Diurnal and Seasonal Variation in Dinitrogen Fixation (Acetylene Reduction) Rates by Field–Grown Soybeans 1

No data exist that represents in situ, continuous observations under field conditions of changes in dinitrogen fixation potential through diurnal cycles. Acetylene reduction responses of soybean [ Glycine max (L.) Merr.] plants to changes during the diurnal cycle and during seasonal developmental changes were monitored using a continuous flow system with very low acetylene concentration. These data were used to assess the relative effects on acetylene reduction rates of environmental and plant ontogeny variables. During the diurnal cycle, acetylene reduction rates generally peaked late in the afternoon and then declined through the night. However, the night rates did not fall below half of the daily maximum. Within individual days, rates were more closely correlated with soil temperature than with photosynthetically active radiation. A consistent link was not observed between acetylene reduction rates and any particular stage of ontogeny. These results, and a reexamination of published reports, suggest that dinitrogen fixation rate in soybeans is not closely coupled to current photosynthetic rates under field conditions.