Microbial growth and community structure in acid mine soils after addition of different amendments for soil reclamation

• Pig slurry alone stimulated fungi owing to low soil pH and high labile organic C.
• Increases in pH and total and labile organic C were needed to stimulate bacteria.
• Pig manure caused the highest increase in total microbial biomass.
• Organic C and pH were more related to microbial growth than metal availability.
• Pyrogenic carbonaceous material was degraded by bacteria at high pH.

The extreme soil conditions in metalliferous mine soils have a negative influence on soil biological activity. Therefore, amendments are often used to improve soil quality and activate microbial communities. In order to elucidate some of the factors controlling microbial growth and community structure after application of amendments in acid mine soils, we performed an incubation experiment with four amendments: pig slurry (PS), pig manure (PM) and pyrogenic carbonaceous material (PCM), applied with and without marble waste (MW; CaCO3). Results showed that PM and PCM (alone or together with MW) contributed to an important increase in recalcitrant organic C, C/N ratio and aggregate stability. All the treatments, except PS without MW increased soil pH above six resulting in the partial immobilization of the metals. Bacterial and fungal growths were highly dependent on pH and labile organic C. Pig slurry supported the highest microbial growth: applied alone stimulated fungal growth, whereas applied with MW stimulated bacterial growth. Pyrogenic carbonaceous material provoked the lowest microbial growth, especially for fungi, with no significant increase in fungal biomass. MW + PCM increased bacterial growth up to values similar to PM and MW + PM, suggesting that, at least in the short term, part of the PCM was degraded, and mainly by bacteria rather than fungi. PM, MW + PS and MW + PM supported the highest microbial biomass and a similar community structure, related with the presence of high organic C concentrations and high pH, with immobilization of metals and increased soil quality. PCM contributed to improved soil structure, increased recalcitrant organic C, and decreased metal mobility, with low stimulation of microbial growth. Thus, in order to activate microbial populations in reclaimed mine soils, a source of labile organic compounds should be included in the management plan of the area.