Exogenous nitrogen alleviates cadmium stress in poplar via restructuring rhizosphere soil properties and fungal communities.

Abstract

Cadmium (Cd) poses a threat to plant growth. Nitrogen (N) has been demonstrated to alleviate Cd phytotoxicity, but the underlying mechanism in woody plants remains unclear. In this study, we aimed to determine whether exogenous NH₄HCO₃ influences rhizosphere soil properties and fungal communities, thereby enhancing plant growth and Cd phytoaccumulation in Populus yunnanensis Dode. A pot experiment was conducted in a greenhouse, with seedlings subjected to four treatments: CK (no additional N and Cd), N (24 mg N kg-1 month-1 supplied as NH₄HCO₃), Cd (20 mg Cd kg-1 month-1), and CN (24 mg N kg-1 and 20 mg Cd kg-1 month-1). Under Cd stress, exogenous NH₄HCO₃ increased the biomass (48.0-78.2%), root development, nutrient content, and Cd accumulation (149.2%). In rhizosphere soil, exogenous NH₄HCO₃ decreased the soil pH and NH4+-N content but increased the NO3--N content, available phosphorus (AP), and soil enzyme activities. Exogenous NH₄HCO₃ also reshaped the composition, structure, and co-occurrence patterns of the rhizosphere fungal community, altering the relative abundances of saprotrophic and ectomycorrhizal fungi, such as Rhizoctonia, Peziza, and Exophiala. Moreover, Rhizoctonia and Peziza were positively and negatively correlated with AP, alkaline phosphatase, root biomass, root surface area (RA), fine root surface area (FRA), and Cd uptake efficiency, respectively. Exophiala was positively correlated with root biomass, RA, and FRA. Overall, exogenous NH₄HCO₃ modulates rhizosphere soil properties and reshapes fungal communities, which may improve plant growth and enhance Cd phytoaccumulation.