SUMMARY: A total of 978 cultures of heterotrophic organisms were isolated from twelve actively nitrifying soils; each isolate was tested for ability to form nitrite or nitrate in glucose peptone broth. None of the isolates yielded substantial amounts of nitrite; concentrations found did not exceed 2 μg. nitrite-N/ml. Almost 7% of the isolates formed nitrite-N in excess of 0·2 μg./ml., while slightly over 2% yielded more than 0·5 μg./ml. Fungus isolates were the most numerous and most active nitrite producers; fifteen of the fungi formed nitrate in addition to nitrite. Concentrations of 5–45 μg. nitrate-N/ml. were recorded for the active fungi. Further isolations of fungi from ten soils of diverse properties resulted in 353 cultures but only three of these formed nitrate. One of these cultures yielded 90 μg. nitrate-N/ml. after 14 days growth in the test medium.

Most of the fungi which produced nitrate (16 of the 18 active cultures) were identified as . A sp., and a sp. also formed low concentrations of nitrate. Each isolate of obtained from soil proved capable of nitrate formation. Strains of were encountered which did not yield nitrate, but these had been obtained from culture collections, and had been carried on artificial media for many years. Some stock cultures of , several cultures of and one of , freshly isolated from wheat seed, also were active nitrate producers. Ammonium sulphate and urea media supported growth of soil fungus isolates but not nitrate formation. Other organic nitrogen substrates in order of increasing effectiveness in nitrate production were: yeast extract, peptone, Protone and casein.


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