Effect of Ammonium and Nitrate Nitrogen upon Photosynthate Supply and Nitrogen Fixation by Soybeans1

Abstract

The N required by soybeans [Glycine max L.) Merr.] is furnished by soil and by symbiotically fixed N. The latter is associated with available photosynthate. A determinate soybean cultivar, ‘Bragg’, was grown in sand in the greenhouse in 1974 and outdoors in 1975 to further evaluate the effect of growth stages and NH4+‐N and NO3‐‐N on 14C translocation, N fixation as measured by acetylene reduction by intact nodules, and accumulation of dry matter and nitrogen.Plants exported very little 14C to the nodules from photosynthesis of 14CO2, and acetylene reduction was very low when significant sink demand by pods became evident. Inorganic N supplied throughout the season or 10 days prior to sampling reduced 14C in nodules and acetylene reduction by nodules. NO3‐‐N generally decreased acetylene reduction more than NH4+‐N. 14C transport to plant parts other than nodules was not influenced by inorganic N. There is indication that in the determinate cultivar of soybeans used, more photosynthate was transferred to nodules during the vegetative and early reproductive stages than that reported previously for indeterminate cultivars. The data further emphasize the high demand for N at the pod‐fill stage where 333 mg N per plant (23% of total) was required during a 20‐day period, mid‐pod to late pod fill stage, and at the same time N fixation was declining.