Redox oscillations in riparian zone stimulate carbon loss by enhancing microbial respiration

Redox oscillations within riparian ecosystems emerge as a critical threat to carbon sequestration, yet the mechanistic coupling between abiotic drivers and microbial metabolism remains elusive. Through controlled incubation experiments, we demonstrate that redox-oscillating conditions significantly reduce microbial carbon use efficiency (CUE), thus accelerating carbon loss compared to static oxic or anoxic conditions. Mechanistically, redox oscillations drove the cycling of iron (Fe) species, thereby reducing the amorphous Fe pool and liberating mineral-associated organic carbon (MAOC) composed of substantial biodegradable organic substrates (e.g., lipids and proteins). Concurrently, hydroxyl radicals (•OH) generated during Fe(II) oxidation depolymerize complex aromatic organic matter into labile forms. Integrated metagenomic and metabolomic analyses further demonstrated that redox oscillations significantly reshaped soil metabolite profiles and microbial community. In particular, microbial catabolic pathways such as pentose phosphate pathway and the tricarboxylic acid (TCA) cycle were activated to efficiently mineralize newly available substrate. Together, these results identify a coupled abiotic-biotic "prime and burn" mechanism in which Fe-driven substrate reorganization primes microbial differentiation toward enhanced respiration. This study highlights redox-oscillating zones as potential carbon leakage hotpots in the terrestrial carbon sink.