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Response to Soil Temperature of Dinitrogen Fixation (Acetylene Reduction) Rates by Field-Grown Soybeans¹

Temperature has a major effect on the dinitrogen (N₂) fixation rate of legume nodules. Experiments were performed under field conditions to assess the influence of soil temperature on acetylene reduction rates by soybean [Glycine max (L.) Merr.]. Data were collected in situ under low acetylene concentrations such that observations of a number of plants were made every 30 min throughout diurnal periods. Significant positive correlations existed between acetylene reduction rates and soil temperature at temperatures below 30 °C. A linear model and an exponential model fit the data equally well over the 10 to 12 °C ranges where data were obtained. Since it was determined that both the nodule gas conductance and maximum activity of the nodule changed in parallel, it was not possible to identify which of these terms was responding directly to the changes in soil temperature. Between 30 and 33 °C little change in acetylene reduction rates with soil temperature was observed. Above 34 °C declines in acetylene reduction rates were observed. The decline was usually to 70% of the values observed in the 30 to 33 °C soil temperature range. Little or no recovery was observed in acetylene reduction rates after this high-temperature-induced decline, indicating that a more permanent alteration had occurred in the nodules.

Key Words: Nitrogen fixation • Glycine max (L.) Merr. • Nodules

² Adjunct professor and graduate research assistant, respectively, USDA-ARS, Agronomy Physiology Laboratory, Univ. of Florida, Gainesville, FL 32611.

Received for publication October 22, 1984.