In the floodplain of the most large and iconic fluvial lake of Quebec (Lake St-Pierre, LSP, Quebec), agriculture landscape switches from 85% of perennial crops and pastures in 1964 to 9% in 2014, while annual crops follows the opposite direction from 15% to 91% (equivalent to 5 and 25% of the all floodplain landscape). In this context, we wanted to know how landuse in the LSP floodplain influence the carbon sequestration of the ecosystem and the atmospheric mitigation of CO2, CH4 and N2O fluxes in the zone of flooding recurrence of 0-2 years. For that, we measured fluxes of soil carbon-nitrogen and the biomasses of soil organic matter, soil microbe and vegetation over 2020-2021 and across 30 plots belonging to six landuse categories. When orders along an anthropogenic disturbance gradient, these categories from more to less disturbed landuse were: 1. conventional soja-corn agriculture, 2. soja-corn agriculture with perennial band and cover crop, 3. temporary grassland, 4. permanent grassland, 5. wet meadows, 6. Floodplain forest.
Results/Conclusions
Landuse has considerably modified the edaphic environment, the standing plant and microbial biomass and ultimately the functions associated with the carbon cycle of the ecosystems in the LSP floodplain. The lower the intensity of disturbance by anthropogenic activities, the more the system was able to extract CO2 and CH4 from the atmosphere, limit their return as well as that of nitrogen in the form of N2O. The relationship between landuse and ecosystem functioning was not linear. The strongest differences occurred when changing landuse from temporary to permanent grassland and from herbaceous to tree vegetation. The incorporation of agri-environmental practices in agricultural fields did not cause any environmental improvement in comparison with conventional practices. Reintroducing more grasslands and forests are landuse opportunities to limit the negative effects of agriculture in the floodplain of lake St-Pierre.
