Dynamics and controls of inland water CH4 emissions across the Conterminous United States: 1860-2019

Inland waters (rivers, lakes, and reservoirs) have been recognized as hotspots of methane (CH4) emissions. However, the magnitude and spatiotemporal pattern of CH4 emissions and their underlying mechanisms remain largely unknown due to a lack of process-based quantification of CH4 production, consumption, and evasion within the aquatic ecosystem. Here we developed a process-based aquatic CH4 module within the framework of the Dynamic Land Ecosystem Model (DLEM) to explicitly simulate inland water carbon fluxes and the associated CH4 processes. We further applied this model to assess the inland-water CH4 emissions across the conterminous United States (CONUS) as affected by the climate variability, land use, fertilizer nitrogen (N) application, atmospheric N deposition, and rising atmospheric CO2 concentration during 1860-2019. The inland water CH4 emissions across the CONUS had doubled from the 1860s (1.65±0.18 Tg CH4-C∙yr-1) to the 2010s (3.73±0.36 Tg CH4-C∙yr-1). In the 2000s, inland water accounts for 8% of the regional CH4 budget that offsets 11∼14% of the terrestrial C uptake across the CONUS. Our study showed that the small headwater streams (1st -3rd order) account for 49% of the diffusive CH4, and reservoirs constitute 50% of the ebullitive CH4 emissions during the 2010s. Climate change and variability played a dominant role in the increased CH4 emissions from rivers and lakes. This study implies that effective mitigation strategies to reduce CH4 emissions should pay much attention to global climate change and headwater stream management.