Molecular composition of dissolved organic matter determines lower-than-expected CH₄ production in Mollisols under warming and atmospheric CO₂ enrichment

CH₄ emissions from agricultural activities in response to anticipated climate changes, such as elevated temperature (warming) and elevated CO₂ levels (eCO₂), remain highly uncertain. In this study, warming, eCO₂, and their combined effects were simulated using open-top chambers to elucidate the underlying mechanisms regulating CH₄ emission potential from paddy soils. We found that single or combined warming and eCO₂ uniformly reduced CH₄ emission potential, decreased by 17.0 to 32.7%. A suite of complementary analyses, including carbon isotopic tracing, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and microbial metagenomic and metabolomic profiling, were conducted to uncover the underlying mechanisms. We discovered only a marginal change of microbial community, metabolism and dead residuals (microbial necromass carbon). In comparison, CH₄ production was primarily mediated by shift in dissolved organic matter (DOM) molecular composition. An increase of lignin-like compounds combined with a decrease of carbohydrate explained the changes in CH₄ production. Variance partition analysis and structural equation model also evidenced the importance of DOM molecular composition rather than microbial traits on regulating CH₄ production. This study highlights an important role of DOM chemical stability in regulating CH₄ emissions in a changing world.