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Ecological Studies and Symbiotic Performance of Soybean Rhizobia Under Different Cropping Systems in Northeast Thailand
Dissertation Abstract:
Greenhouse and field experiments were conducted to study the effect of soil cropping history on soybean response to seed inoculation. Total and introduced rhizobia! populations and nodule occupancy were determined in the field experiments while crop response to reinoculation was studied in the greenhouse experiment. The dynamics of introduced rhizobial as influenced by soiI moisture stress and the presence of host plants was also studied.
The number of indigenous rhizobia in soi ls never planted to soybean was less than 10 cells/g soil. The introduced rhizobia formed 88-97 percent of the nodules in these soils. In the soils that had been grown to soybean, the number of indi genous rhizobia was 110 cells/g soil in one location and 140 cells/g soiI in the other location. Percent nodule occupancies of these were 45 and 20 percent, respectively.
Soil cropping histories had no significant effect on populations of the introduced rhizobia. Generally, total rhizobia! population in the inoculated plots was significantly hi gher than those in the uninoculated plots was significantly higher than those in the uninoculated plots as well as in the soils with soybean cropping history.
Seed inoculation, regardless of inoculation rates. significantly increased nodulation by twice as much as the uninoculated treatments. However, the significant differences were not always obtained in soils previously planted to soybean. Nodulation was also influenced by soil cropping history. The highest nodulation was found in the soils previously planted to soybean.
Seed inoculation, in most cases, significantly increased crop responses in terms of shoot total nitrogen (N), shoot dry weight, and grain yield. On the other hand, soil cropping histories had no significant effect on these parameters except in one location.
Inoculation ofthe preceding soybean crop significantly increased nodule and shoot dry weights of the succeeding soybean. No significant effect, however, was observed in other growth parameters. Reinoculating the next soybean crop greatly improved nodulation, root dry weight, and shoot total N.
In the absence of host plants, the number of introduced rhizobia in the simulated plots gradually decreased from 2.5 x 104 to 2.5 x 103 cells/ g within II weeks. When host plants the population increased to nearly the initial level after a five-week decline. Drastic decreases were observed, especially in the plots without the host plant, when soil moisture stresses were imposed. The initial decreases of population depended on inoculation rate. High rate of inoculation rate showed more drastic decrease in the number of rhizobia during the first week.
