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

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 (Rhet). 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 Rhet had a strong linear relationship to the depth of the water table, and under flooded conditions, Rhet was small and constant. We used this relationship between Rhet and water table depth to estimate carbon export from both Rhet and dissolved organic carbon over the course of a year based on water table records. Rhet 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.