Effects of elevated CO₂ and N addition on growth and N₂ fixation of a legume subshrub (Caragana microphylla Lam.) in temperate grassland in China

It is well demonstrated that the responses of plants to elevated atmospheric CO₂ concentration are species-specific and dependent on environmental conditions. We investigated the responses of a subshrub legume species, Caragana microphylla Lam., to elevated CO₂ and nitrogen (N) addition using open-top chambers in a semiarid temperate grassland in northern China for three years. Measured variables include leaf photosynthetic rate, shoot biomass, root biomass, symbiotic nitrogenase activity, and leaf N content. Symbiotic nitrogenase activity was determined by the C₂H₂ reduction method. Elevated CO₂ enhanced photosynthesis and shoot biomass by 83% and 25%, respectively, and the enhancement of shoot biomass was significant only at a high N concentration. In addition, the photosynthetic capacity of C. microphylla did not show down-regulation under elevated CO₂. Elevated CO₂ had no significant effect on root biomass, symbiotic nitrogenase activity and leaf N content. Under elevated CO₂, N addition stimulated photosynthesis and shoot biomass. By contrast, N addition strongly inhibited symbiotic nitrogenase activity and slightly increased leaf N content of C. microphylla under both CO₂ levels, and had no significant effect on root biomass. The effect of elevated CO₂ and N addition on C. microphylla did not show interannual variation, except for the effect of N addition on leaf N content. These results indicate that shoot growth of C. microphylla is more sensitive to elevated CO₂ than is root growth. The stimulation of shoot growth of C. microphylla under elevated CO₂ or N addition is not associated with changes in N2-fixation. Additionally, elevated CO₂ and N addition interacted to affect shoot growth of C. microphylla with a stimulatory effect occurring only under combination of these two factors.