Environmental drivers of soil organic carbon in the Australian rangelands

Soil organic carbon (SOC) is vital for maintaining healthy ecosystems and providing benefits beyond climate change mitigation. The Australian rangelands occupy a large area and have arid, semi-arid, and tropical climates in the north. They constitute Australia’s most significant carbon sink. Identifying the key drivers of SOC content variation across the different conditions in these regions remains an important knowledge gap. We use a quantitative hierarchical framework and data-driven methods to systematically analyse the controls of SOC variation over the large rangelands scale, regionally and locally, in two soil layers, 0–10 cm and 10–20 cm. At the rangelands scale, vegetation, climate, and topography are the main controls of SOC. At the regional scale, vegetation cover and type, plant species richness, terrain features, soil texture and mineralogy, and microbial diversity determine the variation in SOC content. At the local scale, where the climate does not limit plant carbon inputs, vegetation type is the most significant driver. In regions where a drier and more variable climate restricts carbon inputs into the soil, vegetation type, biomass, and soil mineralogy determine the differences in SOC content. Despite the relatively lower plant species richness in tropical regions, vegetation contributes positively to SOC in both soil layers. In contrast, in areas where aridity increases along climatic gradients, vegetation negatively affects SOC in the 20–30 cm soil layer. Our results provide insights into the drivers of SOC content in Australian rangelands. • A hierarchical framework for understanding rangelands SOC variation across scales. • Robust variable screening methods for local-scale variation with limited samples. • Vegetation, climate, and topography are the main controls of SOC. • SEM analysis reveals drivers on SOC variation in depth along the climatic gradient.