Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes

Riverine carbon-14 measurements show that anthropogenic disturbance of peat swamp forest in southeast Asia is causing increased release of carbon that has been stored in the peat for thousands of years. Tropical peatlands in retreat Tropical peatlands contain a large pool of terrestrial organic carbon, but in some locations disturbance by deforestation, drainage and fire are converting it into a source of atmospheric carbon dioxide. This paper quantifies the annual export of fluvial organic carbon from intact and disturbed peat swamp forests in Indonesia and finds that the total fluvial organic carbon flux from disturbed peat forests is 50% larger than that from intact peat forest, and is dominated by century- to millennia-old carbon from the deeper layers of the peat column. The inclusion of the often overlooked fluvial carbon loss term in the peatland carbon budget increases the estimate of total carbon lost from the disturbed peatlands by 22%. Tropical peatlands contain one of the largest pools of terrestrial organic carbon, amounting to about 89,000 teragrams 1 (1 Tg is a billion kilograms). Approximately 65 per cent of this carbon store is in Indonesia, where extensive anthropogenic degradation in the form of deforestation, drainage and fire are converting it into a globally significant source of atmospheric carbon dioxide 1 , 2 , 3 . Here we quantify the annual export of fluvial organic carbon from both intact peat swamp forest and peat swamp forest subject to past anthropogenic disturbance. We find that the total fluvial organic carbon flux from disturbed peat swamp forest is about 50 per cent larger than that from intact peat swamp forest. By carbon-14 dating of dissolved organic carbon (which makes up over 91 per cent of total organic carbon), we find that leaching of dissolved organic carbon from intact peat swamp forest is derived mainly from recent primary production (plant growth). In contrast, dissolved organic carbon from disturbed peat swamp forest consists mostly of much older (centuries to millennia) carbon from deep within the peat column. When we include the fluvial carbon loss term, which is often ignored, in the peatland carbon budget, we find that it increases the estimate of total carbon lost from the disturbed peatlands in our study by 22 per cent. We further estimate that since 1990 peatland disturbance has resulted in a 32 per cent increase in fluvial organic carbon flux from southeast Asia—an increase that is more than half of t