Biochar (BC) application to low fertility soils is a promising approach to increase crop yield, improve soil quality, and mitigate climate change simultaneously. Only few studies evaluated the combined effects of BC and nitrogen (N) fertilization rates on crop productivity and N losses under field conditions. The objectives were to investigate combined effects of BC (2 rates) and N (5 rates) fertilization on crop productivity and N losses in a long-term field experiment started in 2008 in a winter wheat/summer maize rotation system in the North China Plain. Linear-plateau models best described the responses of wheat and maize yields to N rates. NO fluxes, NH volatilization, and soil mineral N contents increased exponentially with N fertilization rates. Despite the effect of BC on wheat or maize yields was negligible, BC retains of mineral N at 240 kg N ha yr. BC application increased NH volatilization by 31% in wheat season and 26% in maize season because of pH increase. BC reduced NO emissions by 8-23% in the wheat season and by 24% at lower N rates (≤60 kg ha) in the maize season, due to BC induced complete denitrification to N. BC stimulated NO emissions by 18-26% compared to soils without BC in maize season at N rates higher than 60 kg ha. The combination of increased mineral N retention and C availability with BC addition increased nitrification and/or denitrification rates, leading to increased NO emissions. For the wheat/maize rotation system, BC application decreased NO emissions at lower N rates (≤120 kg ha yr) but had no effects at higher N rates.