Interaction between C4 barnyard grass and C3 upland rice under elevated CO2: Impact of mycorrhizae

Atmospheric CO2 enrichment may impact arbuscular mycorrhizae (AM) development and function, which could have subsequent effects on host plant species interactions by differentially affecting plant nutrient acquisition. However, direct evidence illustrating this scenario is limited. We examined how elevated CO2 affects plant growth and whether mycorrhizae mediate interactions between C4 barnyard grass (Echinochloa crusgalli (L.) Beauv.) and C3 upland rice (Oryza sativa L.) in a low nutrient soil. The monocultures and combinations with or without mycorrhizal inoculation were grown at ambient (400 ± 20 μmol mol−1) and elevated CO2 (700 ± 20 μmol mol−1) levels. The 15N isotope tracer was introduced to quantify the mycorrhizally mediated N acquisition of plants. Elevated CO2 stimulated the growth of C3 upland rice but not that of C4 barnyard grass under monoculture. Elevated CO2 also increased mycorrhizal colonization of C4 barnyard grass but did not affect mycorrhizal colonization of C3 upland rice. Mycorrhizal inoculation increased the shoot biomass ratio of C4 barnyard grass to C3 upland rice under both CO2 concentrations but had a greater impact under the elevated than ambient CO2 level. Mycorrhizae decreased relative interaction index (RII) of C3 plants under both ambient and elevated CO2, but mycorrhizae increased RII of C4 plants only under elevated CO2. Elevated CO2 and mycorrhizal inoculation enhanced 15N and total N and P uptake of C4 barnyard grass in mixture but had no effects on N and P acquisition of C3 upland rice, thus altering the distribution of N and P between the species in mixture. These results implied that CO2 stimulation of mycorrhizae and their nutrient acquisition may impact competitive interaction of C4 barnyard grass and C3 upland rice under future CO2 scenarios.