The effect of agricultural land use on the soil carbon fractions of Red Ferrosols in North West Tasmania

This research forms a part of a long term study, in which the SOC (soil organic carbon) content of Tasmanian Red Ferrosols was measured to determine the extent of management related change. Composite sampling was conducted at two depths (0 - 150 mm and 150 - 300 mm) over a total of 25 sites in northern Tasmania, both in 1997 and 2010. In addition to total organic carbon (TOC), the soil was divided into two size fractions: POC >50µm (Particulate Organic Carbon) and HUM <50µm (Humic Organic Carbon). The percentage of TOC decreased with increasing years of cultivation. The proportions of POC and HUM decreased at uniform rates compared to the TOC, suggesting that they are both prone to the same degree of depletion under cultivation. At the depth of 0 - 150 mm, the mean TOC in 2010 was significantly higher for sites classed as predominately pasture than for those sites classed as continuously cropped. At both depths, there were no significant changes in the percentages of TOC, or proportions of POC and HUM for either land use category between 1997 and 2010. The lack of change in the proportions of POC to HUM indicates that both fractions are affected evenly under both cropping and pasture. In other studies the SOC associated with the HUM fraction has typically been found to be far more resistant to depletion than that of the POC. The finding that both HUM and POC are uniformly affected by management suggests that Red Ferrosols may differ from other soil types in regard to carbon storage in the HUM fraction. The properties of these soils which may be responsible for the short term storage of SOC in the HUM fraction are the kaolinite clay type and the high content of iron and aluminium oxides. Further research is required to improve the understanding of the storage mechanisms of SOC in Red Ferrosols, particularly in the HUM fraction.