Effects of elevated CO 2 and N addition on growth and N 2 fixation of a legume subshrub (Caragana microphylla Lam.) in temperate Grassland in China

Abstract

It is well demonstrated that the responses of plants to elevated atmospheric CO 2 concentration are species-specific and dependent on environmental conditions. We investigated the responses of a subshrub legume species, Caragana microphylla Lam., to elevated CO 2 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 C 2H 2 reduction method. Elevated CO 2 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 CO 2. Elevated CO 2 had no significant effect on root biomass, symbiotic nitrogenase activity and leaf N content. Under elevated CO 2, 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 CO 2 levels, and had no significant effect on root biomass. The effect of elevated CO 2 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 CO 2 than is root growth. The stimulation of shoot growth of C. microphylla under elevated CO 2 or N addition is not associated with changes in N 2-fixation. Additionally, elevated CO 2 and N addition interacted to affect shoot growth of C. microphylla with a stimulatory effect occurring only under combination of these two factors. © 2011 Zhang et al.