Nitrogen-rich agricultural waste, soybean straw, was used as a raw material to prepare high efficiency CO₂ adsorbents (nitrogen-doped porous modified biochars). Three different modification methods for the preparation of these adsorbents were compared, i.e. activation with carbon dioxide, ammonification with ammonia (NH₃) and high temperature treatment with the mixture of CO₂ and NH₃. Effects of modification temperature on physicochemical properties of the modified biochars and influences of adsorption temperature on their CO₂ capture performances were both investigated. Activation with CO₂ obviously developed the pore structure of modified biochars, especially micropores, while the ammonification with NH₃ and modification with the mixture not only developed porosity, but also introduced nitrogen functional groups, and the modification with the mixture was better than the ammonification with NH₃. As the modification temperature increased, the micropore surface area and N/C ratio of the modified biochars by the modification with the mixture both increased first, and reached the maximum at 800 °C, and then decreased. Furthermore, at the lower adsorption temperature, the micropore structure played an important role to influence the CO₂ capture performance, while at the higher adsorption temperature, the chemical properties, especially the nitrogen functional groups, contributed more to the CO₂ capture.