The Effect of Biochar and Straw Return on N₂O Emissions and Crop Yield: A Three-Year Field Experiment

To evaluate the effects of application of biochar and straw return for consecutive years on N2O emissions and crop yields in North China, a three-year field experiment of applying biochar and straw following a ten-year application was conducted in a wheat–maize rotation system. Four treatments were set up, including F (NPK fertilizer only); FB (NPK fertilizer + 9.0 t·ha−1 biochar); FS (NPK fertilizer + straw); and FSB ((NPK fertilizer + 9.0 t·ha−1 biochar combined with straw). The results showed that compared with the F treatment, the FB treatment significantly reduced soil N2O emissions by 20.2%, while the FS and FSB treatments increased it by 23.7% and 41.4%, respectively. The FB treatment reduced soil N2O emissions by 15.1% in the wheat season and 23.2% in the maize season, respectively. The FS and FSB treatments increased the N2O emissions by 20.7% and 36.7% in the wheat season, respectively, and by 25.5% and 44.2% in the maize season, respectively. In the wheat season, the soil water content (SWC), NO3−-N content and pH were the main influencing factors of the soil N2O emissions. In the maize season, SWC and NO3−-N content were the main influencing factors. In addition, the FB, FS and FSB treatments increased the crop yield by 4.99%, 8.40% and 10.25% compared with the F treatment, respectively. In conclusion, consecutive application of biochar can significantly reduce N2O emissions and improve crop yield. Although FS and FSB treatments can also improve the crop yield, they are not beneficial to suppressing N2O emissions. Therefore, the successive application of biochar is an effective measure to reduce N2O emissions and maintain crop yield