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 (α_C) 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 α_C 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.