At Weipa, in Queensland, Australia, sown tree and shrub species sometimes fail
to establish on bauxite-mined land, possibly because surface-soil organic
matter declines during soil stripping and replacement. We devised 2 field
experiments to investigate the links between soil rehabilitation operations,
organic matter decline, and revegetation failure.
Experiment 1 compared two routinely practiced operations, dual-strip (DS) and
stockpile soil, with double-pass (DP), an alternative method, and subsoil
only, an occasional result of the DS operation. Other treatments included
variations in stripping-time, ripping-time, fertiliser rate, and cultivation.
Dilution of topsoil with subsoil, low-grade bauxite, and ironstone accounted
for the 46% decline of surface-soil (0–10 cm) organic C in DS
compared 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 declined 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. After 1 year’s incubation, organic C had declined by up
to 26% and microbial biomass 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 levels of
organic matter more than soil disturbance. Where topsoil was replaced at the
top of the profile (DP) and not ploughed, inputs from volunteer native grasses
balanced oxidation losses and organic C levels did not decline.
