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A Field Study of a Soil-Soybean Plant-Rhizobium System Amended with Cadmium¹

ABSTRACT

Soybean plants [Glycine max (L.) Merr.) were cropped over a period of 3 years, under field conditions, in a soil amended with Cd (as CdCl₂·2 H₂O) at rates corresponding to 0, 3.3, 12.5, and 22.8 kg Cd/ha in order to evaluate the effect of Cd on seed yield, Cd accumulation in seeds, and the distribution of indigenous Rhizobium japonicum strains. Cadmium migration from the cultivated layers to the subsoil during 3 years was not significant. Soybean seed yields were affected significantly only when DTPA-TEA-extractable Cd was > 1.57 µg Cd/g in the cultivated layers of the soil, suggesting that only in cases of extreme Cd contamination would significant yield reduction occur. Cadmium content of seeds increased from year to year with values > 1 µg Cd/g being found only when Cd had been added. Grain yield decreased only in the high Cd treatments. Therefore, grain yield would be a poor indicator of Cd contamination in a soil cropped with soybeans. The results also suggested DTPA-TEA-extractable soil Cd does not accurately measure the readily available pool of Cd for soybean plants. Rather Cd content of seeds appears to be a better indicator of the extent of Cd contamination. Rhizobium japonicum strain distribution was not affected by the presence of Cd in the soil. Serogroups 24, 31, 76, and 94 dominated the area surveyed.

Key Words: Rhizobium japonicum • serology • heavy metals

¹ Paper no. 6088 of the Journal Series of the North Carolina Agric. Res. Serv., Raleigh, NC 27650.

² Former Graduate Student and Professor of Soil Science, North Carolina State Univ., Raleigh, N.C., respectively. Dr. Borges is currently Associate Professor, Depto. de Microbiologia, Univ. Fed. de Vicosa, Minas Gerais, Brazil.

Received for publication July 30, 1979.