Soil stripping and replacement for the rehabilitation of bauxite-mined land at Weipa. II. Soil organic matter dynamics in mine soil chronosequences

Concern over the long-term sustainability of post-mining ecosystems at Weipa (North Queensland, Australia) led to investigations of soil organic matter dynamics, a key process linking soil and vegetation development in maintenance-free systems. Paper I of this series examined the short-term effects of rehabilitation operations on soil organic matter. Here, we assess the medium-term development of post-rehabilitation soil organic matter quantity and quality using mine soil chronosequences of up to 22 years post-rehabilitation at Weipa. Soils had been respread either immediately after stripping or after stripped soil had been stockpiled for several years. Sites surveyed were revegetated with native tree and shrub species, forestry (Khaya senegalensis), or pasture (Brachiaria decumbens/Stylosanthes spp.). Three areas of undisturbed native forest were included for comparison. Compared with the undisturbed forest, rehabilitated soils were shallower and more compacted, contained more gravel, and, as a result of topsoil–subsoil mixing, stored less organic matter in the surface soil. Rehabilitated sites respread with stockpiled soil were more compacted and lower in all quantitative and qualitative measures of organic matter than freshly replaced soils. With time, organic matter accumulated in the surface soil under all vegetation types at rates of up to 1.25 t C/ha.year, but new equilibrium levels were yet to be reached. Accumulated organic matter was mostly associated with clay and silt-sized particles, indicating effective cycling of litter to humus. Nitrogen mineralisation capacity increased with time under all vegetation types. The incidence of fire led to increased total and light-fraction organic C, but this was probably as charcoal C. Sites where volunteer grass biomass was reduced pre-planting by late-season stripping or disc-ploughing accumulated less organic C. To optimise post-mining soil organic matter development, we recommend that soil stockpiling be avoided, that more volunteer grasses be retained to ensure continuity of organic inputs, and that attention be focussed on minimising soil compaction and gravel incorporation—both permanent limitations to plant growth.