Redox-Driven C–N–Fe Controls on CH4, CO2 and N2O Dynamics in Lake Sediments

Freshwater sediments play a central role in regulating methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) dynamics, yet the biogeochemical constraints shaping their short-term responses to redox change remain poorly resolved. Here, we used controlled aerobic and anaerobic slurry incubations of natural lake sediments to identify the environmental drivers governing early-stage greenhouse gas (GHG) dynamics. CH4 exhibited minimal variation and no significant differences between live and sterilized treatments, indicating that methane turnover during the first hours of incubation is constrained primarily by rapid geochemical adjustments rather than by detectable microbial activity. In contrast, CO2 and N2O displayed clear biotic signals consistent with fast-responding respiratory and nitrogen-reducing processes. Across multivariate analyses and Random Forest models, redox-sensitive solutes (Fe3+, Fe2+, NO3−, SO42−), together with dissolved organic carbon and NH4+, emerged as key components of the biogeochemical framework structuring early GHG responses, highlighting coupled C–N–Fe controls on short-term gas dynamics. Microbial community analyses revealed the presence of methanogenic archaea (e.g., Methanomicrobiales, Methanofastidiosales), aerobic methanotrophs (Methylomonadaceae, Methylococcaceae) and nitrogen-transforming bacteria; however, their functional expression was limited during the short incubation period. Our results demonstrate that the earliest CH4, CO2 and N2O responses in lake sediments are governed predominantly by rapid geochemical processes that regulate electron-acceptor availability and substrate chemistry, while microbial community composition plays a secondary role at short timescales.