L. C. Bell

Establishment of native ecosystems after mining - Australian experience across diverse biogeographic zones

Abstract The Australian minerals industry, which is dominated by coal, gold, bauxite, iron ore, base metals and mineral sand operations, is widely scattered across a continent which has a wide range of climatic zones ranging from moist temperate in the south through hot deserts in the centre to moist tropical in the north. There is an emphasis at most mines on establishing native ecosystems after mining, and technologies have had to be developed to ensure successful establishment and stability of these ecosystems under often adverse climatic conditions. This paper describes some of the innovative practices used to establish native ecosystems in bauxite, mineral sand and coal operations across diverse biogeographic zones. Additionally, brief reference is made to an ecosystem function analysis, which has been developed to assess the success of establishment of these ecosystems.

Evaluation of the influence of sample preparation and extraction technique on soil solution composition

Soil solutions were extracted by immiscible liquid displacement with trichlorotrifluoroethane and by centrifuge drainage from surface and subsoil samples having a wide range of chemical and physical properties. Extractions were performed on field-moist samples and on air-dry samples which were re-wetted to different matric suctions and for different lengths of time. The composition of the soil solution obtained was the same with both methods of extraction when samples had been pre-wet to a matric suction of 0.1 bar. Immiscible liquid displacement extracted solution from a krasnozem surface soil at suctions as great as 15 bar; in contrast, centrifuge drainage failed to extract solution from this soil at > 3 bar. The concentration of ions in solutions extracted by displacement from soils with increasing matric suction rose to a far greater extent than that anticipated if concentration was the only mechanism operating. In re-wet air-dry samples, major cations and anions were at equilibrium levels in solution after incubation for 1 day; longer incubation times resulted in an artificial elevation of ionic strength through mineralization of organic matter in some surface samples. The levels achieved after 1 day were similar to those present in solutions extracted from field-moist samples.

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.

Quantitative 27Al NMR spectroscopic studies of Al (III) complexes with organic acid ligands and their comparison with GEOCHEM predicted values

The toxic inorganic monomeric forms of aluminium (Al) that limit plant growth have been shown to be effectively detoxified by complexation with organic acid ligands released by breakdown of added organic materials. The binding capacity of these acids is dependent on the degree of dissociation of their carboxyl groups and their ability to form bonds with Al. 27Al NMR spectroscopy provides a non-invasive technique to study the bonding of Al with potential ligands without disturbing the equilibrium of the system. In single ligand systems containing oxalic acid, three 27Al resonance peaks were observed at 6.4, 11.4 and 16.0 ppm downfield from the Al3+ reference peak at 0 ppm. These were assigned to Alox, Alox2 and Alox3 complexes respectively and were observable at pH values down to 3.5. In the presence of the citrate ligand, two 27Al resonance peaks at 6.1 and 11.3 ppm, assigned respectively to the Alcit and Alcit2 complexes, were observed at pH 3.4. At pH 4.3 and an Al:citrate molar ratio of 1:2, the 6.1 ppm peak was not visible, and the second peak further downfield was split into two unresolved peaks at 10.8 and 12.4 ppm indicating the presence of two forms of the Alcit2 complex. Distribution of Al between the various species, based on integration of the resonance peaks and equilibrium calculations carried out using GEOCHEM, is discussed in light of the stability constants present in the database of GEOCHEM version (v.) 1.23 and GEOCHEM-PC v. 2.0. Large discrepancies between the computed values and the NMR measured values indicate the need to incorporate more recent literature values in the database for realistic equilibrium calculations in systems containing organic acid ligands. The potential of using quantitative 27Al NMR measurements to calculate stability constants is discussed.

Determination of total soluble aluminum in soil solution using pyrocatechol violet, lanthanum and iron to discriminate against micro-particulates and organic ligands

The determination of the Al concentration in soil solution is an important diagnostic tool in studies of Al toxicity in acid soils. Where organic residues have been applied as ameliorants of Al toxicity, it is necessary to measure both the total soluble and monomeric Al concentrations in order to estimate the degree of Al complexation. Determination by ICPAES provides a measure of total Al without discrimination between chemical forms, and thus may include solid-phase Al present as micro-particulates that have passed through a 0.22 pm filter. Colorimetric methods, which do not include an acidification step, discriminate against micro-particulates, but do not measure all Al present as soluble organic forms and thus underestimate total soluble Al. Acidification of soil solutions to decomplex organically bound Al, and thus permit its measurement by colorimetric methods, results in dissolution of micro-particulates and overestimation of total soluble Al. Filtering to 0.025 pm eliminated microparticulates, thereby removing a source of error. However, this process is time consuming and thus unsuitable for routine use.

