Nitrite production by Nitrosomonas europaea and Nitrosospira sp. AV in soils at different solution concentrations of ammonium

Although it remains unclear why NH3-oxidizing bacteria (AOB) of the genus Nitrosospira dominate soil environments, and why Nitrosomonas spp. are less common, virtually no studies have compared their behavior in soil. In this study, the NH3 oxidation rates of Nitrosomonas europaea (ATCC 19718) and Nitrosospira   sp. AV were compared in three differently textured soils containing a range of extractable NH4+ contents (2–11 μg NH4+–Ng−1 soil). Soils were adjusted to pH 7.0–7.4 with CaCO3 and sterilized with γ-radiation. Cell suspensions of each bacterium were inoculated into the soils to bring them to two-third of water-holding capacity and cell densities ∼2.5×106 g−1 soil. In virtually all cases, rates of NO2− production for both N. europaea and Nitrosospira sp. AV were linear over 48 h, and represented between 13 and 75%, respectively, of the maximum rates achieved in soil-free bacterial suspensions. Soil solution NH4+ concentrations that supported these rates ranged between 0.2 and 1.5 mM. Addition of 21–36 μg NH4+–Ng−1 soil raised soil solution NH4+ levels to 1.8–2.5 mM and stimulated NO2− production to a greater extent in N. europaea (3.3–6.6-fold) than in Nitrosospira   sp. AV (1–2.1-fold). Maximum rates of NO2− production were obtained by raising soil solution NH4+ levels to 3–4 mM with a supplement of ∼80–90 μg NH4+–Ng−1 soil. Ks values in soil for Nitrosospira sp. AV and N. europaea were estimated as 0.14 and 1.9 mM NH4+, respectively, and estimates of Vmax were about 3.5-times higher for N. europaea (0.007 pmol h−1 cell−1) than for Nitrosospira sp. AV (0.002 pmol h−1 cell−1). The cell density of N. europaea   increased in sterile Steiwer soil independent of supplemental NH4+. In the case of treatments receiving supplemental NH4+, growth yields of N. europaea   calculated from either NO2− produced or NH4+ consumed were similar to those reported in literature (3.5×106–6×106 cells μmol−1NH4+). A higher growth yield was measured in the case of zero added NH4+ (2.7×107 cells μmol−1NH4+), indicating that use of organic carbon compounds might have occurred and resulted in some energy sparing. Our results suggest that Nitrosospira spp. with a Ks similar to Nitrosospira   sp. AV may have an advantage for survival in soil environments where soil solution NH4+ levels are less than 1 mM. However, it is apparent that AOB like N. europaea   are poised to take advantages of modest increases in extractable NH4+ that raise soil solution NH4+ levels to about 2.0–2.5 mM.