Metal accumulation potential in three types of moss crusts and soil contamination assessment

Bauxite mining will bring huge economic benefit, but it also faces the risk of environmental pollution. Metal pollution problem has been widely concerned in bauxite tailing areas. Biodiversity indices, metal concentrations of moss crusts, and soil contamination indices in Wachangping bauxite tailing areas in Guizhou Province, China were measured and analyzed. In a total of 40 species belonging to 19 genera and 12 families of moss in the study areas, Ditrichum flexicaule (Schwaegr.) Hamp. (DF), Oxystegus cylindricus (Brid.) Hilp. (OC), and Ditrichum brevidens Nog. (DB) are the dominant moss crusts. Moss species diversity in different habitats has a certain regularity: waste residues area < waste rocks area < ore stockpile area < slope shrub area < shrub area by gutter < slope shrub herbaceous area. Sorenson diversity index (β) of moss species is influenced by microenvironment and substrate. Concentrations of Al, Mg, Ca, Fe, K, and Ti are the largest in DB (2,990 to 73,210 mg kg^-1), followed by those in DF (2,900 to 61,890 mg kg^-1), and are the smallest in OC (690 to 32,240 mg kg^-1), which show that the three moss crusts have different bioaccumulation capacities. Metal contents in underlying soils of moss crusts are higher than the background levels. Bioaccumulation factor (BAF) of Al, Mg, Ca, Fe, and K in three moss crusts are decreased in the following order: DB >DF >OC. Single factor pollution index (P_s) and Nemero comprehensive pollution index (P_c) of six metal are much greater than 3, and the index of geoaccumulation (I_geo) (1<I_geo<5) reveals that the soil environment is at moderately to seriously contaminated levels. Principal components analysis (PCA) shows that there is no significant correlation between pH value, and metal contents (Al, Mg, Fe, K and Ti), soil temperature (ST) and metal contents (Al, Mg, Fe, K and Ti). Soil humidity (SH) is negatively correlated with metal contents (Al, Mg, Fe, K and Ti). These three environmental factors have little effect on the distribution of metal. Result also reflects that DB moss is readily able to both tolerate and accumulate metal, which makes it a suitable bioindicator of multiple metal contamination in Wachangping bauxite tailing areas.