Terrestrial Organic Matter Inputs Modulate Methane Emissions from a Mega-Reservoir

Reservoirs are hotspots for methane (CH4) emissions. However, to date, the effects of terrestrial organic matter (OM) input and degradation on CH4 emissions from large reservoirs remain largely unknown. From May 2020 to April 2021, we conducted monthly sampling campaigns at 100 sites in Lake Qiandao (580 km2), a mega-reservoir in China, and made monthly vertical profile observations from March to September 2023. We estimated an annual mean FCH4 flux of 0.26 g C m-2 yr-1 (1.51 × 108 g C yr-1). Elevated FCH4 and enriched δ13C-CH4 coincided with low dissolved oxygen (DO) concentrations, high levels of organic suspended solids, terrestrial organic matter, nutrients, depleted δ18O-H2O, and low carbon isotope fractionation (αC) in the inflowing lake regions. Dissolved CH4 (cCH4) correlated positively to the relative abundance of aliphatic compounds. Anoxic bioincubation experiments revealed rapid degradation of riverine organic matter, accompanied by a 56-fold increase in cCH4, δ13C-CH4 enrichment (to -32.25‰), and a significant decrease in αC to 1.02. These findings indicate that acetoclastic CH4 production makes a substantial contribution to cCH4 and thus FCH4. Based on multiple lines of evidence, we conclude that input of terrestrial organic matter and its subsequent degradation lead to DO depletion, and their OM degradation byproducts serve as carbon substrates that promote CH4 emissions.