Medium molecular weight carbon fractions of DOM: Driving soil microbial community differentiation and soil organic carbon sequestration

Soil dissolved organic matter (DOM) is vital for energy and nutrient dynamics in terrestrial ecosystems and significantly influences soil microbial diversity and community structure. However, the impact of long-term organic substitutes (straw and manure) on the molecular composition of DOM and their relationships with organic carbon composition and microbial community structure remain unexplored, particularly in fluvo-aquic soils. This study conducted a systematic investigation utilizing Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), biomarker analyses, microbial high-throughput sequencing, and carbon source metabolism assessments over a 33-year localization experiment. Results showed that long-term organic substitution significantly enhanced the abundance of complex organic molecules (CHONs, CHOSs, and CHONSs) while reducing simple compounds (CHOs). Medium molecular weight carbon fractions (amino sugar, carbohydrate, tannin , and condensed aromatic) increased to an average of 31.1–32.5 %, whereas large (lignin; 6.81–8.10 %) and small (unsaturated hydrocarbon, lipid, and peptide; 1.62–13.0 %) molecular weight carbon fractions decreased. Medium molecular weight carbon fractions showed a significant positive correlation with soil organic carbon (SOC) contents (encompassing particulate, mineral-associated, plant-derived, and bacterial necromass carbon), facilitating SOC sequestration . Furthermore, the modularity of microbial communities (fungi and bacteria) and abundance and diversity of carbon source utilization correlated positively with medium molecular weight carbon fractions, thereby enhancing microbial community stability. Fungal communities exhibited greater sensitivity to these carbon fractions than bacterial communities . The result highlights the critical role of medium molecular weight carbon fractions in shaping microbial community structures and enhancing SOC sequestration , providing a scientific basis for optimizing agricultural management practices and promoting sustainable agricultural development . ● Long-term organic substitution altered soil DOM composition, increasing complexity. ● Medium molecular weight (MM w ) carbon fractions in DOM were particularly increased. ● MM w drive microbial community differentiation and boost SOC protection. ● Fungal communities exhibited greater sensitivity than bacterial communities. ● Plant C contributes the most to POC and ROC, while BNC mainly contributed to MAOC.