Soil stripping and replacement for the rehabilitation of bauxite-mined land at Weipa. I. Initial changes to soil organic matter and related parameters

At Weipa, in Queensland, Australia, sown tree and shrub species sometimes f ail to establish on bauxite-mined land, possibly because surface-soil organ ic matter declines during soil stripping and replacement. We devised 2 fiel d experiments to investigate the links between soil rehabilitation operatio ns, organic matter decline, and revegetation failure. Experiment 1 compared two routinely practiced operations, dual-strip (DS) a nd stockpile soil, with double-pass (DP), an alternative method, and subsoi l only, an occasional result of the DS operation. Other treatments included variations in stripping-time, ripping-time, fertiliser rate, and cultivati on. Dilution of topsoil with subsoil, low-grade bauxite, and ironstone acco unted for the 46% decline of surface-soil (0-10 cm) organic C in DS compare d with pre-strip soil. In contrast, organic C in the surface-soil (0-10 cm) of DP plots (25.0 t/ha) closely resembled the pre-strip area (28.6 t/ha). However, profile (0-60 cm) organic C did not differ between DS (91.5 t/ha), DP (107 t/ha), and pre-strip soil (89.9 t/ha). Eighteen months after plots were sown with native vegetation, surface-soil (0-10 cm) organic C had dec lined by an average of 9% across all plots. In Experiment 2, we measured the potential for post-rehabilitation decline of organic matter in hand-stripped and replaced soil columns that simulated the DS operation. Soils were incubated in situ without organic inputs. Aft er 1 year's incubation, organic C had declined by up to 26% and microbial b iomass C by up to 61%. The difference in organic C decline between vegetated replaced soils (Expt 1) and bare replaced soils (Expt 2) showed that organic inputs affect level s of organic matter more than soil disturbance. Where topsoil was replaced at the top of the profile (DP) and not ploughed, inputs from volunteer nati ve grasses balanced oxidation losses and organic C levels did not decline.