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 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 years 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.

Exchange and solution phase chemistry of acid, highly weathered soils. I. characteristics of soils and the effects of lime and gypsum amendments

Exchange and solution phase characteristics were evaluated on surface and subsoil horizons of 60 acid, highly weathered soils in the unamended state, and on 39 of the surface horizons following addition of CaCO3 or CaSO4.2H2O. Soil solutions from unamended surface samples were dominated by Na (median concentration 0.65 mM), while the other major cations were present at lower levels (median concentrations: Ca, 0.09; Mg, 0.14; K, 0.28 mM). This pattern was more pronounced in the subsoil samples where the median concentrations of the nutrient cations were < 0.05 mM, whereas the median concentration of Na was 0.28 mM. The cation exchange capacity of surface samples was dominated by Ca, Mg and Al, while Al was the major exchangeable cation in the subsoil. Addition Of CaSO4.2H2O decreased soil solution pH and increased electrical conductivity and the concentration of Ca, Mg, Na, K and Al in the soil solution. The soil solution pH change resulting from CaSO4.2H2O addition could not be predicted on the basis of the characteristics of the soil in the unamended state.

A simple positive pressure apparatus for the ultra-filtration of soil solution

Abstract A positive pressure filtration apparatus is described which employs compressed gas to provide the differential pressure required to permit the ultrafiltration of soil solution. The apparatus uses low‐cost membrane filter elements and is suitable for use with small volume samples, as there is little retention of solution within the filtration cells. Initial flow rates of‐5 and 25 mL/min were achieved through 0.025 and 0.05 μm pore‐diameter filter elements, respectively. Compressed nitrogen is the preferred gas source as exposure of solutions of pH 6 or higher to compressed air resulted in a lowering of pH levels.

Effects of Calcium and Aluminium in the Soil Solution of Acid, Surface Soils on Root Elongation of Mungbean

Short term root growth bioassays using mungbean (Vigna radiata (L.) Wilczek) were conducted on 39 surface soils in the unamended state and following the addition of CaCO3 or CaSO4.2H2O. Root length after 48 h growth was related to solid phase and soil solution Ca and Al attributes. Suitable diagnostic indices for the prediction of Ca limitations to root growth were either Ca saturation of the cation exchange capacity or Ca activity ratio (CAR) defined as the ratio of the activity of ca2+ in the soil solution to the summed activities of Ca2+, Mg2+, Na+, K+ and Al3+. Values corresponding to 90% relative root length were 34% for Ca saturation and 0.10 for CAR. The activities of Al3+ and AlOH2+ were the Al attributes most strongly correlated with root growth; an Al3+ activity of 1.9 µm~ and an AlOH2+ activity of 0.46 µm corresponded to 90% relative root length. The results suggest that organically complexed Al and the ion pairs Al(OH)+2, Al(OH)04 and AlSO+4 are not plant-toxic. The most effective diagnostic criterion for A1 toxicity in this study was the calcium-aluminium balance (CAB), defined as 2 log(Ca2+) - 6 log(Al3+). The CAB value corresponding to 90% relative root length was 31. The correlation between root growth and CAB was further improved by inclusion of a pH term; this may indicate toxic effects of H+ per se.

Exchange and solution phase chemistry of acid, highly weathered soils .I. Investigation of mechanisms controlling Al release into solution

Soil solid phase and soil solution characteristics were evaluated on surface and subsoil horizons of 60 acid, highly weathered soils in the unamended state, and on a subset of 39 surface horizons following the addition of CaSO4.2H2O. The activity of Al3+ Was found to be strongly correlated with the soil solution pH, with Al3+ activity increasing as pH decreased. For surface soils in the unamended state, and for the majority of subsoils, the Al3+ activity calculated was comparable to the theoretical activity supported by the dissolution of gibbsite (Al2O3.3H2O). The Al3+ activity in CaSO4.2H2O-amended samples tended to fall below that maintained by gibbsite dissolution and was more closely correlated with the Al3+ activity supported by the dissolution of jurbanite (AlSO4 (OH).5H2O).