CO₂ emissions from an undrained tropical peatland: Interacting influences of temperature, shading and water table...

Emission of CO₂ from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO₂ emissions from chambers root-cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (R_het ). We made two central observations. First, we found strong diurnal cycles in CO₂ flux and near-surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily R_het had a strong linear relationship to the depth of the water table, and under flooded conditions, R_het was small and constant. We used this relationship between R_het and water table depth to estimate carbon export from both R_het and dissolved organic carbon over the course of a year based on water table records. R_het dominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO₂ emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO₂ flux, water table and temperature: water table and daily average flux data suggest that CO₂ is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO₂ fluxes are controlled by very near surface peat. We explore how temperature-, moisture- and gas transport-related mechanisms could cause mean CO₂ emissions to increase linearly with water table depth and also have a large diurnal cycle.