Adsorption-type carbon materials are promising candidates for fast sodium-ion storage via surface physisorption or chemisorption of sodium ions. Post-treatment of carbon such as ammonia can simultaneously strengthen both adsorption effects by increasing the specific surface area for physisorption, and by introducing active nitrogen dopants for chemisorption. Which factor, however, the increment of specific surface area or the introduction of active nitrogen dopants, predominantly contributes to sodium-storage capacity increment, remains a question. Answering this question is of great importance for understanding the sodium storage mechanism of adsorption-type carbon materials and thereby optimizing their sodium storage capabilities. In this work, pristine carbon is thermally treated in ammonia at temperatures from 600 to 1200 degrees C, resulting in a simultaneous increase of specific surface area (8.6-1155 m(2) g(-1)) and active nitrogen dopants (0.75-6.47 wt%). Correlations between sodium storage capacity and specific surface area/active nitrogen dopants are established. It is found that as the post-treatment temperature increases, the capacity increment is contributed first by sodium physisorption on active surface, and then by sodium chemisorption on nitrogen dopants. Our findings enrich the mechanistic understanding of sodium storage in adsorption-type carbon materials, which may guide the rational designs of carbon materials for high-rate sodium-based energy storage systems.