Growth rates of nitrifying chemoautotrophs in soil.

The accurate measurement of growth-rate constants for bacteria in natural environments is generally found to be quite difficult. Estimation of growth rates for the chemoautotrophs concerned with nitrate production, however, may be made. Advantage is taken of the facts that chemoautotrophs are the major, if not the sole, agents concerned with nitrate production in nature, and that the oxidation of ammonium and nitrite compounds is growth-linked. Hence, by measuring the quantity of ammoniumor nitrite-nitrogen oxidized, the rate of growth of the respective bacteria can be ascertained. This was verified using pure cultures of Nitrosomonas europaea by Engel and Alexander (Nature 181:136, 1958). A perfusion system (modified after Lees and Quastel, Biochem. J. 40:803, 1946) was used to determine the growth-rate constants for the nitrifying bacteria in soils. In natural environments, nitrite-oxidizing chemoautotrophs, presumably Nitrobacter spp., are dependent upon Nitrosomonas spp. for substrate (i.e., nitrite). Therefore, the growth rate of the former may be controlled by the activity of the latter. It has been noted that the rate of disappearance of nitrite in a soil that accumulates nitrite early in the nitrification pattern generally exceeds the maximal rate of nitrite formation. Thus it appears possible to adequately characterize the potential growth rate of Nitrobacter spp. only when nitrite is supplied as the energy source. For these experiments, 100-g samples of soil were perfused in triplicate with 400 ml of a solution containing 70 ,ug of either ammoniumor nitrite-nitrogen per ml. With rare exceptions, sluggish nitrifying activity was evidenced when the soil pH was below 5.5 and the responsible chemoautotrophs were found to be present in very small numbers (measured by the most probable number method). Yet these same soils usually showed rapid rates of nitrification after they were adjusted to pH 7.0 or above. Of the measurable environmental factors in soils, only pH, or factors such as calcium, iron, and aluminum which are directly related to pH, was of significance. When growth-rate constants of 116 soils with pH values ranging from 4.4 to 8.8 were correlated with soil pH, the correlation coefficient obtained was 0.768. When pH, calcium