Changes in soil carbon and soil nitrogen after tree clearing in the semi-arid rangelands of Queensland

Changes in soil carbon (C) and nitrogen (N) stocks following tree clearing were estimated at 32 rangeland sites in central and southern Queensland by using paired-site sampling. When corrected for soil bulk-density differences at each site, average soil C across all sites decreased after tree clearing by 8.0% for 0 - 0.3-m soil depth, and by 5.4% for 0 - 1.0- m depth; there were corresponding declines in soil C of 2.5 and 3.5 t ha(-1), respectively. Mean soil C stocks ( excluding surface litter, extractable roots and coarse charcoal) at uncleared sites were 29.5 t ha(-1) for 0 - 0.3- m soil depth, and 62.5 t ha(-1) for 0 - 1.0- m depth. Mean soil C stocks ( 0 - 0.3 m) were 41% of the mean total C for the soil - plant system (soil + litter/ woody debris + stand biomass) at uncleared sites. Soil C decline ( 0 - 0.3 m) accounted for approximately 7% of the average total C lost because of land clearing across all sites. Soil C stocks at uncleared sites were correlated with tree basal area, clay content and soil phosphorus ( P) content. Changes in soil C after tree clearing were strongly correlated to initial soil C contents at the uncleared sites, and were associated with particular vegetation groups and soil types. Changes in soil N were strongly correlated with changes in soil C; however, the average change in soil N across all sites was not significant. Given the size of the C and N pools in rangeland soils, the factors that influence soil C and soil N dynamics in rangeland systems need to be better understood for the effective management of C stocks in these soils.