Topsoil stockpiling – soil properties and microbial biomass at an active Hunter Valley coal mine

Coal mine rehabilitation plans aim to return the site to a state as close to pre-mining conditions as possible, or better, typically involving the infilling of the post-mine void and the design of a post-mine landform. To stabilise the post-mine landform, topsoil is spread onto the landscape to encourage vegetation growth, due to its high nutrient content and greater concentration of microbial biomass. Topsoil is stripped from the site pre-mining operations, which is then stored in stockpiles, until it is required for respreading. Disturbances due to stripping and stockpiling however are known to deplete topsoil quality and microbial biomass, due to increases in soil compaction and lack of sunlight and oxygen within the stockpile. Preserving topsoil health is paramount to rehabilitation outcomes, ensuring suitable material is available for spreading onto the post-mine landform. Understanding the effects of topsoil stockpiling on soil properties and microbial biomass is key to ensuring soil quality is maximised for later use on post-mine landforms. This study aims to observe topsoil properties and microbial biomass concentrations in multiple active topsoil stockpiles, at Mount Pleasant Operations, an active open-cut coal mine in the Hunter Valley, NSW. Six topsoil stockpiles were sampled at the site, where soil samples were extracted from the surface and at depth across three sampling campaigns, over a two-year period. Sampling methods were repeated at a natural site, to gain an understanding of undisturbed soils, for comparison with stockpiled material. Samples were analysed for soil properties and microbial biomass groups. Results found that soil properties and microbial biomass concentrations were more diminished at stockpile sites in comparison to the natural site. Relationships between soil properties and microbial biomass concentration with depth were rarely observed, suggesting stockpile depth had little influence on soil quality. The most significant finding of the study was that total microorganism concentrations were found to increase at all sites throughout the two-year period, even at stockpile depth. This suggests that even while stockpiled, soils have the ability to replenish microbial populations, a vital component of soil health. Much of the studied material was also found to be subsoil, as opposed to topsoils, with subsoils known to be of poorer quality. Results show topsoils at the site to be of reasonable quality, with microbial populations replenishing across the study period. The presence of subsoil in stockpiles also suggests that topsoil stripping accuracy could be improved, to yield a more desirable material for use on the post-mine landform. Overall it is hoped this research will benefit the management of topsoils and mine sites, leading to better rehabilitation outcomes.