The Effects of Oats on the Rhizosphere Microbial Community and Soil Metabolism Under Saline–Alkali Stress

With the increase in soil salinization, global agriculture faces significant threats, compounded by critical challenges in food security. Oats are an important food and feed crop, as well as an excellent crop for saline–alkali soil remediation. Rhizosphere microorganisms play a critical role in helping plants under saline–alkali stress. Understanding the mechanisms between oats and rhizosphere microorganisms is highly significant. In this project, three different tolerant oat varieties were subjected to saline–alkali stress, and then key growth indicators, changes in the rhizosphere microbial community (16S/ITS rRNA sequencing) and rhizosphere soil metabolite profiles (untargeted metabolomics) were examined. The study found that the growth of the sensitive variety C1 was significantly inhibited under saline–alkali stress, while the tolerant variety V9 was less affected, and X17 showed intermediate performance. Corresponding to the above results was that the bacterial diversity (Chao1–Wilcoxon: CT3/VT3 difference = −22, p = 0.0009; CT3/XT3 difference = −15.66, p = 0.0153; Shannon–Wilcoxon: CT3/VT3 difference = −22, p = 0.0009; CT3/XT3 difference = −15.66, p = 0.0153) and the fungal richness (Chao1–Wilcoxon: CT3/VT3 difference = −19.33, p = 0.0185) in V9 rhizosphere soil were significantly higher than C1, although all of them were suppressed to some extent. Further analysis revealed the content of rhizosphere NH4+-N and the levels of rhizosphere neutral phosphatase (NP) (VT3 (265.86 nmol g−1), XT3 (43.56 nmol g−1) and CT3 (54.58 nmol g−1)), urease (UE) (VT3 (1058.19 μg g−1), XT3 (597.85 μg g−1) and CT3 (521.35 μg g−1)), and catalase (CAT) (VT3 (588.0 μmol g−1), XT3 (466.21 μmol g−1) and CT3 (469.39 μmol g−1)) in V9 were maintained at higher levels than in C1 and X17. Meanwhile, some rhizosphere soil metabolites from the unsaturated fatty acid biosynthesis pathway, the pyrimidine metabolism pathway, and the hormones and secondary metabolites synthesis pathway played an important role in the interactions between oat roots and rhizosphere microorganisms under saline–alkali stress. In brief, the growth and development of oats had a positive correlation with their rhizosphere microbial community, and tolerant oat varieties always developed a rhizosphere microenvironment that was more conducive to their growth and to the enhancement of their saline–alkali tolerance. In them, many soil metabolites are involved and play a pivotal role in this interaction process under saline–alkali stress.