Search

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.)

The role of mangroves in the blue carbon stock is critical and requires special focus. Mangroves are carbon-rich forests that are not in steady-state equilibrium at the decadal time scale. Over the last decades, the structure and zonation of mangroves have been largely disturbed by coastal changes and land use conversions. The amount of time since the last disturbance is a key parameter determining forest structure, but it has so far been overlooked in mangrove carbon stock projections. In particular, the carbon sequestration rates among mangrove successional ages after (re)establishment are poorly quantified and not used in large-scale estimations of the blue carbon stock. Here, it is hypothesized that ecosystem age structure significantly modulates mangrove carbon stocks. We analysed a 66-year chronosequence of the aboveground and belowground biomass and soil carbon stock of mangroves in French Guiana, and we found that in the year after forest establishment on newly formed mud banks, the aboveground, belowground and soil carbon stocks averaged 23.56 ± 7.71, 13.04 ± 3.37 and 84.26 ± 64.14 (to a depth of 1 m) Mg C/ha, respectively. The mean annual increment (MAI) in the aboveground and belowground reservoirs was 23.56 × Age -0.52 and 13.20 × Age -0.64  Mg C ha -1  year -1 , respectively, and the MAI in the soil carbon reservoir was 3.00 ± 1.80 Mg C ha -1  year -1 . Our results show that the plant carbon sink capacity declines with ecosystem age, while the soil carbon sequestration rate remains constant over many years. We suggest that global projections of the above- and belowground reservoirs of the carbon stock need to account for mangrove age structures, which result from historical changes in coastal morphology. Our work anticipates joint international efforts to globally quantify the multidecadal mangrove carbon balance based on the combined use of age-based parametric equations and time series of mangrove age maps at regional scales., (© 2018 John Wiley & Sons Ltd.)