Nonmicrobial mechanisms dominate the release of CO2 and the decomposition of organic matter during the short-term redox process in paddy soil slurry

Abstract

Both biotic and abiotic mechanisms play a role in soil CO₂ emission processes. However, abiotically mediated CO₂ emission and the role of reactive oxygen species are still poorly understood in paddy soil. This study revealed that OH promoted CO₂ 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 CO₂ efflux was 10%–33% in topsoil and 40%–77% in deep-soil slurries. Net CO₂ efflux was higher in topsoil slurries, which contained more dissolved organic carbon (DOC). CO₂ efflux correlated well with DOC contents, suggesting the critical role of DOC. Microbial mechanisms contributed 9%–45% to CO₂ production, as estimated by γ-ray sterilization experiments in the short-term reoxidation process. Solid-aqueous separation experiments showed a significant reduction in net CO₂ efflux across all soil slurries after the removal of the original aqueous phase, indicating that the water phase was the main source of CO₂ 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 CO₂ emission in the global C cycle, and the critical influence of the aqueous phase on C loss in paddy environments.