Bradyrhizobium japonicum Inoculant Mobility, Nodule Occupancy, and Acetylene Reduction in the Soybean Root System

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Abstract
In the American Midwest, superior inoculant rhizobia applied to soybeans usually occupy only 5 to 20% of nodules, and response to inoculation is the exception rather than the rule. Attempts to overcome this problem have met with limited success. We evaluated the ability of Bradyrhizobium japonicum, supplied as a seed coat inoculant, to stay abreast of the infectible region of the developing soybean root system. The rhizoplane population of the inoculant strain declined with distance from site of placement, the decrease being more pronounced on lateral than on taproots. This decline was paralleled by a decrease in inoculant-strain nodule occupancy. Inoculant bradyrhizobia contributed little to nodulation of lateral roots, which at pod-fill accounted for more than 50% of nodule number and mass, and were major contributors to acetylene reduction activity. From these data, it appears that inoculant bradyrhizobia are competitive with indigenous soil strains at the point of placement in the soil but have limited mobility and so are incapable of sustaining high populations throughout the developing root system. The result is low nodule occupancy by the inoculant strain in the tap- and lateral roots. Future studies should address aspects of inoculant placement and establishment.