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

Abstract

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.

Post‐fire carbon dynamics in the tropical peat swamp forests of Brunei reveal long‐term elevated CH4 flux

“…At both the intact and burnt PSF transects, the lowest R eco flux rates occurred in January and March, when soil temperatures were at a minimum and the WT was close to the peat surface (Figure 3). Other studies have shown similar trends (Hirano et al., 2009, 2014; Hoyt et al., 2019; Jauhiainen et al., 2012; Jauhiainen, Takahashi, Heikkinen, Martikainen, & Vasander, 2005; Sundari, Hirano, Yamada, Kusin, & Limin, 2012). This is likely due to a reduced oxic zone in the peat upper layer, which results in reduced aerobic decomposition, and hence R eco emissions.…”

“…Other studies have shown similar trends (Hirano et al, 2009(Hirano et al, , 2014Hoyt et al, 2019;Jauhiainen, Takahashi, Heikkinen, Martikainen, & Vasander, 2005;Sundari, Hirano, Yamada, Kusin, & Limin, 2012). This is likely due to a reduced oxic zone in the peat upper layer, which results in reduced aerobic decomposition, and hence R eco emissions.…”

“…While R eco emissions from peatlands positively correlate with temperature increase, particularly at higher latitudes (Lafleur, Roulet, Bubier, Frolking, & Moore, 2003;Mäkiranta, Minkkinen, Hytönen, & Laine, 2008;Minkkinen et al, 2007), the results are less evident in the Tropics. While some recent studies relate observed CO 2 emissions (total or heterotrophic emissions) with peat temperatures at 5 cm depth (Hirano et al, 2009(Hirano et al, , 2014Hoyt et al, 2019;Jauhiainen, Hooijer, & Page, 2012), only two studies to date have directly measured the effect of temperature on tropical peat decomposition in field conditions (Hoyt et al, 2019;Jauhiainen et al, 2014).…”

From canals to the coast: dissolved organic matter and trace metal composition in rivers draining degraded tropical peatlands in Indonesia

“…Additionally, the results of our physical fractionation show that even close to the estuary, DOC remains in the dissolved and fine colloid form (< 0.22 µm) and that flocculation processes might be limited. The large areas of coastal peatlands in the region might explain the relatively high fluvial organic carbon export to the South China Sea (Huang et al, 2017). The decrease in trace metal concentrations after the confluence might be influenced by shifts in physical fractionation and an increased proportion of colloidal form.…”

Scalar Simulation and Parameterization of Water Table Dynamics in Tropical Peatlands

“…And fourth, retardation is likely along banks at high flows, as peatland rivers tend to overflow their channels, leading to backwater effects (Åkesson et al, 2015). A scalar model for runoff generation from peatland subcatchments nonetheless provides a simple building block for inclusion in discharge modeling in tropical catchments, or for estimation of DOC export (Hoyt et al, 2019).…”