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-2d-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.