No. Nitrosomonas sp. oxidizes ammonia to nitrite. This activity (called “nitration”) is part of nitrification (nitrogen’s complete oxidation to nitrate). Here I focus on oxidation and reduction (redox) of nitrogen atoms to distinguish oxygen and hydrogen redox from that of nitrogen, which is the element of interest here.

Nitrogen has either three or five valence electrons, corresponding to the charge states 3- (reduced, ammonia) 0 (neutral, atmospheric or molecular nitrogen), and oxidized to either 3+ (nitrous acid, “nitrite”) or 5+ (nitric acid, “nitrate”). Most nitrogen-cycle interconversions are carried out by soil microorganisms, I’ll begin the cycle with nitrogen fixation, which involves bacteria other than Nitrosomonas. Microorganisms that do not participate in nitrogen cycling are not mentioned here.

Biological nitrogen conversions include

• nitrogen fixation, N0 → N3- (free-living Azotobacter or plant-associated Rhizobium), also known as ammonification
• nitrification,
  • N3- → N3+ (Nitrosomonas) and N3+ → N5+ (Nitrobacter)
  • assimilation into microbial and non-microbial biomass
• denitrification, N5+ → N0 (Pseudomonas, Micrococcus, and othe nitrate-reducing bacteria)

Plants, fungi, and certain other eukaryotes benefit directly from the activity of prokaryotic soil microorganisms. Benefits to animals and animal-like eukaryotes are indirect.