| Summary: | Effects of arbuscular mycorrhizal fungi (AMF), elevated carbon dioxide (eCO<sub>2</sub>), and their interaction on nutrient accumulation of leguminous plants and soil fertility is unknown. Plant growth, concentrations of tissue nitrogen (N), phosphorus (P), and potassium (K) in 12-week-old nodulated faba bean (<i>Vicia faba</i>, inoculated with <i>Rhizobium leguminosarum</i> bv. NM353), and nutrient use efficiency were thus assessed under ambient CO<sub>2</sub> (410/460 ppm, daytime, 07:00 a.m.–19:00 p.m./nighttime, 19:00 p.m.–07:00 a.m.) and eCO<sub>2</sub> (550/610 ppm) for 12 weeks with or without AM fungus of <i>Funneliformis mosseae</i> inoculation. eCO<sub>2</sub> favored AMF root colonization and nodule biomass production. eCO<sub>2</sub> significantly decreased shoot N, P and K concentrations, but generally increased tissue N, P and K accumulation and their use efficiency with an increased biomass production. Meanwhile, eCO<sub>2</sub> enhanced C allocation into soil but showed no effects on soil available N, P, and K, while AM symbiosis increased accumulation of C, N, P, and K in both plant and soil though increased soil nutrient uptake under eCO<sub>2</sub>. Moreover, plant acquisition of soil NO<sub>3</sub><sup>−</sup>–N and NH<sub>4</sub><sup>+</sup>–N respond differently to AMF and eCO<sub>2</sub> treatments. As a result, the interaction between AM symbiosis and eCO<sub>2</sub> did improve plant C accumulation and soil N, P, and K uptake, and an alternative fertilization for legume plantation should be therefore taken under upcoming atmosphere CO<sub>2</sub> rising. Future eCO<sub>2</sub> studies should employ multiple AMF species, with other beneficial fungal or bacterial species, to test their interactive effects on plant performance and soil nutrient availability in the field, under other global change events including warming and drought.
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