Beneficial and adverse effects of bio-inoculation on predicted functional microbial communities in salt-land restoration

The potential impacts of bioinoculants on native soil microbiota—and consequently on soil functioning—represent a key issue for their large-scale adoption in land restoration. Yet, available data remain fragmented, especially for degraded ecosystems that are particularly vulnerable. In Senegal, a restoration program of salt-affected lands has been successfully implemented using Casuarinaceae species in association with salt-tolerant symbionts as bioinoculants. While previous studies demonstrated clear benefits for Casuarinaceae plantations and associated understory vegetation, their effects on soil functioning remain less understood. This study assessed changes in native soil microbiota under this restoration program. Results showed that bioinoculation reduced overall fungal diversity, though rare taxa were unaffected. In contrast, bacterial diversity was primarily shaped by salinity and Casuarinaceae species, with effects limited to dominant taxa. Functional predictions highlighted major shifts, including a bioinoculant-related reduction in bacterial pathogens, but an increase in fungal pathogens linked to salinity. Moreover, soil nitrogen cycling mediated by bacteria appeared sensitive to both salinity and Casuarinaceae species, while nitrogen-fixing bacterial taxa were specifically influenced by bioinoculation. The study demonstrates the environmental impacts of bioinoculation in salt-affected soils, highlighting the needs to routinely integrate the monitoring of soil microbiota in the requirements for the deployment of ecological restoration strategies. In addition, the results supported the importance of N-fixing symbionts as bioinoculant to positively improved N cycling and soil health in degraded landscapes.