Aridity mediates the scale-dependent relationship between plant diversity and soil multifunctionality

Dryland ecosystems are now exposed to intensifying aridity and plant diversity loss, posing significant threats to the maintenance of ecosystem multifunctionality, defined as the capacity of ecosystems to simultaneously sustain multiple functions. Although many experiments and empirical studies have shown that local plant diversity promotes ecosystem multifunctionality, we have little understanding of the scale dependence of plant diversity effect and the roles of local plant loss (α diversity) and spatial homogenization (β diversity) in mediating ecosystem multifunctionality under shifting environmental conditions. Here, we investigate the effects of plant α- and β-diversity on soil multifunctionality across a 3200 km aridity gradient in China's drylands. We measure 12 variables related to soil carbon, nitrogen, and phosphorus cycling and storage to characterize soil functions, and quantified soil multifunctionality using both averaging and multiple threshold approaches. We examine how the relationships between plant α/β-diversity and soil multifunctionality shift along an aridity gradient, and assess the combined effects of plant α- and β-diversity on soil multifunctionality under aridity stress. We show that although both plant α- and β-diversity promote soil multifunctionality, the positive effect of α-diversity increases as aridity intensifies, whereas the contribution of β-diversity diminishes. In addition, the processes of complementarity and facilitation outweigh competition in community assembly as plant water stress tolerance becomes increasingly important. Despite their opposing patterns, the contributions of plant α- and β-diversity are approximately equivalent. Furthermore, plant α-diversity can promote β-diversity, synergistically enhancing soil multifunctionality. Our study highlights the importance of biodiversity conservation across scales in drylands and provides insights for designing aridity-specific biodiversity conservation policies.