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.