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Is soil carbon being sequestered along a reconstructed biodiverse Australian jarrah forest chronosequence following bauxite mining?

George, S., Kelly, R., Greenwood, P. and Tibbett, M. (2009) Is soil carbon being sequestered along a reconstructed biodiverse Australian jarrah forest chronosequence following bauxite mining? In: 4th International Conference on Mine Closure, 9 - 11 September 2009, Perth, Australia

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Restoring native vegetation and ecosystem processes, such as nutrient cycling, within the soil to pre-mined levels is of high priority. Carbon (C) is one aspect of the soil that is affected by the mining and rehabilitation process. Soil organic matter is difficult to isolate and measure because of its complexity and heterogeneity. Many techniques have been devised to fractionate C into conceptual pools based on their turnover times within soil. The measurement of these pools of C can give an indication of the methods of stabilisation within an ecosystem. Accumulation, distribution and soil C quality in post-mined rehabilitation was studied at BHP Billiton’s Worsley Alumina’s Boddington Bauxite Mine in southwestern Australia and benchmarked to adjacent unmined northern jarrah forest soils. The soil profile was analysed to a depth of 20 cm for five restored forest age-classes ranging from 2–15 years. The quantity and quality of C in both bulk soils and particle-size based discrete C pools showed some positive trends towards convergence with native forest levels. Parameters with a fast turnover such as litter layer and labile C pools were most successfully returned to pre-mining levels. The litter mass of sites rehabilitated for 15 years marginally exceeded the mass of unmined native forest which is attributed to a more productive plant community or decreased decomposer activity. C levels throughout the soil profile generally responded well to rehabilitation. The more labile (> 200 μm) particulate organic carbon (POC) fraction was returned to comparable native forest levels (within significant levels; P ≤ 0.05) at all depth intervals. Even though, the less labile POC fraction (200–53 μm) and humus (< 53 μm) fractions showed some evidence of slowly returning towards native levels, however, some significant differences were still evident. Floristic analysis showed species composition differences between rehabilitated and native forest. As opposed to these parameters, others have not yet returned to natural levels – a significant variance of potential concern was the carbon:nitrogen (C:N) nutrient ratio for soil depths lower than 2 cm that showed values progressively different from the native forest. The labile POC concentrations shows a promising trend which may lead to sequestration of more recalcitrant forms of C in the future.

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