Carbon loss from a deforested and drained tropical peatland over four years as assessed from peat stratigraphy

Deforested, drained, and converted tropical peat forest is common in Southeast Asia, and these areas have become a major source of carbon (C) emissions. In this study, measurements of peat properties, i.e., peat thickness, organic matter (OM), bulk density (BD), total organic C (TOC), and total nitrogen (TN), were taken four years apart (2012-2016) at 24 sites in a previously deforested and drained tropical peat in West Kalimantan, Indonesia. We calculated that the average peat subsides at a rate of 3.8 +/- 1.2 cm yr(-1). The estimated net change in C stock ranged from -8 to -41 Mg C ha(-1) yr(-1), with an average of -31 Mg C ha(-1) yr(-1). C loss varied by peat depth and land use. C loss was four times faster on shallow peat (50-100 cm) than deep (>300 cm) peat sites. Fallow sites (bush fern and secondary forest) subsided and lost C faster than oil palm plantations, likely due to land-clearing fires. Percent C and C/N ratio declined significantly in the upper 150 cm of peat, especially in the fallow sites, indicating oxidation within the peat profile. No change occurred in bulk density, showing little effect of compaction, though the density profile migrated downward with subsidence. The current peat topography, modeled from satellite LiDAR data, reflects past peat-loss patterns and confirms the coring results. C loss in this region continues at a high rate since its original deforestation in the early 1970s. This study concludes that C loss due to anthropogenic disturbances on tropical peat is larger than the 2013 Tier 1-IPCC CO2 emission factor for Acacia and oil palms on drained tropical peats, which are 20 and 11 Mg CO2-eq ha(-1) yr(-1), respectively.