Response of N2O emissions to N fertilizer reduction combined with biochar application in a rain-fed winter wheat ecosystem

Optimizing nitrogen application rate and biochar application is an important management measure to reduce N loss and increase yield in farmlands. However, the combined effects and mechanisms of different biochar amendment rates with N fertilizer reduction on soil N2O emissions have not been fully evaluated in rain-fed areas. Therefore, a three-season field trial was conducted in a winter wheat field including six treatments: no fertilizer (N0), conventional N fertilizer (CN: 226.5 kg N ha-1, 100%), reduced N fertilizer (RN: 140.3 kg N ha-1, 62%), RN+ 5 t ha-1 biochar (RNB5), RN+ 10 t ha-1 biochar (RNB10), and RN+ 20 t ha-1 biochar (RNB20). Our results showed that the cumulative N2O emissions ranged from 0.35 kg N ha-1 to 2.33 kg N ha-1 in all fertilizer treatments, of which 45.29-90.32% mainly occurred after N fertilizer application. RN treatment reduced N2O emissions compared with CN treatment. In comparison with the RN, RNB5 treatment significantly (P < 0.05) decreased the cumulative N2O emissions, owing to low inorganic N contents and increase in the abundance of nosZ gene. No significant variation in N2O emissions was found between RNB10 and RN treatments. Whereas, N2O emissions significantly (P < 0.05) increased by 18.42% in 2016-2017 season in RNB20 treatment. Soil moisture, NH4+-N and NO3--N contents, and nirS, nirK, and nosZ abundance were higher on RNB20 treatment than other treatments and showed significant positive correlation with N2O emissions (P < 0.05). In addition, RNB10 and RNB20 treatments significantly (P < 0.05) improved the mean crop N uptake and yield, compared with RN and RNB5 treatments, while no significant variation was found between RNB10 and RNB20 treatments. Biochar addition combined with N reduction reduced yield-scaled N2O emissions by 7.57-12.93%. Given the high production efficiency and sustainability, N fertilizer reduction (140.3 kg ha-1) combined with biochar applied at 10 t ha-1 resulted in increasing yield while minimizing N2O emissions in a rain-fed winter wheat ecosystem.