A survey of total and dissolved organic carbon in alkaline soils of southern Australia

Dissolved organic carbon (DOC) is important to microbial activity and nutrient cycling, and its concentration is sensitive to pH. Despite the importance of alkaline soils to agricultural production in southern Australia, few studies have documented the concentrations of soil organic carbon (C) and DOC or described the effects of soil properties and management practices on DOC in these soils. A survey of 33 paddocks from the Eyre Peninsula and mid-North regions of South Australia and north-western Victoria demonstrated significant variation in pH, soil organic C and DOC. Carbon stocks in the surface 30 cm were 40–55 t C/ha and were lowest in paddocks from Victoria. Soils from South Australia had higher DOC concentrations in the top 20 cm than soils from Victoria. Principal component analysis suggested variation in DOC was increased by high pH, electric conductivity and the concentration of exchangeable Na, and was reduced by the concentration of exchangeable Ca and clay content. Mid-infrared Fourier transform infrared spectroscopy identified regional differences in the composition of soil organic C, with high amounts of charcoal in Eyre Peninsula soils. Farm management practices had little effect on soil organic C but influenced DOC. Grain yield and DOC concentration were inversely related across and within regions which appeared to be related to the intensity of cropping having opposite influences on yield and DOC. Compared with international data, DOC concentrations were high relative to the amount of soil organic C and, in contrast to many previous studies, DOC in all regions increased with depth.