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

Area of Science:

• Environmental Science
• Geochemistry
• Biogeochemistry

Background:

• Reservoirs are significant sources of methane (CH4) emissions, yet the influence of terrestrial organic matter (OM) on these emissions in large systems is poorly understood.
• Understanding the fate of terrestrial OM and its role in CH4 production is crucial for accurate climate modeling and reservoir management.

Purpose of the Study:

• To investigate the impact of terrestrial organic matter input and degradation on methane emissions from a large Chinese mega-reservoir, Lake Qiandao.
• To quantify the annual methane flux and identify the primary drivers and pathways of CH4 production.

Main Methods:

• Conducted monthly sampling at 100 sites over one year and monthly vertical profile observations for seven months.
• Analyzed water chemistry, organic matter composition, and stable isotopes (δ13C-CH4, δ18O-H2O).
• Performed anoxic bioincubation experiments to simulate OM degradation and CH4 production.

Main Results:

• Estimated an annual mean CH4 flux (FCH4) of 0.26 g C m-2 yr-1.
• Observed elevated FCH4 and enriched δ13C-CH4 linked to low dissolved oxygen, high organic matter, and terrestrial OM indicators.
• Anoxic incubation showed rapid riverine OM degradation, a 56-fold increase in dissolved CH4 (cCH4), and significant isotopic shifts, indicating acetoclastic methanogenesis.

Conclusions:

• Terrestrial organic matter input and degradation significantly contribute to methane emissions in large reservoirs.
• The process involves depletion of dissolved oxygen and the formation of carbon substrates that enhance CH4 production, primarily through acetoclastic pathways.
• These findings highlight the critical role of terrestrial OM in regulating greenhouse gas emissions from reservoirs.