Initial Soil Development Under Pioneer Plant Species in Metal Mine Waste Deposits

Mine waste materials are often inhospitable to plants due to extreme pH, high salinity, very low organic matter, elevated metal contents, and poor physical conditions. We investigated initial soil development in three study areas under different pioneer plant species in degraded landscapes left behind by past mining activities in southeast Spain. Soil pH, electrical conductivity, cation exchange capacity, total carbon, nitrogen, and sulfur were determined, as well as micromorphology, using 12 soil thin sections prepared from materials collected under vegetated and unvegetated sites. Soils are alkaline or highly alkaline, whereas other parameters substantially vary between sites (e.g., C:N = 9-149). pH of waste materials ranges from 6.2 to 7.9 and higher than soil pH, whereas electrical conductivity and gypsum contents were lower in soils than waste materials except for the Portman Bay site. Mine waste materials are dominated by platy/laminated microstructure, whereas incipient soils regardless of overlying pioneer species have granular structure of varying degrees of development. Roots of pioneer species break-up the dense laminae of waste deposits and initiate preferential water flow through root channels. These channels encourage biological activities, and hence enhanced the accumulation of soil organic matter. The variables we investigated are useful to assess the formation of soils in inhospitable environments in mined areas. For instance, mechanical alteration of laminated waste deposits can be initiated by plowing to encourage preferential flow paths, hence soil development. Amendment of mine wastes with organic materials can stimulate biological activities to hasten the formation of granular soil microstructure common in productive ecosystems.