Fertilizing effect of the increasing CO2 concentration in the atmosphere

The effect of three atmospheric CO 2 concentrations (ambient, 400 ppm; double, 800 ppm; and triple, 1200 ppm) on the productivity of cottonwood ( Populus deltoides Barr.) and the activity of the soil microbial biomass in the root-inhabited zone was studied. The total biomass of the cottonwood increased with increasing CO 2 concentration (2.28, 5.28, and 3.78 kg/tree for 400, 800, and 1200 ppm, respectively). The strongest responses were observed for the trunk and coarse roots (three and two times higher as compared to the ambient CO 2 concentration). The triple concentration of CO 2 had a greater effect on the roots of the trees, but the growth of leaves and branches was insignificant or absent. The shoot-to-root ratio changed as follows: 2.86, 2.80, and 1.57 at 400, 800, and 1200 ppm, respectively. The rate of C-CO 2 release from the soil samples incubated for 70 days increased in the following order: 400, 800, and 1200 ppm CO 2 ; the average values of the CO 2 emission were 2.02, 2.33, and 2.76 mg/100 g per day, respectively. The greatest content of C mb (75.1 mg/100 g) was observed in the treatment with the triple CO 2 concentration, and the lowest content (53.7 mg/100 g) was found for the ambient CO 2 concentration. This study suggests that the responses of the cottonwood biomass and soil microbial activity vary depending on the CO 2 concentration. The microbial biomass gradually increased with increasing CO 2 concentration; the total plant biomass showed the highest response at the double CO 2 concentration, and the triple CO 2 concentration was likely to suppress the growth of the above-ground plant biomass.