Keystone Microbial Species Drive the Responses of Saline-Alkali Soil to Three-Year Amendment Measures

Saline-alkali soils exhibit ionic toxicities associated with neutral salinity, as well as a high pH that hinders the exclusion of sodium ions and absorption of vital nutrients; thus, obstructing the development of coastal shelterbelts. A three-year field experiment using a high-soil-pH site was conducted for this study to investigate the influences of five prospective amendments on the soil microenvironments of different soil layers compared to a control. Firstly, the bacterial phyla Proteobacteria, Firmicutes, and Actinobacteria were found to be the most predominant in the samples. As for the fungi phylum, Ascomycota was identified as the most abundant. Similar to Module 1's findings, the relative abundances of Ascomycota varied across treatments. Additionally, differences in the ACE index were primarily observed in the deeper soil layers, where all five soil amendments increased the bacterial ACE index compared to the CK (no additive). Only the BA (biochar mixed with arbuscular mycorrhizal fungi) and AM (arbuscular mycorrhizal fungi on its own) treatments significantly increased the fungal ACE index. In the 20-40 cm soil layer, the pH value of the control group was significantly higher than that of all other treatments, except for the AM treatment. However, the AM treatment induced significantly higher soil enzyme activities and available nutrients compared to the CK. Moreover, the Mantel test showed significant correlations between the Module 1 community, the generalist (microbial species that serve as module hubs and connectors, primarily for Acidobacteria) community and soil pH, electrical conductivity, enzyme activities, as well as bacterial and fungal ACE indices. Pearson's correlation revealed a significantly positive association between enzyme activities and available nutrients. Our findings suggested that keystone microbial species have the potential to improve the availability of soil nutrients through the regulation of microbial diversity and stimulation of soil enzyme activities, to ultimately ameliorate saline-alkali soil. Furthermore, the application of AM in combination with an appropriate amount of biochar is a preferred strategy for the improvement of saline-alkali soils.