Nitrogen Fixation by Soybeans as Influenced by Inoculum Placement: Greenhouse Studies
1975
Wilson, D. O.
Maximum summer surface soil temperatures in many areas greatly exceed the optimum temperature for symbiotic N fixation by legumes. Formation of nodules on roots deep in the soil may enhance N fixation because of reduced extremes in soil temperature and moisture. The purpose of the study was to determine whether nodule position could be controlled by inoculum placement and the effect of surface soil moisture on N fixation by such nodules. Soybeans (Glycine max (L.) Merr.) were grown in a greenhouse at Experiment, Georgia, in plastic pots containing preplaced rhizobial inoculum and 1.5 mg molybdenum at 15, 100, and 200 mm below the soil surface. High (90% field capacity) and low (air-dry) surface soil moisture treatments were also included. Two experiments were conducted, one at moderate and the other at relatively high greenhouse temperatures (27 to 30 C and 37 to 41 C daily maxima, respectively). In all treatments, inoculation depth strongly influenced the pattern of nodule development with most of the nodular tissue forming on roots at or near the depth of inoculation. At moderate greenhouse temperatures, neither inoculation depth nor surface soil moisture affected the amount of N fixed. At relatively high greenhouse temperatures, plants in the low surface soil moisture treatments fixed less than 25% of the N fixed by those in the high surface soil moisture treatments at the 15 mm inoculation depth. A decrease in N fixation at the deeper inoculation depths was also observed at the higher greenhouse temperatures.
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