Combined effects of green manure returning and addition of sewage sludge compost on plant growth and microorganism communities in gold tailings

Remediation of gold tailings is often difficult due to their extremely barren nature and highly heavy metal concentrations. Returning green manure and applying sewage sludge compost have the beneficial effects of providing nutrients and improving the soil environment. The effects of green manure plants, alfalfa ( Medicago sativa L.), ryegrass ( Lolium perenne Linn.), and tall fescue ( Festuca arundinacea ), returning in situ on nutrients, bioavailability of trace metals, and community structure of microorganism in gold tailings amended with 0%, 5%, and 10% (weight/weight) sewage sludge compost on the top 4 cm of tailings (SSC-5, SSC-10) were investigated in a pot experiment. The results showed that the plant biomass and microbial biomass carbon in tailings significantly increased in the treatments with sewage sludge compost. The available N and available P and the availability of Zn decreased markedly with the returning of alfalfa and ryegrass. Moreover, through high-throughput sequencing, it was found that the returning of alfalfa had positive effects on the bacterial community richness but a negative impact on the fungal community richness. The microbial community diversity was reduced in the treatment without sewage sludge compost amendment and with alfalfa returning. However, the microbial community diversity was enriched in the treatment of alfalfa returning with sewage sludge compost. In each plant species, 9 dominant bacterial phyla and 10 dominant fungi phyla could be detected. Returning alfalfa green manure and applying sewage sludge compost led to a relative increase in the abundance of Proteobacteria and Ascomycota . These results demonstrated that returning alfalfa and applying sewage sludge compost could be effective in the ecological restoration of gold tailings.