This study shows that nutrient amendment reduced bacterial diversity, centralizing carbon flow through fewer taxa. Key genera (Bradyrhizobium, RB41, Streptomyces) were responsible for 45–57% of carbon flux. Bacteria that used more glucose also utilized more native carbon, emphasizing the role of specific taxa in carbon balance. Mapping carbon flow through these taxa is vital for refining soil carbon models and enhancing climate change forecasts.