Shrub encroachment into grasslands is a global phenomenon with far - reaching implications for ecosystem carbon and nitrogen cycles. Yet, the impacts of anthropogenic shrub encroachment on soil carbon - nitrogen coupling remain largely unknown. Here, we examined a desert grassland - shrubland mosaic formed by nearly 40 years of anthropogenic shrub encroachment. We partitioned the mosaic into four transition sites and selected microhabitats for in - situ mineralization experiments. Our results indicated that transition sites and incubation time significantly influenced soil nitrogen and carbon parameters. As the ecosystem shifted from desert steppe to shrubland, there were notable decreases in inorganic nitrogen (-10.99 %), organic carbon (-97.33 %), and extracellular enzyme activity (-130.77 %), with concomitant reductions in net nitrogen mineralization (-102.47 %), organic carbon conversion rates (-157.14 %), and net nitrification rates (-249.14 %). Contrasting these patterns, soil respiration (+30.49 %) and ammonium nitrogen (+8.93 %) exhibited significant increases, while microbial biomass carbon remained statistically invariant across the vegetation shift. We also observed a shift from microbial mineralization to immobilization during the transition. Strong coupling was identified between carbon and nitrogen contents and conversion rates (p < 0.05). The net nitrification rate emerged as the key factor in carbon - nitrogen coupling due to the decoupling of ammonification and carbon transformation. Our work provides novel insights into the complex interplay of soil carbon and nitrogen under anthropogenic shrub encroachment.