Allelopathic potential of Haloxylon ammodendron against Syntrichia caninervis and comparative analysis of soil microbial differences between inside and outside of the bare patches under its canopies

In the Gurbantunggut Desert, Haloxylon ammodendron and Syntrichia caninervis are often found at the base of the dunes. In these areas, bare patches usually form under H. ammodendron canopy, but not under other shrub canopies. We compared the soil chemical properties under H. ammodendron canopy inside the bare patches (UC1) and of soil under moss crust outside of H. ammodendron canopy bare patches (UM2), and used untargeted metabolomics to analyze soil metabolites and metagenomic sequencing to characterize the soil microbial structure. The results showed that a total of 951 metabolites were identified in the soil samples, and 518 differential metabolites were observed. The content of amide compounds, such as oleamide, in UC soil was significantly higher than that in UM soil, suggesting that the amide compounds may be the main allelochemicals inhibiting S. caninervis. Soil chemical properties and soil metabolites were quite different between inside and outside of the bare patches, and these differences affected soil microorganisms, resulting in significantly lower species richness in UC than in UM, but there were no significant differences in microbial community function. Fungi were minimally affected by the environment, archaea were significantly affected by NO3--N (P < 0.01), bacteria were significantly affected by TN (P < 0.01), and none of the three communities correlated significantly with the major differential metabolites. Therefore, it can be concluded that the amide compounds secreted by H. ammodendron roots create a concentration gradient under its canopy, with high concentrations inhibiting S. caninervis, causing changes in soil chemical factors inside and outside of the bare patches and inhibiting microbial richness inside the bare patches. The results provide important theoretical references for the further study of desert communities in terms of interspecific relationships and species distribution patterns, and are of great significance in promoting comprehensive desertification management and building desert ecological security systems.