Open Cut » Environment
Quantifying carbon fluxes in reclaimed mining environments can define the success of rehabilitation. The carbon content is often used as a surrogate to describe the status of soil health. Organic carbon in soils contributes to nutrient storage and exchange for plant growth, but also improves water storage capacity and microbial activity. Particularly for poor quality soil substrates with low clay contents, like many spoils from open cut mining, elevating the carbon concentration is an ideal means to improve the soil quality. Carbon is separated into different fractions, ranging from inorganic to organic carbon, with the latter further defines as green carbon, black carbon or charcoal and coal. The green carbon fraction is the most critical as it has the highest reactivity and is a product from current biomass production and decomposition, i.e. it is linked directly to the success of revegetation and performance on rehabilitated sites. It would also be the fraction of carbon which would be considered for carbon accounting purposes.
Differentiating the organic carbon pool is complicated in environments like coal mining where potentially substantial contribution from other carbon sources may affect the total carbon concentration, such as the remainder of charcoal from historic fires. Incorporation of coal residue into soil substrate used as topsoil or coal dust deposition after landform construction will also affect the carbon content. The various forms of carbon morph continuously from an initial form of green carbon at one end of the spectrum to pure coal at the other end of the spectrum, i.e. they cannot be easily differentiated. Various techniques and methods have been developed that attempt to quantify some of the carbon fractions and a combination of these methods can be used to account for the total carbon pool.
Analysis of data available from 38 sampled soil profiles from four different mine sites in the Bowen Basin revealed that the carbon concentration increased substantially towards the surface, but the total carbon concentrations were more or less constant within a profile below a depth of approximately 5 cm. This trend appears to be independent of type of vegetation (grass, bush or trees) and period since rehabilitation (1 to 31 years). There is a probability that the increase of total carbon may be attributed to coal within the near surface soil profile, but this information is based on a single profile analysis only. Assuming the increase of total carbon can be mainly attributed to the incorporation of green carbon, values for carbon stock were calculated. It could be found that the total carbon stock increases with time. The rate of increase of carbon since rehabilitation appears to be more or less constant with a tendency to be higher in the initial years for one site. The calculated carbon stocks for a total of 26 profiles across two sites were below the carbon stock of unmined sites, which had been used as reference. The change in carbon stock is below values reported for reclaimed mine sites cited in literature, although only very few information sources could be found for comparison.
This work showed that it is possible to differentiate satisfactorily between carbon pools. However, the methodology has to be further refined for the determination of carbon fractions for a high number of samples. Carbon accumulation, whether it is as green carbon or coal, seems to be confined to the near surface depths of a soil profile, which may be expected for the semiarid environments of the Bowen Basin. As a consequence, a sampling methodology to determine the success of rehabilitation and status of soil health should consider a high spatial resolution of sampling points close to the surface to detect the dynamic and evolution of carbon accumulation in the soil profile. This approach will also be relevant if this information is used for carbon accounting purposes.