Doping of nitrogen (N) on carbon materials often requires complicated synthetic steps or specific machinery. In this study, N-doped biochar was successfully synthesized by simply ball milling pristine biochar with ammonium hydroxide. The content and species of N in the resulting N-doped biochars were carefully characterized, the formation mechanisms of N-related groups were illustrated, and their applications in CO2 uptake and reactive red removal were evaluated. Most of N introduced to biochar was loaded on its surface in forms of -NH2 and C equivalent to N, resulting from the dehydration of -COOH and -OH. Biochars produced at 450 degrees C were doped with more N on the surface (XPS, 2.41%-2.65%) than those produced at 600 degrees C (XPS, 1.18%-1.82%) because the content of O-containing functional groups in biochar decreases with increasing pyrolysis temperature. The basic properties of N-related groups enhanced the sorption performance of biochar to both acidic CO2 and anionic reactive red. For example, the CO2 uptake of the N-doped, 450 degrees C hickory biochar was 31.6%-55.2% higher than the corresponding pristine and ball-milled biochars. The maximum sorption capacity of the N-doped, 600 degrees C bagasse biochar to reactive red was about 3.66-16.2 times of the corresponding biochars without N doping. This paper provides an alternative and facile approach to prepare N-doped biochar that can be extended to other carbon materials to meet the specific needs in different applications, especially adsorption.