CO 2 emissions from an undrained tropical peatland: Interacting influences of temperature, shading and water table depth.

Emission of CO ₂ 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 CO ₂ 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 CO ₂ 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 CO ₂ emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO ₂ flux, water table and temperature: water table and daily average flux data suggest that CO ₂ is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO ₂ fluxes are controlled by very near surface peat. We explore how temperature-, moisture- and gas transport-related mechanisms could cause mean CO ₂ emissions to increase linearly with water table depth and also have a large diurnal cycle.
(© 2019 John Wiley & Sons Ltd.)