Soil mulching influences soil organic carbon (SOC) dynamics and microbial communities, yet the functional potential linking these factors remains unclear. This two-year field study compared biodegradable liquid film (BLF, C:N = 26:1, 161 kg C·ha⁻¹) and grapevine branch mulch (GBM, C:N = 51:1, 2790 kg C·ha⁻¹) applied in-row in a vineyard, with clean tillage as control. The Vitis vinifera cv. Meili was used as the test material, SOC fractions were determined and metagenomic sequencing was performed. The results showed that GBM had the highest SOC content and significantly increased the levels of total organic carbon, all five labile fractions, and the three recalcitrant fractions. BLF significantly increased the levels of recalcitrant fractions, while its effect on labile fractions varied by year. Metagenomic analysis revealed that the two mulching treatments significantly influenced the abundances of Acidobacteria, Verrucomicrobia, and Bacteroidetes. Redundancy analysis identified soil moisture, pH, SOC, and total nitrogen as key drivers of community structure. Mulching downregulated carbon fixation and methane metabolism genes but upregulated carbohydrate metabolism pathways, including O-glycan biosynthesis, which correlated positively with SOC. Glycosyl transferases were the dominant carbohydrate-active enzymes across all treatments. These results demonstrate that GBM and BLF differentially affect SOC fractions and microbial functional traits, providing empirical evidence for mulch selection in vineyard carbon management.