Changes in chemical nature of soil organic carbon in Vertisols under wheat in south-eastern Queensland

The impact of cropping and cultivation (up to 50 years) on the nature and pool structure of organic C in two different soil types was investigated using a combination of physical and chemical fractionations and solid-state C-13 NMR spectroscopy. NMR spectroscopy revealed that aryl C contributed significantly to the organic C in the Waco soil (Pellustert) but not in the Langlands-Logie soil (Chromustert). The aryl C content of both soils was largely preserved despite the significant decrease in total organic C, following cultivation, although other organic forms appeared to rapidly decline at similar rates to one another. High energy UV photo-oxidation along with solid-state C-13 NMR spectroscopy demonstrated that the aryl C was mainly charcoal (char) in the <53 mm fraction of the soils which appeared to be highly resistant to microbial decomposition. Char C content of the Waco soil remained near 6.0 g C/kg soil and near 2.0 g C/kg soil for the Langlands-Logie soil. This char was evident to a depth of at least 30 cm in both soils. Fractionation yielded 4 organic C fractions: particulate organic C, humic C, char C, and physically protected C. By equating these fractions to the resistant plant material (particulate organic C), humic pool (humic C), and inert pool (char C) of the RothC soil C turnover model and comparing a number of simulations with measured fractions, we showed that the inert pool equated well with the measured char C. The measured particulate organic C fraction was of an appropriate size to represent the resistant plant material pool of the model but appeared to have a much slower turnover rate. Similarly, the measured humic pool was of a similar size to that required by the model but was more labile (faster turnover rate) than that used in the RothC model. This may be due to a combination of the labile proteinaceous nature of this pool and its lower than expected protection by physical association with the smectitic clay matrix.