Hierarchical drivers shaping the global patterns of soil organic carbon

Recent advancements in observational data and experimental research have significantly enhanced our understanding of the mechanisms governing the magnitude, distribution, and dynamics of soil organic carbon (SOC). However, few studies have systematically explored the hierarchical drivers of SOC. This study addresses this gap by integrating multiple independent datasets—covering productivity, carbon allocation, carbon turnover, and carbon fractions—to construct a comprehensive framework for the hierarchical drivers shaping global SOC patterns. Using this framework, we examine the impact pathways and contributions of primary drivers. Our findings show that climate, as the fundamental primary driver, mainly influences SOC through carbon input pathways, while soil properties, as a secondary driver, predominantly affect SOC via carbon output pathways. Further analysis reveals that carbon input plays a key role in shaping topsoil SOC distribution, while carbon output is more influential in regulating subsoil SOC. In both cases, these drivers exert their effects primarily through stable organic carbon fractions, particularly mineral-associated organic carbon (MAOC), whose influence on particulate organic carbon (POC) is the opposite. This study provides the first quantification of the impact pathways and relative strengths of the hierarchical drivers shaping global SOC. It also underscores the need to consider the hierarchical structure of these drivers when assessing the magnitude, distribution, and dynamics of SOC, particularly in relation to its fractions.