4. Peat carbon and greenhouse gas dynamics

The greenhouse gas (GHG) dynamics of tropical peat ecosystem involve carbon dioxide (CO2) uptake via photosynthesis and losses through autotrophic respiration of vegetation, CO2, methane (CH4) and nitrous oxide (N2O) emissions from soil organic matter decomposition run by microbiota, and fluvial exports of dissolved and particulate organic carbon compounds (e.g. Jauhiainen et al. 2005, 2008, 2012; Jauhiainen & Silvennoinen 2012; Hirano et al. 2009; Moore et al. 2013). The position of the water table controls the thickness of the potentially aerobic zone in the upper part of peat and in the lower water-saturated anaerobic peat. The organic polymers in the surface peat are decomposed largely aerobically by bacteria and fungi, and as the peat substrate becomes water-saturated a succession of bacteria breaks down emerging degradation products anaerobically and at the end methanogens produce methane from acetate or CO2 and hydrogen (H2). At the same time methanotrophic bacteria consume upwards diffusing CH4 in the aerobic peat zone. This means that actual, into the atmosphere emerging, GHG-emissions are determined by activity levels of fungi and aerobic microbes, with specific simultaneous activities of methanogens and methanotrophs in the peat profile. N2O emissions are produced in anaerobic processes by denitrifiers.

Jauhiainen, J., Vasander, H., Heikkinen, J. & Martikainen, P.J. (2002). Carbon fluxes in pristine and developed Central Kalimantan peatlands. In Rieley, J.O. & Page, S.E. (Ed.), Proceedings of the International Symposium on Tropical Peatlands. Jakarta, Indonesia.

Major peat carbon stocks, pathways and processes found in tropical peat (from Jauhiainen et al. 2002)

An increase in oxygen availability during the dry season may lead to enhanced litter decomposition and thus also nitrogen mineralization in the deposited organic matter. Drainage of tropical ombrotrophic forest systems increases organic matter mineralization rates in peat, although the litter deposition into peat may remain relatively unaffected. This results in enhanced GHG emissions, especially as CO2 formed in oxic conditions in the topmost peat and N2O formed in anoxic conditions closer to the water table level. Organic matter mineralization has recently found to be connected in tempertures also in tropical peatland conditions, and potential effects from diurnal temperture variation and land use dependent effects on peat decomposition can be marked (Jauhiainen et al. 2014). Overview to temperture impacts on peat is on page ‘Reclaimed peatlands‘.

For clear-felled sites (low fertilization amounts receiving agricultural areas and deforested burned peat) N20 emission rates from organic matter mineralization may remain modest, and this is associated with reduced N-availability in the peat substrate (Jauhiainen et al. 1012a, 2014). Therefore, enhanced N2O emission rates from drained peat after land use change can be a transient phenomenon when the nitrogen availability is not maintained at a high level from biological or artificial sources. Fertilization, for example in oil palm plantations, form potentially high temporal N2O emission source by maintaining and enhancing peat decomposition processes.

CO2 monitoring inside Acacia stand in Riau (Sumatra, Indonesia) (Photo: J. Jauhiainen)

CO2 monitoring inside Acacia stand in Riau (Sumatra, Indonesia) (Photo: J. Jauhiainen)