The objective of this experiment was to measure the impact of molybdenum (Mo) addition to the soil on changes in rate of photosynthesis (PN), specific root nodule nitrogenase activity (SNA), chlorophyll ‐ (Chl‐a) and (Chl‐b) and biomass of soybean (Glycine maxL. Merrill), exposed to carbon dioxide (CO2) (350, 400, or 450 μL/L) for 5 d during its sensitive growth stage (R2). Plants grown in soil with Mo (0, 0.5, or 1.0 mg/kg soil dry weight) were exposed to carbon dioxide (CO2) in open‐top field chambers for 12 h/d for 5 d with a nitrogen (N)‐free fertilizer. Increasing CO2levels reduced SNA and increased both Chl‐a and Chl‐b. Addition of 1 mg/kg Mo significantiy reduced SNA for plants grown with 350 μL/L CO2with no change in PN.