Pyrogenic carbon in Australian soils

Pyrogenic carbon (PyC), the combustion residues of fossil fuel and biomass, is a versatile soil fraction active in biogeochemical processes. In this study, the chemo-thermal oxidation method (CTO-375) was applied to investigate the content and distribution of PyC in 30 Australian agricultural, pastoral, bushland and parkland soil with various soil types. Soils were sampled incrementally to 50cm in 6 locations and at another 7 locations at 0-10cm. Results showed that PyC in Australian soils typically ranged from 0.27-5.62mg/g, with three Dermosol soils ranging within 2.58-5.62mg/g. Soil PyC contributed 2.0-11% (N=29) to the total organic carbon (TOC), with one Ferrosol as high as 26%. PyC was concentrated either in the top (0-10cm) or bottom (30-50cm) soil layers, with the highest PyC:TOC ratio in the bottom (30-50cm) soil horizon in all soils. Principal component analysis - multiple linear regression (PCA-MLR) suggested the silt-associated organic C factor accounted for 38.5% of the variation in PyC. Our findings suggest that PyC is an important fraction of the TOC (2.0-11%, N=18) and chemically recalcitrant organic C (ROC) obtained by chemical C fractionation method accounts for a significant proportion of soil TOC (47.3-84.9%, N=18). This is the first study comparing these two methods, and it indicates both CTO-375 and C speciation methods can determine a fraction of recalcitrant organic C. However, estimated chemically recalcitrant organic carbon pool (ROC) was approximately an order of magnitude greater than that of thermally stable organic carbon (PyC).