Temperature affects the kinetics of nitrite oxidation and nitrification coupling in four agricultural soils

Abstract

The oxidation of ammonia (NH3) to nitrite (NO2−) by archaea and bacteria (AOA and AOB) in soil has been studied intensively for >140 y. However, relatively little attention has been focused on the activity of nitrite oxidizing bacteria (NOB), which catalyze the oxidation of NO2−, despite ample published evidence of soil NO2− accumulation, and the temperature dependent accumulation of NO2− in wastewater and aquatic environments. This study evaluated the response of NO2− oxidizing activity to temperature in four Oregon agricultural soils. Rates of NO2− oxidizing potentials (NOP) were measured at 4–42 °C in all soils. Evaluation of the thermodynamic parameters found that the NOB have a similar temperature range and optimal temperature as the NH3 oxidizer response observed in Taylor et al. (2016). A determination of kinetic parameters of NO2− oxidation at 17, 30, and 37 °C found a trend for the apparent maximum velocity (Vmax) to be greater at 30 than 17 °C, and significant, soil-specific changes in substrate affinity (Km) of NO2− consumption. Although the potential rates of NO2− oxidation exceeded those of NH3 oxidation, NO2− accumulated under at least one temperature in all soils. There were only weak correlations between NO2− accumulation and rates of NH3 or NO2− oxidation, the abundance of Nitrobacter nxrA or Nitrospira nxrB genes or the total (nxrA + nxrB)/amoA gene ratio. The apparent Km did not correlate with NO2− accumulation at any temperature; however, utilizing the obtained Vmax and Km we found that Michaelis-Menten kinetics predict that an accumulation of measurable NO2− is required to drive equal rates of NH3 and NO2− oxidation. Implications of reductions in NO2− affinity by NOB at some temperatures that result in increased NO2− accumulation in soils include plant toxicity and the loss of soil N through increased production of reactive N-oxide gases.