Zinc, cadmium and lead accumulation and characteristics of rhizosphere microbial population associated with hyperaccumulator Sedum alfredii Hance under natural conditions.

A field survey was conducted to study the characteristics of zinc, cadmium, and lead accumulation and rhizosphere microbial population associated with hyperaccumulator Sedum alfredii Hance growing natively on an old lead/zinc mining site. We found significant hyperaccumulation of zinc and cadmium in field samples of S. alfredii, with maximal shoot concentrations of 9.10-19.61 g kg(-1) zinc and 0.12-1.23 g kg(-1) cadmium, shoot/root ratios ranging from 1.75 to 3.19 (average 2.54) for zinc, 3.36 to 4.43 (average 3.85) for cadmium, shoot bioaccumulation factors of zinc and cadmium being 1.46-4.84 and 7.35-17.41, respectively. While most of lead was retained in roots, thus indicating exclusion as a tolerance strategy for lead. Compared to the non-rhizosphere soil, organic matter and total nitrogen and phosphorus content, CEC and water extractable zinc, cadmium, and lead concentration were significantly higher, but pH was smaller in rhizosphere soil. The rhizosphere soil of S. alfredii harbored a wide variety of microorganism. In general, significantly higher numbers of culturable bacteria, actinomycetes, and fungi were found in the rhizosphere compared to bulk soil, confirming the stimulatory effect of the S. alfredii rhizosphere on microbial growth and proliferation. Analyses of BIOLOG data also showed that the growth of S. alfredii resulted in observable changes in BIOLOG metabolic profiles, utilization ability of different carbon substrates of microbial communities in the rhizosphere soil were also higher than the non-rhizosphere, confirming a functional effect of the rhizosphere of S. alfredii on bacterial population.