Unravelling depth-dependent pedoclimatic controls on measurable soil organic carbon fractions across climatic gradients in Australian agricultural soils.

Soil organic carbon (SOC) accumulation and persistence are predominantly governed by an intricate interplay among biomass inputs, microbial activity, and soil mineralogy, all of which are strongly modulated by climate-driven moisture availability. However, how climate gradients influence pedoclimatic controls on particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) remains unclear, limiting reliable assessments of SOC vulnerability to climate change and the development of targeted sequestration strategies across contrasting climate zones. Here, using a continent-wide dataset from 2256 Australian farm paddocks across diverse climate zones (ranging from dry to very humid) and soil depths (0-10, 10-20 and 20-30 cm), we applied XGBoost and SHAP, coupled with piecewise structural equation modeling, to disentangle the key drivers and mechanisms governing POC and MAOC stocks in agricultural ecosystems. Our results show that land-use effects on SOC fractions varied with aridity and soil depth, with pastures (modified pastures including annual, perennial and mixed annual-perennial pastures dominated by grasses, legumes or their mixture) maintaining higher POC stocks across all depths in Mediterranean and semi-humid zones but mainly in the surface layers in both drier and wetter regions, while cropping systems (including cereals, oilseeds, grain legumes and other commercial crops) enhanced subsoil MAOC storage in humid climates. Total nitrogen (TN) was the predominant driver of both SOC fractions across most aridity-depth combinations, explaining 15-50% of spatial variation. The TN threshold for surface SOC accumulation increased from ~ 1.0 mg/g soil in dry zones to ~ 2.8 mg/g soil in very humid zones. TN was more influential in surface soils of relatively dry areas (34-50% for POC, 33-37% for MAOC) but shifted to subsoils in humid zones. POC was also strongly associated with Mean Annual Temperature (MAT) and pH under relatively dry climates and with clay content in surface soils, whereas MAOC was sensitive to both MAT and Mean Annual Precipitation (MAP) in dry zones and became increasingly influenced by oxide minerals with depth and humidity, occasionally surpassing TN in subsoils of relatively arid climates or in surface layers of humid regions. These findings underscore the need for climate-smart SOC management strategies: in dry zones, increasing organic inputs and improving soil structure are essential to overcome input limitations and minimize carbon losses, whereas in humid areas, enhancing surface MAOC formation and increasing subsoil carbon inputs is critical for promoting SOC accumulation and stability.