Effects of Nitrogen and Exogenous Rhizophagus irregularis on the Nutrient Status, Photosynthesis and Leaf Anatomy of Populus x canadensis 'Neva'

The productivity of poplar is associated with large nitrogen (N) requirements. Exogenous arbuscular mycorrhizal fungi (AMF) show potential for use as bio-fertilizers. Understanding the interaction between N and exogenous AMF has theoretical and practical significance for poplar plantation. A pot experiment was conducted to assess the effects of N and exogenous Rhizophagus irregularis on plant growth, nutrient uptake, photosynthesis, water status, and leaf anatomical properties of Populus x canadensis 'Neva' in natural soil. The results showed that N fertilization increased plant growth, net photosynthesis, water status and the conduit diameter of midribs. The concentrations of carbon (C) and N in leaves were increased, but the phosphorus (P) concentration was decreased by N fertilization. The effectiveness of exogenous R. irregularis varied under different N levels. Under low N levels, exogenous R. irregularis-inoculated plants grew faster and exhibited superior photosynthetic capacity, water status and leaf conduit diameters than non-inoculated plants. Under high N levels, C, N and P concentrations were enhanced by exogenous R. irregularis inoculation. Furthermore, the average conduit diameter of midribs presented a significant positive correlation with plant growth parameters, photosynthesis, relative water content (RWC) and leaf C and N concentrations. It was concluded that exogenous R. irregularis exerted the strongest positive effects under low N levels by promoting plant growth and photosynthesis, and the fungus promoted plant nutrition decoupled from the level of N fertilization. Moreover, the improvement of plant physiological traits due to N fertilization or exogenous R. irregularis inoculation was accompanied by changes in internal anatomical properties.