CH4 emissions from biomass burning of shifting cultivation areas of tropical deciduous forests – experimental results from ground-based measurements

Context Abstract: Tropical deciduous fires from shifting cultivation process in India are characterized by the highly differential nature of fire behavior due to fragmented burning patterns. Our study from ground-based experiments from biomass burning of tropical deciduous forest fires suggests smoldering combustion as the dominating process during biomass burning which leads to evolution of more incompletely oxidized products such as Methane when compared to other ecosystems such as Savannas. In the study, we report emission ratios and emission factors for methane from biomass burning of tropical deciduous forests. As tropical deciduous forests in India cover more than 50% of overall forests, the emission factors obtained in the present study can be used widely for methane emission estimation from forest biomass burning studies in other parts of India and in modeling studies of Methane from forest biomass burning in India.

Main Abstract: Biomass burning is an important source of trace gas emissions to the atmosphere. Methane emitted from the biomass burning contributes to the atmospheric greenhouse effect and is sufficiently long-lived to enter the stratosphere and take part in the stratospheric ozone cycles. In India, though CH₄ emissions from the different sources such as rice paddy fields and domestic animals have been well studied, there are relatively no field-based studies with respect to CH₄ emissions from biomass burning. In the present study, we report for the first time, the CH₄ emissions from biomass burning of tropical deciduous forests cleared for shifting cultivation purposes. Trace gas emissions from the biomass burning plumes have been collected through grab sampling in canisters as well as from online measurements through instruments. Site characteristics with respect to species composition, amount of biomass burnt and relative amounts of combustion, viz., flaming, mixed and smoldering have been determined. Modified combustion efficiency has been used to differentiate relative amounts of combustion. Emission ratios were calculated with respect to CO₂ and emission factors based on the amount of biomass consumed. Results of the study with respect to biomass estimations prior to burning suggested values of 12–14 t ha⁻¹ at the first site and 13.5–15.3 t ha⁻¹ at the second site. The mean modified combustion efficiencies during flaming, mixed and smoldering combustion phases for the first site were found to be 95.7%, 91.1% and 74.4% and 95.31%, 90.63% and 72.89%, respectively, for the second site. The average biomass consumed during the fire ranged from 4.7 t ha⁻¹ (site 1) to 3.4 t ha⁻¹ (site 2), indicating low amount of biomass burnt during the first phase of burning in shifting cultivation areas. Results suggested the CH₄ emission ratios of 1.29% at the first site and 1.59% at the second site. The CH₄ emission ratios obtained in the present study are closer to the most accepted estimates of 1.2±0.5% obtained for tropical forests elsewhere. Using the emission ratios obtained in the study and estimating the amount of methane emissions from biomass burning suggests that nearly 0.99 Tg of methane is emitted annually from shifting cultivation process in India. Also, in the study, a detailed comparison of emission ratios and emission factors of CH₄ has been made.