Changes in Cytoplasmic Carbon and Nitrogen Pools in a Soil Bacterium and a Fungus in Response to Salt Stress

The effects of water potential on the cellular compositions of a soil bacterium and a fungus were examined by growing the organisms in media with various KCl concentrations. In media containing up to 1 M KCl, C/N ratios in Aspergillus flavus increased significantly, while those for a Pseudomonas sp. did not. For both organisms, the proportions of cellular C and N in cytoplasm increased by a factor of 10 as salinity increased from 0 to 1 M KCl. Such compositional changes have implications for microbial biomass dynamics in soils of varying water potential and for biomass measurement by chloroform fumigation. </jats:p

Identification of Heterotrophic Nitrification in a Sierran Forest Soil

A potential for heterotrophic nitrification was identified in soil from a mature conifer forest and from a clear-cut site. Potential rates of NO 2 − production were determined separately from those of NO 3 − by using acetylene to block autotrophic NH 4 + oxidation and chlorate to block NO 2 − oxidation to NO 3 − in soil slurries. Rates of NO 2 − production were similar in soil from the forest and the clear-cut site and were strongly inhibited by acetylene. The rate of NO 3 − production was much greater than that of NO 2 − production, and NO 3 − production was not significantly affected by acetylene or chlorate. Nitrate production was partially inhibited by cycloheximide, but was not significantly reduced by streptomycin. Neither the addition of ammonium nor the addition of peptone stimulated NO 3 − production. 15 N labeling of the NH 4 + pool demonstrated that NO 3 − was not coming from NH 4 + . The potential for heterotrophic nitrification in these forest soils was greater than that for autotrophic nitrification. </jats:p