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Formation of Free Amino Acids in Rhizosphere and Nonrhizosphere Soil¹

ABSTRACT

Untreated samples of nonrhizosphere and soybean rhizosphere soils each contained about 15 identified free amino acids totaling 2 to 4 µg. per g. of soil; lysine was the most prevalent amino acid in each preparation. Numerous additional unidentified compounds occurred at concentrations estimated as 0.1 to 0.5 µg. per g. Treatment with glucose and potassium nitrate increased the amount of free amino acids to about 100 µg. per g. after 3 days. Concentrations declined after 3 days but still were 4 to 5 times that of the untreated control after 2 weeks' incubation. Glutamic acid was the dominant amino acid in all treated soils. Rhizosphere soil did not differ quantitatively from nonrhizosphere in samples treated with glucose, although a greater variety of ninhydrin reacting compounds was encountered in rhizosphere soil. Treated soils incubated at 20% field moisture capacity differed little in free amino acids from those held at 30%. The features of the free amino acid fraction are discussed.

¹ Paper No. 4485, Scientific Journal Series, Minnesota Agr. Exp. Sta., University of Minnesota, St. Paul. Contribution from Department of Soils and Department of Bacteriology. Data taken from a thesis submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree. Presented before Div. III, Soil Science Society of America, Cincinnati, Ohio, Nov. 18, 1959.

² Formerly Research Assistant in Soils, University of Minnesota, now Assistant Professor, Department of Soil Science, University of Saskatchewan, Saskatoon, Can., and Associate Professor, jointly, Department of Soils and Department of Bacteriology, University of Minnesota, respectively.

Received for publication October 21, 1960. Accepted for publication March 27, 1961.