Can the co-application of biochar and different inorganic nitrogen fertilizers repress N2O emissions in acidic soil?

The sole application of nitrogen (N) fertilizer with lower N2O emission potential or combined with biochar may help for mitigating N2O production. However, how biochar applied with various inorganic N fertilizers affected N2O emission in acidic soil remains unclear. Thus, we examined N2O emission, soil N dynamics and relating nitrifiers (i.e., ammonia-oxidizing archaea, AOA) in acidic soil. The study contained three N fertilizers (including NH4Cl, NaNO3, NH4NO3) and two biochar application rates (i.e., 0% and 0.5%). The results indicated that the alone application of NH4Cl produced more N2O. Meanwhile, the co-application of biochar and N fertilizers enhanced N2O emission as well, especially in the combined treatment of biochar and NH4NO3. Soil pH was decreased with the application of various N fertilizers, especially with NH4Cl, and the average decrease rate was 9.6%. Meanwhile, correlation analysis showed a negative relationship between N2O and pH, dramatically, which might indicate that the alteration of pH was one factor relating to N2O 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 NH4NO3, the lowest net nitrification rate and net mineralization rate appeared during days 16–23. Meanwhile, the highest emission rate of N2O in the same treatment also appeared during days 16–23. The accordance might indicate that N transformation alteration was another factor relating to N2O emissions. In addition, compared to NH4NO3 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 N2O emission. Moreover, in future studies, it is necessary to explore the soil N dynamics controlled by microorganisms.