The Effect of Heavy Metal Speciation on Arbuscular Mycorrhizal Fungi Associated with Phoenix dactylifera L. Growing in Moroccan Urban and Peri-Urban Environments

Impacts of metal pollution on arbuscular mycorrhizal fungi (AMF) in rhizopheric soils of date palms in urban and peri-urban areas were studied. The objective was to assess the impact of rhizospheric soil contamination. Various chemical species of heavy metals on the AMF spore density associated with date palms were evaluated. A collection of date palm rhizospheric soil samples from eight sites including three boulevards, three gardens and two distinct areas of the Marrakesh palm grove was under study. These samples were used for counting endomycorrhizal fungal spores, for estimating mycorrhization state of root system and for physico-chemical analyses. A five-stage sequential extraction scheme was used to evaluate the fractionation of some heavy metals like lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn) and iron (Fe). Pearson’s correlation coefficients between AMF spore’s density and metal species were determined and a multiple linear regression was tested to predict AMF spore density from the chemical species content of soil. A mycorrhizal dependency of date palm was indicated, since a high frequency from 61 to 98% and a middle intensity from 10 to 47% of root colonization by AMF were recorded. The spore density from all sites was found in increasing order of boulevards, garden then palm groves. A significant correlation between AMF spore density and some metal species rhizospheric soil content was recorded; negative for sulfide-bound lead (r = -0.81) and zinc (r = -0.70) and for exchangeable fraction copper (Cu), (r = -0.79) whereas it is positive for exchangeable fraction zinc (r = 0.70). AMF spore density predictions from sulfide-bound Zinc and exchangeable fraction copper exhibited a good fit, with higher R2 value (0.91, p = 0.002). Since Date palm has a mycotrophic nature, the sustainability of the microbial populations associated with their roots might be ensured by modifying some chemical forms of heavy metals like sulfide-bound zinc and exchangeable fraction copper.