Forest Degradation without Drainage Increases Tropical Peat Greenhouse Gas Emissions

Across Southeast Asia (SEA) and Latin America (LAC) conversion and drainage of peat swamp forests is an important source of anthropogenic greenhouse gas (GHG) emissions. Differences in peat GHG emissions from intact and degraded undrained forests can also be substantial, but the GHG impact of disturbance without drainage is poorly characterized, and current IPCC guidelines for GHG inventories do not distinguish between intact and degraded undrained peat forests. We reviewed the scientific literature related to peat GHG (CO2, CH4, and N2O) emissions and controlling environmental variables in undrained tropical peat forests. Methods and criteria of the 2013 Wetland Supplement to the 2006 IPCC guidelines were applied to select studies and compute CO2, CH4, and N2O soil emission factors (EF). We used the process-based model, DeNitrification DeComposition (DNDC), to further explore differences in peat GHG fluxes in primary (intact) and secondary (degraded) undrained peat forests in Central Kalimantan, Indonesia. Relationships among EF and drivers were investigated using empirical and modeled data. In intact forests the average peat carbon (C) budget tended to be positive (sequestering C). In contrast, in degraded undrained forests it tended to be zero or negative (emitting C), owing to elevated heterotrophic respiration and/or decreased C inputs from vegetation. Average peat CH4 and N2O emissions in SEA intact forests, expressed in CO2 equivalents, were offset by C sequestration, while in degraded forests the peat tended to be a net GHG source, due to larger soil CH4 and N2O emissions in addition to reduced peat C budget. In situ soil respiration increased with decreasing peat C:N ratio, while peat N2O fluxes increased with increasing pH, and methane fluxes increased with decreasing depth to water table. Site-specific modeled results indicated higher peat CO2 and N2O emissions in secondary than primary forests linked to lower peat C:N ratio and higher mineral N availability. Our results indicate that the lack of distinction between intact and degraded undrained peat forests in IPCC guidelines has important implications for national GHG accounting. Research is critically needed to support development of more accurate EF and determine the extent of degradation without drainage in tropical peatlands.