CARBON SEQUESTRATION IN MINERAL-SOIL WETLANDS: A WATERSHED PERSPECTIVE FROM SOUTH-CENTRAL CANADA

Although peatlands hold most of the carbon (C) stored in wetland soils, freshwater mineral soil wetlands – a dominant wetland type in south-central Canada - also store globally significant amounts of C. In North America, freshwater mineral soil wetlands account for 18% of the wetland C pool (Bridgham et al. 2006), which is equivalent to 5 times the average annual amount of C emitted through the burning of fossil fuels globally.

In this study, we estimated the historical and current rates of soil C accumulation across the entire spectrum of wetland types in the Lake Simcoe Watershed (LSW) of south-central Canada. First, we estimated current rates of C sequestration as a 3-year average (g C m^-2 yr^-1) through measurements of total biomass and lateral (flood) solid C inputs at 12 representative sites. The gross C sequestration values were adjusted for annual C loss through decomposition. Second, we estimated net C accumulation rates during the last 100 years (g C m^-2 yr^-1) using high-resolution ²¹⁰Pb and ¹³⁷Cs sediment chronologies and sediment C content at 11 sites across the watershed.

The current rate of C sequestration potential in the LSW is 413 g C m^-2 yr^-1. Among wetland types, Phragmites and shallow marshes dominated by Typha spp. have the highest capacity to sequester C (881 and 855 g C m^-2 yr^-1, respectively), while bogs have the lowest (146 g C m^-2 yr^-1). At watershed scale, LSW wetlands sequester on average a net total of ≈590,000 t of CO2-equivalent per year.

The process of wetland development results in long-term accumulation of C in the form of organic-rich sediments. However, the initial soil C content is progressively reduced by continuing decomposition. The long-term wetland C accumulation rate in the watershed for the last 100 years was 138.36 g m^-2 yr^-1. Among wetland types, bogs have the highest long-term C accumulation rate (365 g C m^-2 yr^-1). The difference between current and long-term rates of C accumulation is on average 275 g C m^-2 yr^-1. This indicates that the long-term net C sink is a fraction (33%) of the initial value, illustrating the important role played by decomposition. In the LSW, decomposition processes are controlled primarily by temperature and hydrological regime, which in turn are controlled by climate. Consequently, climate change could play a crucial role in the future of the LSW wetland C sink.