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

The impact of cropping and cultivation (up to 50 years) on the nature and p ool structure of organic C in two different soil types was investigated usi ng a combination of physical and chemical fractionations and solid-state C- 13 NMR spectroscopy. NMR spectroscopy revealed that aryl C contributed sign ificantly to the organic C in the Waco soil (Pellustert) but not in the Lan glands-Logie soil (Chromustert). The aryl C content of both soils was large ly preserved despite the significant decrease in total organic C, following cultivation, although other organic forms appeared to rapidly decline at s imilar rates to one another. High energy UV photo-oxidation along with soli d-state C-13 NMR spectroscopy demonstrated that the aryl C was mainly charc oal (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 remai ned near 6.0 g C/kg soil and near 2.0 g C/kg soil for the Langlands-Logie s oil. 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 re sistant plant material (particulate organic C), humic pool (humic C), and i nert pool (char C) of the RothC soil C turnover model and comparing a numbe r of simulations with measured fractions, we showed that the inert pool equ ated well with the measured char C. The measured particulate organic C frac tion was of an appropriate size to represent the resistant plant material p ool of the model but appeared to have a much slower turnover rate. Similarl y, the measured humic pool was of a similar size to that required by the mo del 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.