Ammonia-oxidizing archaea (AOA), bacteria (AOB) and complete ammonia oxidizers (comammox) take an essential part in soil nitrogen (N) cycling. In case of 8 years biochar and N fertilizer application, nevertheless, the comparative contribution of comammox, AOA and AOB to nitrification is unclear. Our long-term field study on alkaline soil (pHwater 8.49) investigated the impact of biochar and N fertilizer on nitrification rate and potential ammonia oxidation in terms of the soil properties, both physical and chemical, together with the abundance and diversity of AOA, AOB and comammox. The findings revealed that biochar and N fertilizer did not affect the amoA abundance of AOA but increased it in AOB and commamox clade A and clade B, but decreased the diversity of AOA and comammox. Nitrogen fertilization significantly declined the diversity of AOB. The AOB abundance was controlled mainly by pH, whereas the AOA, AOB and comammox Chao1 index were governed by soil microbial biomass carbon (MBC). Additionally, biochar increased the soil nitrification rate with or without N fertilizer, but N fertilizer decreased the soil nitrification rate and potential ammonia oxidation (with nitrite oxidation inhibited) under the same biochar application. Biochar and/or N fertilizer increased the corresponding contribution of AOA and comammox to ammonia oxidation by 143-156 % and 33-395 %, respectively, but suppressed the contribution of AOB. Although the abundance of AOA amoA was obviously greater than that of AOB and comammox, the highest contribution to ammonia oxidation was by AOB in soil not fertilized by N and by comammox in the N-fertilized soil. Our findings suggest that changes in soil properties brought about by biochar and N fertilizer addition can influence the diversity of AOA, AOB and comammox, affecting potential ammonia oxidation and the nitrification rate. The AOB and comammox make a bigger contribution to nitrification than AOA in the alkaline soil.
