Karstification forms tremendous karst carbon sinks in the Earth. Whether terrestrial higher plants can absorb and utilize bicarbonate, there is a key testimony that karst carbon sinks can be transformed into carbon sequestrations by terrestrial higher plants. The uptake and use of root-derived bicarbonate, photosynthesis, phosphoenolpyruvate carboxylase and ribulose-1,5-bisphosphate carboxylase contents of Broussonetia papyrifera (Bp) and Morus alba L. (Ma) were measured. This study provides the most direct and primary evidence for the transformation using the bidirectional isotope tracer technique. The transformation may result from the synergism in the absorption and utilization of photosynthetic and nonphotosynthetic pathway, and simultaneously strengthen karst carbon sink and carbon sequestrations of plants, while it had no effect on photosynthetic CO2 assimilation in leaves. Differences in the transformation result in the discrepancies of Bp and Ma in the adaptation to karst environments. Karst-adaptable plants can more regulate the entire carbon cycle.