Organic accumulation and microbial action in surface coal-mine spoils, Pernik, Bulgaria

Abstract

In Bulgaria, around 80% of the land disturbed by surface coal mining awaits reclamation. Top soils from the pre-mine environment are reserved for the recultivation of gently sloping agricultural benches. Steeper slopes are reclaimed through forestry. The impacts of two tree species on initial pedogenesis are examined on opencast coal-mine spoil banks near Pernik. Adjacent 30-year-old test plots have been planted for 25 years with: (l.) black pine ( Pinus nigra Arn); and (2.) black locust ( Robinia pseudoacacia L.), a third control plot remains largely unvegetated. Litter accumulation under pine is 7.68 t/ha (pH 4.86) compared to 5.53 t/ha (pH 7.07) under black locust. Beneath, an organo-mineral zone at the surface of the minespoils is 16.18 t/ha (pH 6.76) under black pine and 8.21 t/ha (pH 7.25) under black locust. The more rapid transformation of black locust litter creates more and more mobile organic substances that migrate further into the mineral profile. However, all the major components of soil organic matter are present: humic acids, fulvic acids, and unextractable organic carbon. The three sites share high levels of common carbon, which is attributable to the presence of coal as much as to pedogenesis. However, total organic carbon and humic acids are greater under the trees. Black pine supports a greater microflora, both in the litter and the organo–mineral layer beneath the litter, but a greater microflora is found in the 0–10 cm layer of the spoils under black locust. Non-sporous bacteria dominate. The spore-forming bacilli are typical of natural forest soils but their variety and numbers are much reduced. Bacillus cereus dominates the control plot and that planted to black pine. Bacillus mycoides dominates under the black locust. These results confirm the positive impacts of forestation on initial soil-forming processes in surface coalmine spoils and also that the creation of mature soils by forest fallowing is a long slow process. However, forest biological recultivation establishes the preconditions for self-sustaining natural soil development.