Methane flux measurements along a floodplain soil moisture gradient in the Okavango Delta, Botswana

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? (part 1)’.