Effects of plant photosynthetic type on methane flux in reservoir drawdown zones

Wetlands are major natural sources of methane (CH₄), and reservoir drawdown zones represent distinctive wetland systems in which reverse seasonal water-level fluctuations create favorable conditions for CH₄ production. Reservoir operation can also drive vegetation turnover in drawdown zones, with plant communities shifting from C3-dominated to C4-dominated assemblages. Nevertheless, the effects of this photosynthetic-pathway transition on CH₄ fluxes and the biogeochemical mechanisms underlying these effects, remain poorly resolved, limiting accurate estimation and parameterization of CH₄ emissions from reservoir drawdown zones. This study quantified CH₄ fluxes, soil physicochemical properties, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), and enzyme activities across plant communities with different photosynthetic types and water-level fluctuation zones to determine how vegetation photosynthetic type impacts CH₄ flux. Both C3 and C4 plant communities independently inhibited CH₄ emissions, whereas mixed C3–C4 communities had only a weak overall effect. The effect of mixed C3–C4 communities was limited in the moderate flooding zone (MFZ), but became negative in the severe flooding zone (SFZ), which can be attributed to the different drivers of CH₄ response ratio among different photosynthetic-type plants. In C3 plant population, SOC, EOC, NH₄⁺, SoilT, and nitrate reductase are the main factors; in C4 plant population, TP, NH₄⁺, and NO₃⁻ are the main factors; in combined C3 and C4 plant population, MBN and SOC are the main factors. These findings clarify how plant photosynthetic type interacts with flooding regime to regulate CH₄ efflux from reservoir drawdown zones and provide critical process-level evidence for improving greenhouse gas inventories and mitigation assessments in managed wetland ecosystems.