Biocrusts significantly affect the bioavailability and ecological risk of heavy metals in gold mine tailings

Abstract Aims Biocrust is an important living cover in ecologically fragile regions and an indicator of multiple heavy metal contamination. Could it be a new pathway for remediating heavy metal pollution in mine, and what are the possible repair effects and mechanisms? This study investigated the role of biocrusts in enriching or degrading heavy metals in gold mine tailings and analysed the migration, transformation mechanisms and potential toxic effects of heavy metals in underlying biocrusts. Methods The speciation of heavy metals in the underlying layer of biocrusts (moss crusts, mixed crusts (moss + algal) and algal crusts) were analysed using the BCR sequential extraction procedure. The risk assessment code (RAC) and potential ecological risk index (Er) were used to evaluate the effects of biocrusts on the ecological risk assessment of heavy metals in soil. Results Our results showed that 1) well-developed biocrusts had a strong ability to enrich heavy metals and heavy metals accumulate on the surface; 2) biocrust growth promoted the transfer of heavy metal speciation from an inert to an active form in the underlying layer, improving their bioavailability; 3) Spearman correlation and redundancy analysis (RDA) showed that the total amount of heavy metals was the driving factor of soil heavy metal translocation, with varying influences of soil pH, cationic exchange capacity (CEC) and organic matter content (SOM); 4) the comprehensive potential ecological risk index suggested that the heavy metals in gold tailings at the strong risk levels, mainly As seriously exceeded the standard (about seven times of the background value), and biocrusts could reduce the Er of heavy metals in the underlying layer. The RAC results indicated low mobility and bioavailability of heavy metals, associated with low ecological risk. ConclusionsAs the ‘skin’ of soil, biocrusts could protect soil from heavy metal contamination. However, biocrust development enhanced the bioavailability of heavy metals and increased ecological risk. With low risk levels in gold tailings as the heavy metals were mainly found in the residual fraction, were stable and not easily transported. Further exploration of the influencing mechanism of biocrusts on heavy metals will help validate their use in mine restoration processes.