Sustainable residue management is crucial for enhancing soil carbon sequestration in agroecosystems, yet the microbial mechanisms governing coupled carbon and nitrogen cycling under different tillage regimes in semiarid regions remain inadequately understood. Through a 29-year field experiment (1992-2021) on China's Loess Plateau, we compared no-tillage (NT) with surface residue retention against conventional tillage (CT) with residue incorporation in a rain-fed wheat system. We combined soil chemical analyses with shotgun metagenomics to characterize soil properties and microbial functional potentials. The study demonstrated that long-term NT significantly increased topsoil (0-20 cm) organic C and total N by 29.7 % and 18.5 %, respectively, relative to CT (average of 2020-2021). Metagenomic analysis revealed that NT reshaped the soil microbiome, enriching Proteobacteria (7.1 %-13.3 %) and Acidobacteria (9.7 %-42.7 %), while suppressing genes involved in carbon fixation (-18.1 % on average) and the decomposition of complex plant polymers (-7.5 % on average). Concurrently, NT enhanced the genetic potential for denitrification (narG, +25.4 %; napB, +20.3 %) while maintaining higher nitrate levels. Partial least squares path modeling confirmed that the restructured microbial community under NT directly suppressed carbon fixation (-0.65, P < 0.01) and carbon degradation (-0.92, P < 0.01) pathways. In summary, long-term no-tillage in semiarid croplands fosters a distinct soil-microbe system. This system enhances carbon stabilization but also drives a nitrogen cycle characterized by elevated nitrate levels and high denitrification potential. Consequently, it raises the risks of nitrate leaching and nitrous oxide emissions, a trade-off that requires coordinated nitrogen management.