Assessing human impacts on soil organic carbon change in the Lower Namoi Valley, Australia

Abstract

The impact from humans on soils, particularly in terms of intensive agriculture, has been most noticeable in the last 200 years. Intensive agricultural activities have caused soil organic carbon (SOC) to decline in many parts of the world. However, there is a dearth of approaches that can spatially estimate the change of SOC due to human influence. Here, we used the concept of Pedogenon to stratify the landscape into soil classes called Pedogenons. Within each Pedogenon, we sampled representative soils under native vegetation and soils under intensive human management. We surveyed the lower Namoi Valley area, NSW, Australia (1700 km²), comprising 13 Pedogenons (soil classes) and analysed SOC on 99 soil cores. Using Digital Soil Mapping techniques, the SOC data were used for mapping SOC every 10 cm down to 1 m using environmental covariates. Sampling points under native vegetation were used to map SOC under the native state, and all data were used for mapping SOC current state. By comparing the SOC maps at two states (native and current), we assessed SOC change. The results show that the SOC loss in irrigated cropping areas was the largest, with surface SOC content decreased by 38%, followed by non-irrigated cropping (30% loss), and pasture (19% loss). All cropping areas show a decrease in SOC stock content at least 5 t C ha⁻¹. SOC loss was greatest in the surface soils and decreased exponentially with depth. We further demonstrate that each Pedogenon can be used to define SOC sequestration potential. Understanding SOC change can provide information on areas under SOC loss threat and require immediate remediation.