The application of manure and straw is beneficial for improving the content and stability of DOM in paddy soil.

While organic manure application effectively increases soil organic carbon (SOC) content, it may elevate greenhouse gas emissions. Crop straw, a widely available agricultural residue, enhances SOC through gradual decomposition. The effect of organic manure combined with crop straw on the organic carbon components of paddy soil is still unclear. Here, we conducted a field experiment to investigate the effects of organic manure alone versus combined application with crop straw on soil organic carbon (SOC) fractions, with a focus on dissolved organic matter (DOM) in paddy soil. The treatments included the following: organic manure replaced 100 % nitrogen fertilizer (LOM), 150 % nitrogen fertilizer (HOM), LOM combined with crop straw (LOMS), HOM combined with crop straw (HOMS), and conventional fertilization (NPK). The study quantified SOC fractions, DOM concentrations and optical properties, and iron/manganese (Fe/Mn) fractions etc., revealing that combined organic manure significantly enhanced microbial biomass carbon (MBC) and dissolved organic carbon (DOC) in topsoil (0-20 cm). Notably, compared to organic manure alone, the combined application of manure and crop straw increased DOM molecular weight in topsoil (0-20 cm) while elevating chromophoric and aromatic DOM content in subsoil (20-40 cm). The combined application of organic manure and crop straw facilitated the transformation of iron Fe/Mn fractions, and Fe/Al oxides cooperated to promote the stability of soil DOM. Additionally, the changes in Fe/Mn oxide morphology appear to exert distinct effects on soil DOM stabilization processes. Furthermore, crop straw incorporation significantly increased plant-derived DOM containing aromatic structures. These findings demonstrate that the combined application of organic manure and crop straw not only enhances DOM content and stability throughout the soil profile, but also mitigates potential environmental risks (especially CO emission) associated with organic manure alone.