Can the co-application of biochar and different inorganic nitrogen fertilizers repress NO emissions in acidic soil?

The sole application of nitrogen (N) fertilizer with lower NO emission potential or combined with biochar may help for mitigating NO production. However, how biochar applied with various inorganic N fertilizers affected NO emission in acidic soil remains unclear. Thus, we examined NO emission, soil N dynamics and relating nitrifiers (i.e., ammonia-oxidizing archaea, AOA) in acidic soil. The study contained three N fertilizers (including NHCl, NaNO, NHNO) and two biochar application rates (i.e., 0% and 0.5%). The results indicated that the alone application of NHCl produced more NO. Meanwhile, the co-application of biochar and N fertilizers enhanced NO emission as well, especially in the combined treatment of biochar and NHNO. Soil pH was decreased with the application of various N fertilizers, especially with NHCl, and the average decrease rate was 9.6%. Meanwhile, correlation analysis showed a negative relationship between NO and pH, dramatically, which might indicate that the alteration of pH was one factor relating to NO 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 NHNO, the lowest net nitrification rate and net mineralization rate appeared during days 16-23. Meanwhile, the highest emission rate of NO in the same treatment also appeared during days 16-23. The accordance might indicate that N transformation alteration was another factor relating to NO emissions. In addition, compared to NHNO 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 NO emission. Moreover, in future studies, it is necessary to explore the soil N dynamics controlled by microorganisms.