Drivers of organic carbon dynamics in surface and subsurface agricultural soils of New South Wales, Australia

Context: Understanding of organic carbon (OC) stock in surface (0–30 cm) and subsurface (30–60 cm) soils and its determinants are crucial for managing OC in agricultural lands. Aim: Our aim was to examine how land use, soil type, and environmental drivers influence OC stocks in the 0–60 cm soil layers of agricultural regions in New South Wales (NSW), Australia. Method: Soil OC (SOC) and nitrogen (N) stocks were measured across diverse soil types from 49 farms representing pastures and cropping in NSW. The dominant drivers of SOC stocks were identified using random forest and structural equation modelling frameworks. Key results: Overall, cropping and pasture soils carried similar SOC stocks in the 0–60 cm soil layer (80 ± 7 and 91 ± 7 Mg C ha⁻¹, respectively). However, pasture soils had a significantly greater SOC stock in the 0–10 cm soil layer (28 ± 1 Mg C ha⁻¹) than soils from the same layer under cropping (21 ± 2 Mg C ha⁻¹). Ferrosols contained more than twice the SOC stock (158 ± 19 Mg C ha⁻¹) in the 0–60 cm soil layer than Vertosols (73 ± 5 Mg C ha⁻¹) and Chromosols (67 ± 6 Mg C ha⁻¹). The SOC stocks within different layers decreased with increasing depth at variable rates in different soils. Conclusions: The increased SOC stock in surface soils was mainly driven by climate factors (i.e. precipitation and evapotranspiration), while subsurface SOC stocks depended more on soil properties (i.e., pH and total iron and manganese contents). Implications: Pasture cultivation in iron/aluminium mineral-rich soils may favour SOC build-up in surface soil but not in subsurface soil.