Contrasting effects of symbiont inoculation on soil microbiota functionalities in a rehabilitation programme of salt-affected lands

Soil salinization has become a major global ecosystem sustainability issue. In Senegal, promising ecosystem restoration of salt-affected lands has been achieved by the implementation of ecological engineering strategies based on beneficial associations between Casuarina species and salt-tolerant symbiotic microorganisms. However, the extent of the impacts of symbiont inoculants on the native soil microbiota and, consequently, soil functioning is fragmentary. The current study aimed to evaluate the changes in the native soil microbiota associated with the use of a symbiont inoculant in mixed Casuarinaceae plantations in salt-affected lands. Symbiont inoculation led to a decrease in the diversity of soil fungi but not bacteria. However, the whole soil microbiota structure was impacted by symbiont inoculation, as was the case for salinity and Casuarina species. Casuarina species also impacted the diversity of the dominant taxa constituting the soil microbiota, but only salinity affected bacterial diversity. Important modifications of soil microbiota functionalities were revealed, notably, a decrease in the abundance of bacterial pathogens related to symbiont inoculation and an increase in the abundance of fungal pathogens with increasing salinity. Soil nutrient cycling, mostly micronutrient cycling and N fixation, was also impacted by symbiont inoculation, but salinity and Casuarina species were the main factors affecting N cycling in the soil.