Abrupt temperature shifts alter DOM composition and electron transfer capacity: Unraveling the impact of climate extremes on greenhouse gas emissions in Yangtze River floodplain sediments.

Frequently abrupt temperature shifts driven by global climate change significantly disrupt carbon sequestration and CO2 emissions in wetland ecosystems. The condition has raised an important question to clarify ambiguity in the regulatory mechanism of greenhouse gas (GHG) emissions from some of the world's ecologically important large river floodplain wetland systems. This study investigated how abrupt temperature changes could modulate GHG emissions from surface sediments of the China's Yangtze River floodplain wetlands (YRW). A microcosm experiment was conducted in YRW with four temperature treatments: control (20 °C), abrupt cooling (8 °C), abrupt heating (32 °C), and abrupt heating followed by cooling (32 °C to 8 °C) by simulating the latest daily temperature fluctuations (≥12 °C) in the region. Dissolved organic matter (DOM) composition was analyzed in each treatment using the three-dimensional fluorescence spectroscopy followed by the measurement of concentrations of dissolved organic carbon (DOC), total carbon (TC), and total nitrogen (TN) using a TOC analyzer. DOM redox properties were then evaluated through mediated electrochemical techniques. Our results showed that the abrupt heating increased CO2 emissions, while heating followed by cooling suppressed CO2 emissions but significantly increased CH4 emissions. These responses were clearly linked to changes in the carbon concentration and DOM composition in YRW. Specifically, the abrupt heating may have enriched microbial-derived fulvic acid and increased DOC levels, enhancing the electron-accepting capacity (EAC) of DOM by 40-60 %. Fluorescence spectroscopy measurement showed a 12-30 % increase in microbial-derived DOM components under heating. These results demonstrate that temperature-induced alterations in the DOM structure and redox reactivity in large river floodplain wetland sediments strongly regulate GHG fluxes of both CO2 and CH4, providing an important insight into carbon cycling and help developing improved wetland management strategies under climatic warming in YRW.