Introduction: Almost all elevated atmospheric CO2 (eCO2) studies have focused on plant aboveground characteristics under mono-cultures, whereas limited considerations of interactions with arbuscular mycorrhizal fungi (AMF) under intercropping, let alone under the same, yet unrealistic, daytime/nighttime CO2 concentrations.

We thus addressed how auto-simulated field daytime and/or nighttime eCO2, AMF and N2-fixing/non-N2-fixing intercropping could interactively affect plant biomass production and major physiological characteristics.

Methods: Non-N2-fixing winter wheat (Triticum aestivum cv. Yunmai) and N2-fixing fababean (Vicia faba cv. 89-147) inoculated with AM fungus (Funneliformis mosseae) were grown in mono-cultures and intercropping in soil (Eutric Regosol) filled pots inside environmentally controlled glass-made chambers. Those chambers had the same growth conditions (nitrogen-fertilization, AMF inoculation, etc.), except CO2 concentrations: ambient CO2 (ACO2, 400ppm daytime/450ppm nighttime), daytime elevated (DeCO2, 550ppm/450ppm), nighttime elevated (NeCO2, 400ppm/600ppm), and daytime/nighttime elevated (D+NeCO2, 550ppm/600ppm).

Plant physiological parameters were examined. Results: Regardless AM-mycorrhization, root and shoot biomass and total soluble sugar were significantly increased, but tissue NPK, leaf gas exchange, total chlorophyll, soluble protein and free amino acids were significantly decreased under eCO2 in mono-wheat, mono-fababean or wheat/fababean intercropping. Significantly different responses of plant growth parameters (tissue N, total chlorophyll, soluble protein and free amino acid) to eCO2 patterned as: DeCO2 > D+NeCO2 > NeCO2 > ACO2, while to AM-mycorrhization as D+NeCO2 > DeCO2 > NeCO2.

Conclusions: Mycorrhization and/or eCO2 have varied interactive effects on plant biomass production and physiological characteristics, no matter whether plants were grown in mono-cultures or intercropping. Such interactive effects are probably plant host-species or cropping pattern dependent.