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Effect of Cadmium on Soil Bacteria and Actinomycetes¹

ABSTRACT

Soil procaryotic populations were evaluated from a soybean field (Glycine max L.) that had been amended 3 years previously with Cd (as CdCl₂·2H₂O) at rates corresponding to 0, 6, 22, and 40 kg Cd/ha. Replicate aliquots of soil suspensions from the treated plots were plated on media containing increasing concentrations of Cd. This was done to determine if soil bacteria and actinomycetes (procaryotes) that were tolerant of Cd had developed in Cd-treated soils. It was found that the level of DTPA-TEA extractable soil Cd had no influence on the development of Cd-tolerant soil populations on media amended with Cd. Rather, it was found that low levels of media Cd retarded procaryote growth, whereas high levels did not. This also occurred regardless of the level of soil extractable Cd. Soil moisture at the time of sampling was found to be inversely related to procaryote numbers on media containing 0 ppm Cd and directly related on media containing 10, 20, and 40 ppm Cd. On media containing 5 ppm Cd, numbers of soil organisms were low with respect to the control and other Cd media levels and had no relationship to soil moisture. Growth rate constants of 58 pure cultured bacteria were determined at Cd concentrations of 0, 5, 10, 20, and 40 ppm in liquid media. The organisms exhibited tolerant, intolerant, and stimulated responses of Cd; however, 50% were intolerant to 5 ppm Cd but tolerant at concentrations > 5 ppm.

Key Words: soil moisture • organic matter • serial dilution

¹ Paper no. 5970 of the Journal Series of the North Carolina Agric. Exp. Stn., Raleigh, NC 27607 and a contribution of the Univ. of Wyoming Agric. Exp. Stn., Laramie, WY 82071. Portions of this paper were presented before Div. S-3, Am. Soc. of Agron., Houston, Tex., 2 Dec. 1976.

² Former Graduate Student at North Carolina State Univ., now Assistant Professor of Soil Science at the Univ. of Wyoming, Division of Plant Science, Laramie, WY 82071, and Professor, Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27607, respectively.

Received for publication May 18, 1979.