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Forest degradation without drainage increases tropical peat greenhouse gas emissions

  1. Forest Degradation without Drainage Increases Tropical Peat Greenhouse Gas Emissions Erin Swails1, Kristell Hergoualc’h1, Jia Deng2, Steve Frolking2 Introduction Methods Results Conclusions Tropical peat swamp forests play an important role in regulating global climate change through their capacity to store tremendous amounts of carbon in biomass and waterlogged soils. Forest degradation can substantially decrease biomass stocks, alter organic matter dynamics, and modify peat greenhouse gas (GHG) emissions (Hergoualc’h et al., 2020; Swails et al., 2021; Sanchez et al., 2017) (Fig 1) even without drainage (Fig 2). Despite extensive areas of undrained degraded peat swamp forests across the tropics, accounting of their soil GHG emissions is lacking or inaccurate as current Intergovernmental Panel on Climate Change (IPCC) guidelines (Drösler et al., 2014) do not provide default emission factors (EF) for anthropogenically degraded undrained organic soils. We address the following questions: - How do peat onsite CO2 and net GHG budgets differ between undrained tropical peat forests that are undegraded and degraded, and between geographic regions? - How do environmental variables control peat GHG fluxes in these ecosystems? - What are research needs for further refinement of EF for anthropogenically degraded undrained tropical peat forests? We reviewed the literature on field measurements of peat GHG fluxes and controlling environmental variables in undrained degraded (DF) and undegraded (UF) peat swamp forests in Southeast Asia (SEA) and Latin America and the Caribbean (LAC) (Fig 3). We calculated peat onsite CO2 budgets as the difference of mean annual CO2-C outputs from heterotrophic soil respiration (SRh) and mean annual C inputs from litterfall and root mortality. Net onsite peat GHG budgets were calculated as the balance of peat annual GHG emissions using the rate of onsite peat CO2 emission or uptake and the N2O and CH4 emission rates. We investigated relationships among total soil respiration, peat CH4 and N2O fluxes and water table level, air and soil temperature, soil pH, C:N ratio, cation exchange capacity, base saturation, mineral nitrogen content, and peat minerotrophy/ombrotrophy status as indicated by the Ca:Mg ratio. The process-based model DeNitrification DeComposition (DNDC) (Fig 4) was used to investigate the relationship between heterotrophic respiration and vegetation C inputs in undrained secondary peat forest and pristine peat forest in Central Kalimantan, Indonesia. - In both regions, degradation without drainage tended to shift the peat from a net CO2 sink to a source (Fig 5a). - In SEA the peat in degraded forests tended to be a net GHG source as compared to undegraded conditions, while In LAC peat was a net GHG source in both degraded and undegraded forests (Fig 5). - Across forest conditions and regions, total soil respiration was negatively correlated with peat ammonium content and Ca:Mg ratio (Fig 6a and 6d, respectively). It was positively correlated with peat nitrate content and C:N ratio (Fig 6b and 6c, respectively). Peat CH4 fluxes increased with increasing base saturation (Fig 6e). - In undrained Indonesian peat swamp forests, DNDC predicted that heterotrophic respiration was related to vegetation C inputs to the soil (Fig 7). - The observed increase in peat GHG emissions in degraded undrained tropical peat swamp forest as compared to undegraded conditions calls for inclusion of undrained degraded organic soils as a new class in the IPCC guidelines to support countries in refining their GHG inventories. - Although water table is widely considered the dominant biogeophysical control on soil GHG emissions in tropical peatlands, variations in peat GHG fluxes in undrained peat swamp forests were linked to peat chemistry (observed) and vegetation inputs of C to peat (simulated). - Additional measurements that adequately cover spatial and temporal variability in peat GHG fluxes and controlling environmental variables in anthropogenically degraded undrained tropical peatlands are needed, particularly in Africa. Acknowledgements: This research was conducted under the Sustainable Wetlands Adaptation and Mitigation Program (SWAMP) and was generously supported by the governments of the United States of America (Grant MTO-069033) and Norway (QZA-21/-124). It was undertaken as part of the CGIAR research program on Climate Change, Agriculture and Food Security (CCAFS). References Drösler, M., et al. (2014). Drained inland organic soils. In 2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands: Methodological Guidance on Lands with Wet and Drained Soils, and Constructed Wetlands for Wastewater Treatment. Intergovernmental Panel on Climate Change; Gumbricht, T., et al. (2017). An expert system model for mapping tropical wetlands and peatlands reveals South America as the largest contributor. Global Change Biology, 23(9), 3581-3599.Hergoualc’h, K., et al. (2020). Spatial and temporal variabilityof soil N2O and CH4 fluxes along a degradation gradient in a palm swamp peat forest in the Peruvian Amazon. Global Change Biology, 26(12), 7198-7216; Sánchez, M. E., et al. (2017). Carbon dioxide and methane fluxes in grazed and undisturbed mountain peatlands in the Ecuadorian Andes. Mires and Peat, 19(20); Swails, E., et al. (2021). Spatio-temporal variabilityof peat CH4 and N2O fluxes and their contribution to peat GHG budgets in Indonesian forests and oil palm plantations. Frontiers in Environmental Science, 48. Figure 2. Conceptual diagram of anthropogenic activities that degrade tropical peat swamp forests. Not all degradation includes drainage. Figure 1. Summary of peat onsite GHG fluxes as defined by the IPCC. Figure 3. Study locations (green circles) and peatland extent (black areas, (Gumbricht et al., 2017) in Southeast Asia (n = 16) (a) and Latin America and the Caribbean (n = 2) (b). Study locations in Thailand and Micronesia are not shown (n = 2). There were no data for African peat swamp forests. Figure 4. DeNitrification DeComposition (DNDC) model conceptual framework. Figure 5. Peat onsite CO2 and net GHG budgets for undegraded (UF) and degraded (DF) undrained peat swamp forests in Southeast Asia (SEA) and Latin America and the Caribbean (LAC). The number of sites is indicated in parentheses. 20-yr GWP values are used to convert CH4 and N2O into CO2- equivalent. Negative values indicate an emission reduction or removal. Figure 7. Relationship between modeled heterotrophic respiration (SRh) and C inputs to peat from litterfall and root mortality in undrained secondary peat forest (DF) and pristine peat forest (UF) in Central Kalimantan, Indonesia (p < 0.05). Figure 6. Relationships among peat GHG fluxes and controlling environmental variables (p < 0.05). Relationships between total soil respiration (SR) and peat NH4 + and NO3 - content and C:N and Ca:Mg ratio are shown in (a), (b), (c), and (d), respectively. The relationship between peat CH4 fluxes and base saturation (BaseSat) is shown in (e). a b 1Center for International Forestry Research, 2University of New Hampshire a b
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