FIELD RESPONSE OF SOYBEAN TO INCREASED DINITROGEN FIXATION

A number of studies have shown an association between soybean [Glycine mar (L.) Merr.] seed yield and plant N accumulation, suggesting that yield could be increased by nodulation by superior strains of Bradyrhizobium japonicum. Attempts to evaluate the response of field-grown soybean to bradyrhizobia-mediated increases in N accumulation have been hindered by an inability of introduced strains to compete successfully with indigenous strains. Therefore, to determine the effect of increased N2 fixation on soybean growth and seed yield, an experiment was conducted using soybean seedlings that were nodulated under greenhouse conditions and then transplanted to the field. Two inoculation treatments, USDA 122 and a mixture of bradyrhizobia indigenous to the field site (control), were applied to the seed and seedlings were developed for 21 d in the greenhouse. Prenodulated seedlings were then transplanted to 1.8 m2 field microplots on a Matapeake silt loam (fine-loamy, mixed, mesic Typic Hapludult) with a most probable number of indigenous bradyrhizobial population of 6.1 x 10(8) cells kg-1 soil. Relative to the control, nodulation by USDA 122 increased both N2 fixed and total shoot N by a 2-yr average of 29%. Plants nodulated by USDA 122 also assimilated more soil N. The increase in N2 fixation and total N accumulation occurred prior to, but not during, seedfill. The USDA 122 treatment increased seed yield by an average of 31% (90 g m-2). The yield increase was associated with increases in pod number, seedfill rate, remobilized dry matter, remobilized N, and vegetative biomass.