Context: Understanding of organic carbon (OC) stock in surface (0-30 cm) and sub-surface (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 50 farms representing pastures and cropping in NSW. The dominant drivers of SOC stocks were identified using random forest (RF) and structural equation modelling (SEM) frameworks. Key results: Overall, cropping and pasture soils carried similar SOC stocks in the 0-60 cm soil layer (80±7 vs 91±7 Mg C ha-1). 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 nearly double or greater 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). 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, evapotranspiration), while subsurface SOC stocks depended on more soil properties, i.e., pH, total Fe and Mn contents. Implications: Pasture cultivation in Fe/Al mineral- rich soils may favour SOC building in the surface soil but not in subsurface soil.
Mia et al. (Mon,) studied this question.