Soil Populations of Rhizobium japonicum in a Cotton-Corn-Soybean Rotation
1985
Hiltbold, A. E. | Patterson, R. M. | Reed, R. B.
The ability of legume root nodule bacteria to live in soil in the absence of host plants has great practical value. Since soybean [Glycine max (L.) Merrill] is extensively grown and its value is largely dependent upon effective N₂ fixation, it is important to develop an understanding of the ecology of its nodule bacteria, Rhizobium japonicum. In order to determine seasonal trends in R. japonicum populations in relation to cropping and soil fertility conditions that severely limit crop production, selected treatments in a long-term field experiment were sampled at approximately 30-d intervals over a 2-yr period for R. japonicum numbers in soil of the plow layer. Soil was serially diluted and applied to surface-sterile soybean seed in growth pouches to determine most probable number (MPN). Numbers of R. japonicum in soil were maximum during the winter after the soybean crop, exceeding 10⁶ per gram soil. Populations declined rapidly during the following summer under cotton (Gossypium hirsutum L.). Where lime, P, and K were adequate, R. japonicum numbers stabilized at 10⁴ to 10⁵ per gram, while in soil deficient in P or K numbers fell to 10³ to 10⁴ per gram. In acid soil (pH 4.6), R. japonicum decreased to less than 10² per gram. Minimum populations occurred about 1 October under cotton with increasing numbers into the winter, particularly where lime, P, and K were adequate. In the following year under corn (Zea mays L.), similar cycles were found where populations declined from winter to summer and then increased into fall and winter. After rye (Secale cereale L.) harvest and planting soybeans in the third year, R. japonicum numbers increased rapidly to approach 10⁶ per gram. Vetch (Vicia dasycarpa Tenore) and crimson clover (Trifolium incarnatum L.), while influential on crop yields, had no effect on R. japonicum numbers. Similarly, fertilizer N did not affect R. japonicum populations in soil where pH was maintained by liming. Soil acidity was the most severe stress on R. japonicum, followed in order by P and K deficiencies.
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