The relative influence of topography and land cover on inorganic and organic carbon exports from catchments in southern Quebec, Canada

Export of carbon (C) from watersheds represents a key component of local and regional C budgets. We explored the magnitude, variability, and drivers of inorganic, organic, and total C exports from 83 temperate catchments in southern Quebec, Canada. The average dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and total C (TC) exports from these catchments were 4.6, 5.1, and 10.2gm(-2)yr(-1), respectively. Multiple regression models, using a combination of topographical variables (catchment area, shape, and slope), along with land cover variables (%vegetation, %wetland, %lake, and building density), explained 34%, 62%, and 53% of the variability in the DIC, DOC, and TC exports, respectively. Variance partitioning in the models revealed that topography is slightly more important than land cover in explaining the variance in DIC export (19% versus 15%), whereas land cover is much more important than topography in determining DOC export (44% versus 18%). Interestingly, %vegetation had a negative effect on DIC export but a positive effect on DOC export, suggesting that a change in land cover that reduces vegetation (e.g., deforestation) would lead to modest decreases in TC export but large increases in DIC/DOC export ratio. We conclude that topography and land cover together determine DIC, DOC, and TC exports. While topography is static, land cover can be altered, which will determine the quantity, form, and fate of C exported from these catchments. Finally, annual differences in export values that are related to temperature and precipitation suggest that climate change also have an impact on C export.