Biochar effects on crop yields and nitrogen loss depending on fertilization

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. N2O fluxes, NH3 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−1 yr−1. BC application increased NH3 volatilization by 31% in wheat season and 26% in maize season because of pH increase. BC reduced N2O emissions by 8–23% in the wheat season and by 24% at lower N rates (≤60 kg ha−1) in the maize season, due to BC induced complete denitrification to N2. BC stimulated N2O emissions by 18–26% compared to soils without BC in maize season at N rates higher than 60 kg ha−1. The combination of increased mineral N retention and C availability with BC addition increased nitrification and/or denitrification rates, leading to increased N2O emissions. For the wheat/maize rotation system, BC application decreased N2O emissions at lower N rates (≤120 kg ha−1 yr−1) but had no effects at higher N rates.