Can the co-application of biochar and different inorganic nitrogen fertilizers repress N 2 O emissions in acidic soil?

The sole application of nitrogen (N) fertilizer with lower N2 O emission potential or combined with biochar may help for mitigating N2 O production. However, how biochar applied with various inorganic N fertilizers affected N2 O emission in acidic soil remains unclear. Thus, we examined N2 O emission, soil N dynamics and relating nitrifiers (i.e., ammonia-oxidizing archaea, AOA) in acidic soil. The study contained three N fertilizers (including NH4 Cl, NaNO3 , NH4 NO3 ) and two biochar application rates (i.e., 0% and 0.5%). The results indicated that the alone application of NH4 Cl produced more N2 O. Meanwhile, the co-application of biochar and N fertilizers enhanced N2 O emission as well, especially in the combined treatment of biochar and NH4 NO3 . Soil pH was decreased with the application of various N fertilizers, especially with NH4 Cl, and the average decrease rate was 9.6%. Meanwhile, correlation analysis showed a negative relationship between N2 O and pH, dramatically, which might indicate that the alteration of pH was one factor relating to N2 O emission. However, there was no difference between the same N addition treatments with or without biochar on pH. Interestingly, in the combined treatment of biochar and NH4 NO3 , the lowest net nitrification rate and net mineralization rate appeared during days 16-23. Meanwhile, the highest emission rate of N2 O in the same treatment also appeared during days 16-23. The accordance might indicate that N transformation alteration was another factor relating to N2 O emissions. In addition, compared to NH4 NO3 alone application, co-applied with biochar had a lower content of Nitrososphaera-AOA, which was a main contributor to nitrification. The study emphasizes the importance of using a suitable form of N fertilizers and further indicates that two factors, namely alteration of pH and N transformation rate, are related to N2 O emission. Moreover, in future studies, it is necessary to explore the soil N dynamics controlled by microorganisms.