P respond to flooding.This study examines the variability in flood response of soil N, P, and C dynamics across an ecosystem disturbance gradient around the Lake Saint-Pierre watershed in Québec, Canada that experiences regular spring flooding. We collected soil samples across five land use types: conventional agriculture, conservation agriculture, managed prairies, wetlands and forests. To capture spatial and temporal variation in flood response we sampled at three different flood elevations within each land use in the spring, summer and late fall. We measured soil bioavailable nutrient concentrations, dissolved and total organic C, extracellular enzyme activity for C, N and P cycling, and microbial biomass C and N.

Results/Conclusions

We expected that the natural sites (forest and wetland) would have the most adapted microbial community for flooded conditions and therefore would have less spatial and temporal variability in terms of moisture, soil C and inorganic N concentrations compared to the agriculture treatments. Preliminary results for soil moisture, soil C and inorganic N show no consistent trends in spatial or temporal variability (based on coefficient of variance) in response to flood conditions across the land use gradient. Nonetheless, we saw soil moisture significantly increased by 77% from the agricultural fields to the natural sites (p< 0.001). There were also significant increases in soil C (p< 0.001) and in N and C cycling extracellular enzyme activities (p< 0.05 for both) across the land use gradient (lowest in the conventional agriculture fields, highest in the natural sites). Soil ammonium was highest in the forests and wetlands, while Olsen-P concentrations were highest in the agricultural sites (p< 0.05). Our preliminary results demonstrate that land use has a strong effect on C, N and P cycling potential and further data analysis will attempt to uncover how these dynamics are responding to flooded conditions.