Regulation of nitrate assimilation by ammonium in soils and in isolated soil microorganisms

Abstract

The regulation of NO − 3 assimilation by NH + 4 and amino acids was studied to better define the conditions under which raicrobial NO − 3 assimilation could be expected to take place in nature. The assay system was microbial isolates from soil and slurries of two soils enhanced in NO − 3 assimilating capacity by previous exposure to glucose for 18–24 h. The NO − 3 assimilation rate was monitored by a NO − 3 electrode and the inhibition determined by the change in rate following addition of NH + 4 or amino acids. NH + 4 inhibited (by 60%) NO − 3 assimilation immediately (< 1 min) and at very low NH + 4 concentrations (O.1 μg N g −1 soil). At higher NH + 4 concentrations (10 μg N g − soil) a second state of inhibition (80%) occurred after 5 min. The speed of the first inhibition stage suggests that the site of inhibition is the transport of NO − 3 into the cell. Asparagine was the only amino acid to act as rapidly and effectively as NH + 4 in the soil, and thus the amino acids are unlikely to be important regulators of NO − 3 assimilation in soils. Of the three soil isolates studied, Saccharomyces cerevisiae and Pseudomonas fluorescens responded to NH + 4 and amino acids in a manner similar to the soil but NO − 3 assimilation in the third organism, Azotobacter vinelandii, was generally insensitive to these N substrates in this short-term assay. Only high concentrations of NH + 4 caused a partial inhibition of NO − 3 assimilation in A. vinelandii. Due to the effectiveness of low NH + 4 concentrations in inhibiting NO − 3 assimilation in soils, microbial immobilization is not expected to be an important fate of NO − 3 unless there are NH + 4-free microsites in the soil.