【Objective】Microplastics and herbicides are ubiquitous contaminants in agricultural soils; however, their combined impact on soil carbon cycling and dissolved organic matter (DOM) remains underexplored.【Method】In this study, polylactic acid (PLA) microplastics and fomesafen were used as model contaminants to investigate their effects on soil carbon dioxide (CO2) emission and characteristics of DOM through soil incubation experiments as well as ultraviolet/fluorescence spectroscopy techniques.【Result】The results revealed that both PLA microplastics and fomesafen enhanced the CO2 emission rates in Mollisols and red soil during the 7–45 day incubation period. However, by the 60th day, individual treatment of fomesafen reduced the CO2 emission rate in red soil by 14.8% to 21.6%. Notably, the combined presence of fomesafen and PLA microplastics exhibited a synergistic effect, further suppressing the CO2 emission rate in red soil by 54.3% to 79.7%. The addition of 0.1% and 1% PLA microplastics increased the DOM content in both soil types. Ultraviolet/fluorescence spectroscopy analyses indicated that PLA microplastics enhanced the DOM humification and aromaticity of Mollisols. Also, the high level of combined contamination accelerated the transformation of organic matter in red soil. PARAFAC analysis identified three fluorescent components: terrestrial humic-like, short-wave humic, and fulvic acid-like substances. Polylactic acid microplastics significantly increased the content of these components in Mollisols, whereas the combined contamination did not markedly alter the DOM composition in red soil.【Conclusion】This study provides critical scientific data and insights into the effects of microplastics and herbicides on soil carbon cycling, contributing to a deeper understanding of soil ecological health and informing management strategies.