Abstract:【Objective】Biochar is an important soil amendment material, as it plays an increasingly prominent role in carbon sequestration and greenhouse gases mitigations, especially in nitrous oxide (N2O) mitigation. To investigate the effects and the mechanisms of the biochar aging process on soil N2O emissions under a rice-wheat rotation system, an in situ field experiment was conducted.【Method】Five treatments were established as follows: CK (without urea and biochar), N (urea), NB0y(urea with fresh biochar), NB2y(urea with 2-year aged biochar) and NB5y(urea with 5-year aged biochar). Soil N2O emission dynamics were monitored during rice and wheat annual rotation. Also, soil physicochemical characteristics and the abundance of relevant microbial functional genes during the N2O production process were determined after rice and wheat harvest.【Result】Biochar treatment significantly reduced the cumulative N2O emissions by 32.4%-54.0%, with the reduction capacity following NB0y> NB2y> NB5y. Compared with the N treatment, NB0y, NB2y and NB5y treatment significantly increased soil pH by 0.6-1.2 units, soil NO3--N content by 1.7%-31.3%, and soil organic carbon content by 21.4%-58.6%. Nevertheless, the ability of biochar to improve soil pH was decreased with aging. Also, NB0y, NB2y and NB5y treatments significantly increased the abundance of nosZ gene by 54.9%-249.4%, and the soil (nirS +nirK)/nosZ ratio increased with the biochar age. Meanwhile, the cumulative N2O emissions showed a significant negative correlation with soil pH, and a positive correlation with NO3--N content and amoA -AOB gene abundance.【Conclusion】In summary, both fresh and aged biochar can significantly improve soil physical and chemical properties and decrease soil N2O emissions, but the effect of fresh biochar was better than that of aged biochar. The mitigation capacity of aged biochar decreased mainly due to the increase of soil NO3--N content and (nirS +nirK)/nosZ ratio.