Nonmicrobial mechanisms dominate the release of CO and the decomposition of organic matter during the short-term redox process in paddy soil slurry.

Both biotic and abiotic mechanisms play a role in soil CO emission processes. However, abiotically mediated CO emission and the role of reactive oxygen species are still poorly understood in paddy soil. This study revealed that •OH promoted CO emission in paddy soil slurries during short-term oxidation (4 h). •OH generation was highly hinged on active Fe(II) content, and the •OH contribution to CO efflux was 10%-33% in topsoil and 40%-77% in deep-soil slurries. Net CO efflux was higher in topsoil slurries, which contained more dissolved organic carbon (DOC). CO efflux correlated well with DOC contents, suggesting the critical role of DOC. Microbial mechanisms contributed 9%-45% to CO production, as estimated by γ-ray sterilization experiments in the short-term reoxidation process. Solid-aqueous separation experiments showed a significant reduction in net CO efflux across all soil slurries after the removal of the original aqueous phase, indicating that the water phase was the main source of CO emission (>50%). Besides, C emission was greatly affected by pH fluctuation in acidic soil but not in neutral/alkaline soils. Fourier transform ion cyclotron resonance mass spectrometry and excitation-emission matrix results indicated that recalcitrant and macromolecular dissolved organic matter (DOM) components were more easily removed or attacked by •OH. The decrease in DOM content during the short-term reoxidation was the combined result of •OH oxidation, co-precipitation, and soil organic matter release. This study emphasizes the significance of the generally overlooked nonmicrobial mechanisms in promoting CO emission in the global C cycle, and the critical influence of the aqueous phase on C loss in paddy environments.