Abstract
Data-poor tropical wetlands constitute an important source of atmospheric CH4 in the world. We studied CH4 fluxes using closed chambers along a soil moisture gradient in a tropical seasonal swamp in the Okavango Delta, Botswana, the sixth largest tropical wetland in the world. The objective of the study was to assess net CH4 fluxes and controlling environmental factors in the Delta's seasonal floodplains. Net CH4 emissions from seasonal floodplains in the wetland were estimated at 0.072 ± 0.016 Tg a−1. Microbial CH4 oxidation of approximately 2.817 × 10−3 ± 0.307 × 10−3 Tg a−1 in adjacent dry soils of the occasional floodplains accounted for the sink of 4% of the total soil CH4 emissions from seasonal floodplains. The observed microbial CH4 sink in the Delta's dry soils is, therefore, comparable to the global average sink of 4–6%. Soil water content (SWC) and soil organic matter were the main environmental factors controlling CH4 fluxes in both the seasonal and occasional floodplains. The optimum SWC for soil CH4 emissions and oxidation in the Delta were estimated at 50% and 15%, respectively. Electrical conductivity and pH were poorly correlated (r2 ≤ 0.11, p < 0.05) with CH4 fluxes in the seasonal floodplain at Nxaraga.This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part1)'.
Highlights
Global warming is associated with increasing atmospheric concentrations of greenhouse gases (GHGs) such as nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) [1,2,3]
A much higher soil electrical conductivity (EC) value of 578.3 ± 75.7 μS cm−1 was recorded at Site 1 compared to a mean of 91.6 ± 8.3 μS cm−1 observed at the other measurement sites along the floodplain transect
The observed CH4 oxidation which was upscaled to the whole dry occasionally flooded swamp area of the Okavango Delta accounted for approximately 4% of the total CH4 emissions from the Delta’s seasonal floodplains
Summary
Global warming is associated with increasing atmospheric concentrations of greenhouse gases (GHGs) such as nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) [1,2,3]. The reasons for the stabilization and the renewed growth of approximately 6 Tg CH4 a−1 or approximately 3% increase in atmospheric CH4 concentration per year [9,10] since 2007 remain poorly understood due to seemingly contradictory findings, especially pertaining to the magnitudes of CH4 sources estimated using different methods, by various research works on the issue [6,9,10]. According to Dlugokencky and colleagues [8,10], the 2007 renewed growth in atmospheric CH4 concentration is consistent with abrupt increases in CH4 emissions from biomass burning and wetlands as well as a reduction in the tropospheric hydroxyl radical (OH) sink of CH4. Tropical and subtropical wetlands remain the world’s largest natural source of CH4 to the atmosphere [11,12], accounting for 70% of the global wetland emissions budget [12,13]. Methanotrophic CH4 oxidation, the only known biological sink of atmospheric CH4, is poorly understood [16] despite accounting for 4–6% of the total atmospheric CH4 sink [17]
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