Soil biodiversity plays a critical role in supporting ecosystem functions such as nutrient cycling, organic matter decomposition, and plant production. This study investigates how soil biodiversity's role in supporting ecosystem functions (soil BEF) changes as ecosystems develop over time. The research analyzed 87 globally distributed ecosystems across 16 soil chronosequences, ranging in age from centuries to millennia. The findings show that within-site soil biodiversity and function relationships are negatively correlated with soil age, indicating that younger ecosystems rely more heavily on soil biodiversity to support functions. As ecosystems age, the accumulation of plant cover, soil carbon, and microbial biomass reduces the need for biodiversity to sustain ecosystem functions. The study highlights the importance of soil biodiversity in drier and geologically younger ecosystems with low microbial biomass. The results suggest that soil function becomes less dependent on biodiversity in older, more productive ecosystems due to the legacy of organic matter and microbial biomass accumulated over millions of years. The study also found that microbial biomass is the most important driver of soil BEF relationships, with increased plant cover and litter inputs promoting microbial biomass and thus ecosystem functions. The findings emphasize the need to consider soil biodiversity in Earth system models and for conservation efforts in the face of environmental changes. The study provides insights into the dynamics of soil BEF relationships during long-term soil development, highlighting the role of climate, plant cover, soil carbon, and microbial biomass in driving ecosystem functions. The research underscores the importance of soil biodiversity in maintaining ecosystem functions, particularly in younger and drier ecosystems.Soil biodiversity plays a critical role in supporting ecosystem functions such as nutrient cycling, organic matter decomposition, and plant production. This study investigates how soil biodiversity's role in supporting ecosystem functions (soil BEF) changes as ecosystems develop over time. The research analyzed 87 globally distributed ecosystems across 16 soil chronosequences, ranging in age from centuries to millennia. The findings show that within-site soil biodiversity and function relationships are negatively correlated with soil age, indicating that younger ecosystems rely more heavily on soil biodiversity to support functions. As ecosystems age, the accumulation of plant cover, soil carbon, and microbial biomass reduces the need for biodiversity to sustain ecosystem functions. The study highlights the importance of soil biodiversity in drier and geologically younger ecosystems with low microbial biomass. The results suggest that soil function becomes less dependent on biodiversity in older, more productive ecosystems due to the legacy of organic matter and microbial biomass accumulated over millions of years. The study also found that microbial biomass is the most important driver of soil BEF relationships, with increased plant cover and litter inputs promoting microbial biomass and thus ecosystem functions. The findings emphasize the need to consider soil biodiversity in Earth system models and for conservation efforts in the face of environmental changes. The study provides insights into the dynamics of soil BEF relationships during long-term soil development, highlighting the role of climate, plant cover, soil carbon, and microbial biomass in driving ecosystem functions. The research underscores the importance of soil biodiversity in maintaining ecosystem functions, particularly in younger and drier ecosystems.