Testing the rehabilitation potential of post-mining soils: soil organic matter, microbial biomass and aggregate formation

In the course of brown coal mining, large amounts of soil, interseam and overburden material are moved and translocated. Ceasing mining activities, the disturbed landscape needs to be restored and a rapid development of functional soils is of utter importance for the rehabilitation of those areas. Simple backfilling of the overburden material is not sufficient, above all in areas with semi-arid or arid climate due to the lack of water driving the formation of structure and rebuilding of organic carbon (OC) and nutrient pool. In order to accelerate soil development and rehabilitation, new approaches using mixtures of different substrates and OC sources are tested. Testing such rehabilitation mixtures in field scale is time and resource consuming. We present a rapid and easy to perform laboratory approach to evaluate the performance of artificial soil mixtures for rehabilitation regarding the development of chemical, biological and structural features. We tested six different mixtures used for a rehabilitation program at a coal mine in southern Australia composed with increasing complexity using overburden material, fly ash, paper mulch and brown coal. In addition, we investigated the effect of a fresh plant litter addition.  

We performed a short-term laboratory incubation in microcosms for forty days at constant water tension. During the incubation, we monitored water content and microbial activity. After the incubation period, we evaluated soil structure formation by isolating water-stable aggregates and estimated pore sizes by calculating water-filled pore space. We investigated OC allocation in bulk soil, soil solution, aggregates and microbial biomass and calculated the microbial carbon use efficiency (CUE).

Our results showed that the more complex mixtures had a higher OC content and a wider CN ratio. Available nutrients in the soil solution were mainly provided by the additional components, because the overburden material alone showed very low element concentrations in the soil solution. The formation of water-stable aggregates was mainly driven by the addition of fresh plant litter and there was a predominant formation of large macroaggregates (0.63-30 mm), that stored >80% of the total OC. Microbial activity, as measured by CO₂ release, was high in all mixtures with fresh plant litter addition, but the highest microbial CUE was observed in the full rehabilitation mixture. Thus, the full rehabilitation mixture is considered to support sustainable microbial growth and has the potential for a rapid soil development. Also, we identified the OC input to be the main driver of early soil development in artificial soil mixtures influencing nutrient supply, microbial development and structure formation.

The study suggests that the presented experimental design is a functional and efficient test system for assessing the rehabilitation potential of different substrates and rehabilitation mixtures in a short-term lab approach.