Abstract: Soil water content and nitrogen availability are important factors regulating nitrification rate and ammonia-oxidizing microorganisms. In this study, a microcosm experiment with three soil water content levels (40% WFPS, 60% WFPS, 80% WFPS; WFPS, water filled pore space) × four nitrogen addition levels (0, 25, 50 and 100 mg N·kg^-1), was carried out using Ultisol to study the effects of soil water content and nitrogen addition on net nitrification rate and the abundance of functional gene (amoA) of ammonia-oxidizing microorganisms. Results showed that soil net nitrification rate increased with increasing soil water content and nitrogen addition levels. Increasing soil water content significantly increased the abundance of AOA and AOB amoA genes, but decreased the abundance of comammox Nitrospira clade A amoA gene, indicating that increasing soil water content suppressed the growth of comammox Nitrospira clade A. In contrast, nitrogen addition significantly increased the abundance of AOA and AOB, while did not affect the abundance of comammox Nitrospira clade A. Taken together, increasing soil water content was detrimental to the growth of comammox Nitrospira in upland agricultural Ultisol, while nitrogen addition did not influence the abundance of comammox Nitrospira.

Key words: Ultisol, soil moisture content, nitrogen addition, nitrifiers, comammox Nitrospira