Carbon footprints of peatland Degradation : Impacts on soil carbon leaching, aquatic CO2 emissions and the marine carbonate system in Indonesia

Abstract

In Southeast Asia and Indonesia, land use change (LUC) occurs in the form of large-scale deforestation and peatland degradation for agricultural purposes, which causes terrestrial CO2 emissions from peat soils as a consequence of oxidation, subsidence and forest fires. However, the consequences of this peatland degradation for the aquatic and marine environment and carbon cycle are less well known. In the framework of the SPICE III a CISKA subproject 1, the impact of land use change in Indonesia was determined by the quantification of the inorganic and organic carbon fluxes and CO2 emissions from the rivers, estuaries and coastal ocean into the atmosphere as well as the marine carbonate system in order to develop sustainable mitigation strategies to reduce CO2 emissions. The findings of this PhD study bring attention to the fact that the impacts of tropical peatland degradation in Indonesia are not limited to direct CO2 emissions due to drainage and deforestation, but also greatly affect the carbon cycle of adjacent freshwater and marine environments through a variety of processes. By means of a mixing model it was shown that dissolved organic carbon leaching from disturbed peat soils has increased by 200% from 62 to 183 g m-2 yr-1 as a consequence of hydrological changes and secondary vegetation. Increased freshwater fluxes due to reduced evapotranspiration account for 38% of the increase in carbon leaching, whereas the labile leaf litter from secondary vegetation is responsible for the remaining 62% increase. Once the organic carbon has reached the rivers, it is either respired and emitted to the atmosphere (river outgassing) or exported to the coastal ocean (riverine carbon export). Dissolved organic carbon (DOC) and pCO2 concentrations in the rivers increase as the share of disturbed peatland coverage in the catchment increases as a consequence of increased carbon leaching and decomposition. Based on the regression between peat coverage and CO2 yield, the CO2 fluxes from rivers in Indonesia have been estimated as well as in Malaysia and extrapolated to Southeast Asia, and amount to 53.9 A /-12.4, 6.2A /-1.6 and 66.9A /-15.7 Tg C yr-1, respectively. However, these fluxes are rather moderate due to the short residence time of the river waters and the location of peat close to the coast, which shorten the time available for decomposition. Circa 53% of the carbon that enters the freshwater system in Southeast Asia is emitted as CO2 to the atmosphere, whereas the remaining 47% is exported to the coastal ocean. Based on total alkalinity (TA), dissolved inorganic carbon (DIC) and pCO2 measurements in the estuaries and coastal ocean of Sumatra, it was shown that the majority of the exported carbon is respired in the estuaries. Circa 62.7% of the exported, respired CO2 is emitted to the atmosphere, whereas 6.4% is assumed to be buried in the sediments and the remaining 30.6% is absorbed in the water column. Here, the respired CO2 contributes to ocean acidification and lowers the aragonite and calcite saturation states (I(c)AR / I(c)CA). This induces carbonate dissolution of sediments, but also coral reefs and other calcifying organisms and can therefore be viewed as the invisible carbon footprint, but is currently overlooked in climate mitigation strategy policies. In Indonesia, the terrestrial direct CO2 emissions due to LUC via secondary vegetation (10.9 Tg C yr-1), peat oxidation (109.9 Tg C yr-1) and forest fires (82.1 Tg C yr-1) amount to 192.0 Tg C yr-1. Carbon loss due to indirect emissions from the rivers (53.9 Tg yr-1, ref. (Wit et al. 2015)), estuaries and coastal ocean (49.4 Tg yr-1), as well as the invisible carbon footprint (24.1 Tg yr-1) and excluding the natural emissions from pristine peatlands (13.0 Tg C yr-1) amounts to 114.3 Tg C yr-1. Therefore, the total carbon loss due to LUC amounts to 306.3 Tg yr-1, which represents an increase of 60% with respect to the direct terrestrial emissions currently considered in greenhouse gas mitigation policies. With respect to the development of climate change mitigation strategies as one of the overarching goals of SPICE and CISKA, the advice is to include the aquatic and marine CO2 emissions, as well as the invisible carbon footprint in order to cover the carbon losses with respect to LUC in Indonesia. In addition, carbon leaching and fluvial carbon export should be reduced to mitigate the impact of ocean acidification and carbonate dissolution.