Influence of soil salinity and benzoxazinoids on the composition and ecological functions of fungal and nematode communities in the maize rhizosphere

Soil salinization is a major constraint on global agricultural sustainability, as it directly inhibits crop growth and compromises soil ecosystem stability. Although substantial research has examined the isolated effects of salt stress on either crop or soil properties, the mechanistic pathways through which salinity influences soil biological barriers via the modulation of benzoxazinoid (BXs) metabolism in the maize rhizosphere remain poorly understood. In this study, maize rhizosphere soils from three distinct regions of the North China Plain, including Cangzhou, Dongying, and Handan, were analyzed using an integrated approach combining field surveys and laboratory analysis to systematically examine how soil salinity regulates BXs metabolism and affects fungal and nematode community structures. High-throughput sequencing was used to characterize the fungal and nematode community composition. The major BXs components, 6-methoxy-2-benzoxazolinone (MBOA), 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), and 2-hydroxy-1,4-benzoxazin-3-one (HBOA) were quantitatively analyzed, and key soil physicochemical parameters were determined. Multivariate statistical approaches, including Mantel tests, Pearson correlation analysis, and one-way analysis of variance, were used to evaluate the complex relationships among the salinity parameters, BXs contents, and biological community structures. There were significant disparities in α- and β-diversity and community composition of both fungal and nematode communitites across the three regions. Mantel analysis identified soil salinity and BXs composition as the primary drivers of these differences, with soluble salt content (SSC) and electrical conductivity (EC) showing significant correlations with fungal communities, whereas DIMBOA emerged as a key factor affecting nematode community variations. Correlation analysis focusing on plant pathogenic fungi, herbivorous nematodes, BXs contents, and salinity indicators revealed significant relationships. Salinity parameters (EC and SSC) showed substantial correlations with BXs contents, with DIMBOA exhibiting a positive correlation with soil salinity, while MBOA demonstrated a negative correlation. Further investigation established strong connections between BXs levels and key indicators of soil biological barriers, where DIMBOA displayed a positive correlation with ectoparasitic herbivorous nematode abundance but a negative correlation with the pathogenic fungus Penicillium, whereas MBOA exhibited inverse relationships with these organisms. We identified a significant positive correlation between the abundance of the pathogenic fungus Fusarium and the abundance and infection rates of herbivorous nematodes, suggesting potential synergistic interactions that collectively exacerbated soil biological barriers in saline environments. From the perspective of rhizosphere metabolite-microbe interaction networks, this study elucidates that soil salinity stress alters the synthesis and accumulation patterns of BXs in maize, thereby influencing the assembly of rhizosphere fungal and nematode communities and promoting synergistic effects between pathogenic fungi and herbivorous nematodes, ultimately exacerbating soil biological barriers. The synergistic effect of these soil biological barrier factors may further exacerbate the incidence of crop diseases, posing a potential threat to crop production. This study provides novel insights for understanding crop-soil-microbe interactions in saline farmlands and provides a theoretical foundation for developing ecological strategies to mitigate soil biological barriers in maize cropping systems.