The effects of elevated CO2 on leaf area index (LAI) vary among studies. We hypothesized that the interactive effects of CO2 and nitrogen on leaf area loss have important roles in LAI regulation. We studied the leaf area production and loss using nodulating soybean and its non-nodulating isogenic line in CO2-controlled greenhouse systems. Leaf area production increased with elevated CO2 levels in the nodulating soybean stand and to a lesser extent in the non-nodulating line. Elevated CO2 levels accelerated leaf area loss only in nodulating plants. Consequently, both plants exhibited a similar stimulation of peak LAI with CO2 elevation. The accelerated leaf loss in nodulating plants may have been caused by newly produced leaves shading the lower leaves. The nodulating plants acquired N throughout the growth phase, whereas non-nodulating plants did not acquire N after flowering due to the depletion of soil N. N retranslocation to new organs and subsequent leaf loss were faster in non-nodulating plants compared with nodulating plants, irrespective of the CO2 levels. LAI regulation in soybean involved various factors, such as light availability within the canopy, N acquisition and N demands in new organs. These effects varied among the growth stages and CO2 levels.
