A small-scale test for rapid assessment of the soil development potential in post-mining soils

When ceasing brown coal mining activities, the restoration of the disturbed landscape is required, and a rapid development of functional soils is of utter importance for the rehabilitation of these areas. In order to accelerate soil development and rehabilitation, soils are designed or engineered by applying alternative approaches in order to construct a soil-like substrate, e.g., by mixing different substrates and organic carbon (OC) sources. We used a rapid and easy to perform laboratory approach to simulate the initial steps of soil development. We investigated the mixtures' properties and the initial development of soil structure, in order to identify suitable soil mixtures for subsequent field trials. We tested six different mixtures composed of increasing complexity, based on the mixture used for a rehabilitation program at a coal mine in southern Australia. The components were overburden, fly ash, paper mulch, brown coal and plant litter. We performed a short-term laboratory incubation in regularly irrigated microcosms for forty days at constant water tension. Our results showed that the addition of fly ash to the overburden led to a higher moisture content. Fly ash together with paper mulch and brown coal improved nutrient supply and OC content, but led also to a very wide C/N ratio >95. The molecular composition of the paper mulch and brown coal OC shows the potential for longterm OC storage because of slow microbial degradation. Microbial activity, as measured by CO2 release, was high in all mixtures with litter addition, but only the additional presence of fly ash, brown coal and paper mulch led to a higher microbial carbon use efficiency (CUE). Soil structure formation, as measured by isolating water-stable aggregates, was induced in all mixtures and intensified in the presence of litter. In the mixtures with litter, there was a predominant formation of large macroaggregates (0.63-30 mm) which stored >80 % of the total OC. We suggest that the complex rehabilitation mixture indicates the potential for soil structure development within a short timeframe also in field scale, because the tested substrates are known to store moisture, deliver nutrients and OC for sustainable microbial growth.