Autumn destabilization of deep porewater CO2 store in a northern peatland driven by turbulent diffusion

The CO2 stored in deep peat porewater is viewed as a fixed component of peatland C cycling. Here, the authors reveal a hitherto unknown hydro-physical process that results in sudden losses from this CO2 store every autumn. The deep porewater of northern peatlands stores large amounts of carbon dioxide (CO2). This store is viewed as a stable feature in the peatland CO2 cycle. Here, we report large and rapid fluctuations in deep porewater CO2 concentration recurring every autumn over four consecutive years in a boreal peatland. Estimates of the vertical diffusion of heat indicate that CO2 diffusion occurs at the turbulent rather than molecular rate. The weakening of porewater thermal stratification in autumn likely increases turbulent diffusion, thus fostering a rapid diffusion of deeper porewater CO2 towards the surface where net losses occur. This phenomenon periodically decreases the peat porewater CO2 store by between 29 and 90 g C m(-2) throughout autumn, which is comparable to the peatland's annual C-sink. Our results establish the need to consider the role of turbulent diffusion in regularly destabilizing the CO2 store in peat porewater.

Automated summary

The study reveals large and rapid fluctuations in deep porewater CO2 concentration occurring every autumn in a boreal peatland, likely due to weakening of porewater thermal stratification leading to increased turbulent diffusion and rapid diffusion of deeper porewater CO2 towards the surface resulting in net losses. This phenomenon periodically decreases the peat porewater CO2 store by between 29 and 90 g C m(-2) throughout autumn, comparable to the peatland's annual C-sink.