Abstract
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.
Highlights
Increasing atmospheric carbon dioxide (CO2) concentration caused by combustion of fossil fuels and enhanced nitrogen (N) deposition by human activities are two factors associated with global climate change
The stimulatory effect of N addition on Asat was different between the two CO2 concentrations, with a 40% increase occurring at the elevated CO2 concentration
Numerous studies have found that stimulation of photosynthetic rates induced by elevated CO2 will decrease or even disminished over time as plants acclimate to elevated CO2 concentrations through a process known as down-regulation [36,38,40]
Summary
Increasing atmospheric carbon dioxide (CO2) concentration caused by combustion of fossil fuels and enhanced nitrogen (N) deposition by human activities are two factors associated with global climate change. These factors are likely to have a widespread influence on individual plant communities and their interactions with each other [1]. The results from a six-year field study of perennial grassland species showed that the positive effect of CO2 without N addition is reduced substantially [5]. Plant growth in scrub -oak woodland showed a sustained increase after 11 years of atmospheric CO2 enrichment with enhanced inorganic N absorption from deep soil [6]
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