Spatial distribution and molecular evidence of CO2 dark production from lake sediments during anoxic-oxic transitions across a watershed

Abstract

Anoxic-oxic interfaces that occur in lake sediments serve as hotspots for CO₂ production. Traditionally, in lake sediments, the oxidative production of CO₂ has been primarily attributed to microbial mechanisms. Although abiotic formation of hydroxyl radicals (OH) can oxidize organic carbon to CO₂ in soils, the specific role of OH in CO₂ production and its spatial distribution remains unexplored in lake sediments. Particularly, the underlying mechanism of CO₂ production from lake sediments during the anoxic-oxic transition is lacking. Here, by integrating field measurements, laboratory incubations, and advanced characterizations, we provide compelling evidence that the coupled iron‑carbon cycles drive the extensive CO₂ production during oxygenation. Surface-adsorbed Fe(II) and structural Fe(II) in sediments are the major drivers of CO₂ production. CO₂ production exhibited significant spatial variabilities across the watershed, with shallow sediments displaying higher CO₂ production due to the abundant Fe(II). The spatial distribution of CO₂ closely mirrors that of OH generated from Fe(II) oxygenation, with OH accounting for a CO₂ flux of 95.61 g C m⁻² d⁻¹. Spectroscopic and microscopic characterizations reveal that OH-mediated carboxyl addition initiates the ring opening of aromatic structures, leading to their depolymerization, demethoxylation, and fragmentation, ultimately producing condensed aromatic compounds and CO₂. Besides, OH facilitates the oxidative decomposition of larger macromolecules to bioavailable low-molecular-weight acids. Altogether, we propose that the Fe(II)-driven OH formation represents a previously unrecognized and prevalent abiotic mechanism of CO₂ production in lake sediments. This new knowledge is valuable for improving the ability to predict CO₂ emissions at hotpots.