Water-level operation creates redox windows that govern greenhouse-gas pathways in reservoir drawdown soils

Reservoir drawdown zones repeatedly alternate between inundation and exposure, yet the mechanisms linking hydrologic fluctuations to greenhouse gas (GHG) emissions remain unresolved. Here we tested whether water-level-driven redox transitions and associated electron-acceptor dynamics could explain observed variations in soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes, and whether vegetation management modifies these relationships. Across an annual water-level cycle, we monitored (i) soil physicochemical and redox conditions (moisture, redox potential (Eh), iron/manganese (Fe/Mn) redox proxies, inorganic electron acceptors), (ii) microbial functional potential using compact qPCR-derived pathway indices (log₁₀(mcrA/pmoA), log₁₀(mtrB/mcrA), and log₁₀((nirS + norB)/(nosZ-I + nosZ-II))), and (iii) in situ chamber-based CO₂, CH₄, and N₂O fluxes under contrasting vegetation treatments. Rising water levels rapidly drove soils toward wetter, more reduced states, reorganizing inorganic electron acceptors consistent with sequential terminal electron-accepting processes. Microbial indices primarily reflected hydrologic and redox context rather than vegetation management alone. Among GHGs, CO₂ flux displayed the most consistent sensitivity to hydrologic fluctuations, whereas CH₄ and N₂O emissions occurred episodically within distinct redox windows. Overall, our findings support a unified, water-level-regulated redox framework that can be used as a risk-based diagnostic to prioritize monitoring and inform drawdown-zone management in regulated reservoirs.