FIELD RESPONSE OF SOYBEAN TO NODULATION BY A RHIZOBITOXINE-PRODUCTING STRAIN OF BRADYRHIZOBIUM

Nodulation by rhizobitoxine-producing (RT+) strains of Bradyrhizobium japonicum has been shown to decrease shoot growth of greenhouse-grown soybeans. The potential impact of nodulation by a RT+ strain on the productivity of field-grown soybeans has not been determined. Therefore, a field experiment was conducted for 2 yr on a Matapeake silt loam (fine-silty, mixed, mesic Typic Hapludults) to determine the response of soybean cv. 'Forrest' to nodulation by one RT+ strain (USDA 94). Inoculation treatments were (i) USDA 94, (ii) a mixture of bradyrhizobia indigenous to the field site (Control), and (iii) a combination of Treatments 1 and 2 (USDA 94/Soil). Inoculation treatments were applied to the seed and the seedlings were grown for about 21 d under greenhouse conditions. Pre-nodulated seedlings were transplanted to 1.8-m2 field microplots. Inoculation with USDA 94 reduced plant N derived from fixation (as measured by isotope dilution) by 43% relative to the Control (2-yr mean). Inoculation with USDA 94 decreased seed yield by 38%. Yield reductions were associated with reductions in pod number. The USDA 94 inoculation decreased vegetative growth, delayed the onset of full pod, beginning seed and full seed stages, and doubled the interval between beginning pod and full pod stages. For most of the parameters measured, USDA 94 and USDA 94/Soil treatments produced similar results. This research demonstrated that nodulation by USDA 94 can significantly alter shoot growth and reduce seed yield of field-grown soybeans.