Mediation Of Arbuscular Mycorrhiza Fungi On Plant Interaction Under Elevated Carbon Dioxide

Carbon dioxide (CO₂) enrichment in the atmosphere in general stimulates photosynthetic activity and growth of plants. This in turn has the potential to impact the competitive relationships among coexisting plant species directly through altering plant growth and also indirectly by modifying plant-microbial interactions such as plant-rhizobial symbiosis and mycorrhizae. To examine the response of plant growth, nutrient uptake and mycorrhizae to elevated CO₂ and to investigate the influence of AMF on the competition between crop and weed under elevated CO₂, pot experiments of elevated CO₂ (700±50 μmol mol^-1) effects on various plant species and plant species combination were conducted in environment controlled growth chamber during 2002²004. Main results obtained in these experiments are as follows:1 Responses of various plant species to elevated CO₂Fourteen native plant species in orchard ecosystem, Poa annua, Lolium perenue, Avena fatua, Oryza sativa, Trifolium repens, Medicago lupulina, Glycine max, Veronica didyma, Plantago asiatica, Gnaphalium affine, Oxalis corniculata, Echinochloa crusgalli var.mitis, Eleusine indica and Amaranthus spinosus were used in the experiment, and they were divided into five different functional groups. Compared to control (ambient CO₂), the plant height of all species was promoted by elevated CO₂. Total biomass tended to increase under elevated CO₂. However, changes of total biomass under elevated CO₂ significantly differed among functional groups. Under elevated CO₂, C₃ legume showed the greatest increase in total biomass of all functional groups, following the order C₃ forb> C₃ grass> C₄ grass (Fig. 1) , whereas C₄ forb decreased by 20.2%. Ratio of root biomass to shoot biomass tended to increase under elevated CO₂, but there was no significant difference among 5 functional groups.Elevated CO₂ did not enhance total phosphorus and nitrogen concentration (including aboveground and belowground) , but increased total N and P uptake of C₃ species and decreased total N uptake of C4 species. Mycorrhizal colonization varied significantly among functional group under elevated CO₂. Mycorrhizal colonization of C₃ legume, C₃ forb, and C₄ forb decreased significantly, while C₄ grass increased significantly. Under ambient CO₂ there was no significant relation between mycorrhizal colonization and N/P content of the rhizosphere soil, but under elevated CO₂ mycorrhizal colonization was significantly related with total P content of the rhizosphere soil.2 Responses of two co-existed species to elevated CO₂ and simulated nitrogen deposition Competitive interaction of C₃ crop Oryza sativa and C₄ weed Echinochloa crusgalli var.mitis wasinvestigated under elevated CO2, and elevated CO2 plus simulated nitrogen deposition. Results from the experiments showed that both treatments of elevated CO2 and elevated CO2 plus simulated nitrogen deposition enhanced plant height and biomass of Ory^a sativa and Echinochloa crusgalli v&x.mitis, and the enhancement of plant height and biomass promoted by elevated CO2 was higher in Echinochloa crusgalli vai.mitis than that in Ory^a sativa. Both elevated CO2 and elevated CO2 plus simulated nitrogen deposition increased the Root/shoot biomass ratio of Echinochloa crusgalli vai.mitis but did not change Ory%a sativa. Compared to control, both treatments elevated CO2 and elevated CO2 plus simulated nitrogen deposition significantly increased N uptake (including aboveground and belowground ) of Ory^a sativa and Echinochloa crusgalli vat.mitis. Elevated CO2 significantly decreased mycorrhizal colonization of Ory%a sativa but significantly increased that of Echinochloa crusgalli -va.i.mitis. Both treatments of elevated CO2 and elevated CChplus simulated nitrogen deposition significantly enhanced mycorrhizal colonization of both Echinochloa crusgalli vzt.mitis and Ory%a sativa, but the enhancement of mycorrhizal colonization of Echinochloa crusgalli vzt.mitis was significantly greater than that of Ory^a sativa. No significant relation between mycorrhizal colonization and N/P content of the rhizosphere soil was found in the experiments.3 Mediation of arbuscular mycorrhizal fungi on C3 crop and C4 weed competitive interactions Elevated CO2 significantly decreased mycorrhizal colonization of Ory^a sativa but significantly increased that of Echinochloa crusgalli vzi.mitis. However, elevated CO2 plus simulated nitrogen deposition significantly enhanced mycorrhizal colonization of both Echinochloa crusgalli vzx.rnitis and Ory^a sativa, and mycorrhizal colonization of Echinochloa crusgalli vzr.mitis was significantly higher than that of Ory^a sativa. Mycorrhizal inoculation enhanced plant height and biomass of both Ory^a sativa and Echinochloa crusgalli under both treatments of elevated CO2 and elevated CO2 plus simulated nitrogen deposition. Mycorrhizal inoculation increased the root/shoot biomass ratio of Echinochloa crusgalli vzt.mitis, but decreased that of Ory\a sativa under both treatments of elevated CO2 and elevated CO2 plus simulated nitrogen deposition. Compared to non-inoculating AMF, P and N uptake ratio of Echinochloa crusgalli vm.mitis to Oryt^a sativa were significantly increased under inoculating AMF condition. The effects of the treatment of elevated CC>2plus simulated nitrogen deposition on P and N uptake ratio of Echinochloa crusgalli vax.mitis to Ory%a sativa were higher than that of the treatment of elevated CO2. Results from the experiments suggested that mycorrhizae might play a major role in determining the outcome of C3 crop- C4 weed interactions under future CO2 scenarios.