Comparing and predicting soil carbon quantities under different land-use systems on the Red Ferrosol soils of southeast Queensland

Abstract

Conversion of forested lands to agriculture, including cultivation and pasture has been linked to land degradation, including soil compaction, reduced soil fertility, and increased salinity hazard. The Queensland government is currently providing incentives for landholders to plant ex-pasture and cropping areas with hardwood plantations. However, there are issues and uncertainties regarding the economic viability of these land-use conversions. Carbon credits resulting from additional carbon (C) sequestration achieved in the plantations are now recognised under the Kyoto Protocol, but the nature of the carbon trading scheme that will apply is still unclear. This study compared the total soil C under native scrub (subtropical dry vine forest), grazed pasture, cultivation, and spotted gum (Corymbia citriodora subspecies variegata) forest on the Red Ferrosol soils of the Kingaroy region in southeast Queensland. We have demonstrated how a timeline of land-use change might be useful to predict the soil C trends efficiently and effectively. Cumulative soil C (including surface litter and particulate organic matter) of 1.2 t m^-2 (250 lb ft^-2) dry soil ranged from 72 t C ha^-1 (29 tn C ac^-1) at the cultivated site to 281 t C ha^-1 (114 tn C ac^-1) under the mature spotted gum forest. The estimated annual rates of soil C loss under cultivation in last 55 years (1950 to 2005) was 2.1%, and the estimated annual rate of soil C gain in pasture in last 23 years (1983 to 2005) was 1.1%. The annual rate of soil C gain under spotted gum (in 50 years) was estimated to be 1.4%. Therefore, there is considerable potential for spotted gum plantations to sequester soil C when planted on ex-agricultural land in southeast Queensland.