Linking CO2 and CH4 emissions to the microbial community and land cover in tropical headwater streams of Madagascar

Headwater streams play a crucial role as significant contributors of greenhouse gases, yet CO₂ (carbon dioxide) and CH₄ (methane) fluxes from tropical African streams are under-represented, resulting in uncertainties in global estimations. We conducted a three-month sampling of 25 sites in the Ikopa River, Madagascar, where we quantified fluxes and assessed their regulating physicochemical, and biological drivers. Ikopa’s headwater streams were a net source of greenhouse gas, with soluble CO₂ at 2483 µatm and dissolved CH₄ concentration at 1.44 µmol L^-1. Low-order steams released a total of 2.58 Gg CO₂ yr^-1 and 0.19 Gg CH₄ yr^-1. Notably, the average CH₄ ebullition was 55.66 mmol m⁻² d⁻¹, approximately 30 times higher than the global average, and streams draining through grassland areas exhibited higher CH₄ emissions. Dissolved oxygen and sediment texture emerged as the primary predictors for CH₄ ebullition rates, while partial pressure of CO₂ (pCO₂) and dissolved CH₄ directly impact CO₂ and CH₄ fluxes. Water temperature, pH level, and mcrA gene abundance directly influenced dissolved CH₄ concentration, while soil organic carbon (SOC) content, water temperature, and mcrA gene abundance indirectly impacted CH₄ fluxes. The results highlight the overlooked role of grasslands in influencing CH₄ emissions in the African tropical river. This study confirms the major role of tropical headwater streams in CH₄ emissions and provides new insight into The link between land cover and greenhouse gas emissions.