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SEASONAL FLUCTUATION OF Rhizobium japonicum UNDER A VARIETY OF FIELD CONDITIONS IN NORTH CAROLINA

Little is known about native populations of rhizoba in soils of the southeastern United States. We undertook this study to observe the influence of season on fluctuations of native Rhizobium japonicum populations in different field environments in physiographically different regions of North Carolina during a 16-month period. Soil samples were collected from the Ap horizons of fields in four physiographic regions of North Carolina every 30 days for a 16-month period. Rhizobium japonicum populations were enumerated using a most probable number (MPN) technique and soybeans (Glycine max (L.) Merr.) as the host for plant infectivity tests. Larger soil populations of R. japonicum exist in cultivated fields with a soybean history than in cultivated fields without soybean histories, pastures, orchards, or forest environments. Seasonal fluctuation was evident in North Carolina soybean fields, with populations being greatest in November 1978 and lowest in March and April 1979. Similar trends but of lower magnitude were observed in cultivated fields without a soybean history. Seasonal distribution differences were not noted in pasture, orchard, or forest environments. Soybean rhizobia were found to constitute from 0.01 to 9.1 percent of the total soil extract, agar-plateable aerobic bacteria population in cultivated fields with a soybean history, from less than 0.001 to 0.01 percent in cultivated fields without a soybean history, and less than 0.001 percent in pastures, orchards, and forested environments. Rhizobium japonicum populations ranged from less than 1 to more than 100000 per gram of soil, depending on the crop present. In general, R. japonicum populations in soybean, corn, wheat, and tobacco fields exceeded 10000 per gram of soil. Peanut, cotton, pasture, and orchards were intermediate in numbers of R. japonicum, and counts of fewer than 10 per gram occurred in the forest environments.