The influence of arbuscular mycorrhizae (AM) on plant growth and Zn and Pb uptake by Lygeum spartum and Anthyllis cytisoides was studied in soils with different levels of these heavy metals. A. cytisoides is highly dependent on AM for optimal growth, while L. spartum is a facultative mycotroph.Mycorrhizal and nonmycorrhizal plants were grown in soil supplemented with 0, 10, 100 and 1000 mg of Zn kg -l soil or 0, 100 and 1000 mg of Pb kg -1 soil. Two different mycorrhizal fungi were separately studied: Glomus macrocarpum isolated from a non contaminated site and a strain of Glomus mosseae isolated from a soil contaminated with these metals. The infectivity of the fungi was not affected by the presence of Zn or Pb in the soil. In unamended soil, both fungi were equally effective in promoting plant growth, but when Zn or Pb were added to soils, G. mosseae was more efficient than G. macrocarpum in stimulating plant growth of A. cytisoides. A. cytisoides was unable to grow unless mycorrhizal. Metal addition to the soil induced a reduction in the biomass of L. spartum and of mycorrhizal A. cytisoides, and a decrease in shoot P concentration of mycorrhizal plants. The concentration of metals in the plants varied according to the amount added to the soil and to the inoculation treatment: at low doses, mycorrhizal plants showed equal or higher concentration of Zn or Pb than nonmycorrhizal ones; at higher doses, however, metal concentrations in the plants inoculated with G. mosseae were lower than those found in the corresponding controls, while the plants inoculated with G. macrocarpum showed similar (L. spartum) or even higher (A. cytisoides) levels than the controls.

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Patterns in plant-soil biota interactions could be influenced by the spatial distribution of species due to soil conditions or by the functional traits of species. Gypsum environments usually constitute a mosaic of heterogeneous soils where gypsum and nongypsum soils are imbricated at a local scale. A case study of the interactions of plants with arbuscular mycorrhizal fungi (AMF) in gypsum environments can be illustrative of patterns in biotic interactions. We hypothesized that (i) soil characteristics might affect the AMF community and (ii) there are differences between the AMF communities (modules) associated with plants exclusive to gypsum soils (gypsophytes) and those associated with plants that show facultative behavior on gypsum and/or marlylimestone soils (gypsovags). We used indicator species and network analyses to test for differences between the AMF communities harbored in gypsophyte and gypsovag plants. We recorded 46 operational taxonomic units (OTUs) belonging to nine genera of Glomeromycota. The indicator species analysis showed two OTUs preferentially associating with gypsum soils and three OTUs preferentially associating with marly-limestone soils. Modularity analysis revealed that soil type can be a major factor shaping AMF communities, and some AMF groups showed a tendency to interact differently with plants that had distinct ecological strategies (gypsophytes and gypsovags). Characterization of ecological networks can be a valuable tool for ascertaining the potential influence of above-and below-ground biotic interactions (plant-AMF) on plant community composition.

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