Soil microbiome-mediated salinity tolerance in poplar plantlets is source-dependent.

Soil salinization is a global environmental problem and one of the most common land degradation processes. To effectively utilize saline lands, it is crucial to improve plant growth and stress tolerance, particularly through the microbiome intervention strategy. However, less is known about the interactions of microbes with trees than those with crops or herbaceous plants. Here, we examined how natural soil microbes affected the performance of salt-sensitive Populus deltoides × P. euramericana 'Nanlin895' (NL895) under salt stress. Gnotobiotic NL895 plantlets were inoculated with soil microbiome extracted from no-salt (NS; soluble salt: 0.71 g/kg), low-salt (LS; 5.14 g/kg), and high-salt (HS; 23.07 g/kg) lands, and then exposed to salt treatments. Compared to control, 33.8%, 18.0%, and 29.9% of the aboveground biomass was increased by NS, LS, and HS inoculation, respectively. The salt injury index was lower in LS and HS than in NS treatments. Rhizosphere microbial communities of all treatments were taxonomically and functionally different across multiple stages, while the variation extent was larger in bacterial than in fungal communities. FUNGuild and PICRUSt2 analysis demonstrated the changes of fungal trophic modes and bacterial metabolic pathways, respectively. In summary, our findings revealed the stronger potential of NS than LS and HS inoculants in growth promotion, while weaker strength than LS and HS inoculants in enhancing salt tolerance of NL895 plantlets. This source-dependent effect should be considered in future microbiome engineering, aiming at harnessing soil microbes to create predictable plant phenotypes.