Dryland ecosystems span a wide aridity gradient, support diverse soil organisms, and rely on soil biodiversity to maintain essential soil functions. It is widely recognized that increasing aridity is critical in reducing soil biodiversity and disrupting belowground organism associations that ultimately lead to the collapse of soil functions. However, the relative contributions of the diversity of different taxonomic groups and distinct types of multi-trophic associations under aridity gradients remain poorly understood. In this study, we surveyed 120 dryland sites along a 4500 km aridity gradient in northern China and assessed soil multifunctionality by measuring eight soil attributes related to carbon, nitrogen, and phosphorus cycling. Diversity of eight microbial and microfaunal groups was analyzed to assess their relationships and relative contributions to soil multi-functionality. To further explore potential biotic associations, we constructed co-occurrence networks and assessed how different association types across trophic levels contributed to soil multifunctionality. Although soil biodiversity was a strong predictor of soil multifunctionality across the aridity gradient, the contributions of individual taxa varied markedly. Among microbes, bacterial and saprotrophic fungal diversity showed stronger positive correlations, while among microfauna, rotifer diversity was most strongly correlated. Aridity weakened biotic associations, and their functional contributions to soil multifunctionality differed among types. Associations among microbes, both within and across groups, had the strongest positive relationships with soil multi-functionality, followed by microbe-microfauna associations, whereas associations involving only microfaunal groups showed weak or non-significant relationships. Importantly, biotic associations not only directly promoted soil multifunctionality but also mediated biodiversity's positive effects on multifunctionality. Our results unveil the relative importance of soil biotic diversity and specific ecological associations in relation to soil multi-functionality across the aridity gradient, highlighting the need to conserve not only the diversity of functionally important soil biotic groups but also ecological associations with high functional importance.