The Respone Of Two Herbaceous Plants And Its Soil Microbial Properties To Doubles CO₂Concentration And Drought Stress

The concentration of carbon dioxide (CO2) in atmosphere has been increased year by year. It is forecast that the concentration of CO2will increase from379μmol·mol-1at present to540-970μmol·mol-1at the end of this century. The increase of CO2concentration will greatly influence the plant growth, climate, human being, and so on. Therefore, the present study aimed at investigating the effects of CO2concentration elevated from375μmol·mol-1to750[μmol·mol-1on the photosynthesis physiological characteristics, substance accumulation and root soil environment of C4plant, Bothriochloa ischaemum and C3plant, Medicago sativa using pot experiments, as well as exploring the response mechanisms to elevated CO2concentration. The main results are as follows:Under the condition of water sufficient, doubled CO2concentration had no significant effect on the photosynthesis of B. ischaemum, but reduced stomatal conductance (Gs) and transpiration rate (Tr), and improved intercellular CO2concentration (Ci), Light Saturation Point (LSP), water use efficient (WUE) and maximum photosynthetic rate (Pmax) of the leaves in Medicago sativa, this result suggested that the photosynthesis of M. sativa was increased significantly. Under the condition of drought stress, doubled CO2concentration improved Gs, photosynthetic pigment, Pmax, apparent quantum efficiency (AQE) and WUE of the B. ischaemum. Therefore, the photosynthesis was increased by doubled CO2concentration, while did not influence the M. sativa. The result showed that the B. ischaemum was more sensitive than M. sativa to respond to doubled CO2concentration and drought stress.Doubled CO2concentration significantly increased the maximal fluorescence (Fm) of B. ischaemum, while reduced minimal fluorescence (F0) and increased maximum photochemical efficiency(Fv/Fm), potential photochemical efficiency (Fv/F0) of M. sativa under the condition of water sufficient, this indicated that the ability of protecting photosynthetic system and photo-chemical activity were significantly improved, and also promoted the photosynthesis. Under the condition of drought stress, the Fm、Fv/Fm and Fv/F0of B. ischaemum and the non-photochenmical quenching coefficient (NPQ) and photochemical quenching coefficient (qP) of M. sativa were increased significantly by double CO2 concentration. The result showed that doubled CO2concentration can improve photo-chemical activity of B. ischaemum. The open part of PS Ⅱ and extra photo-energy consumed of M. sativa were increased, suggesting that the protection ability of photo synthetic system was increased.The effects of doubled CO2concentration were closely related to plants types and soil moisture. Under the condition of drought stress, doubled CO2concentration increased plant height, single plant biomass, aboveground and root biomass. The ratio of root to shoot was reduced, while there was no significant difference under the condition of water sufficient, showing a significant interactive effects under doubled CO2concentration and drought stress. M. sativa was not affect by doubled CO2concentration under drought stress, while its biomass was increased under water sufficient, and the interactive effects only influenced the ratio of root to shoot.The malondialdehyde (MDA) and proline (Pro) of B. ischaemum and M. sativa were reduced by the doubled CO2concentration, but they were different under different soil moisture. The protection effect of doubled CO2concentration for plant was obviously reflected under the condition of drought stress. The effect of doubled CO2concentration on soil microbial properties was related to the plants types. For the B. ischaemum, the benefit of doubled CO2concentration on soil biomass C, microbial N, urease, invertase, basal respiration, induce respiration and respiration quotient (qCO2) were more obvious under drought stress than that under the water sufficient. For the M. sativa, soil microbial biomass C and microbial N were increased, while basal respiration and qCO2were reduced significantly by the doubled CO2concentration under the condition of water sufficient. Urease and induce respiration were significantly increased, while basal respiration and qCO2were reduced significantly by double CO2concentration under the condition of drought stress. In conclusion, doubled CO2concentration could improve soil fertility and promote soil environment developing to mature and stabilization.