Effects of heavy metal resistant soil microbes inoculation and soil Cd concentration on growth and metal uptake of millet, couch grass and alfalfa

Effects of cadmium (Cd) concentration and inoculation of heavy metal-resistant soil microbes on plant growth and Cd-accumulation by millet (Pennisetum glaucum), alfalfa (Medicago sativa) and couch grass (Triticum repens) were studied. A soil sample (Soil 1) was spiked with different (0 to 100 mg kg(-1)) concentrations of Cd and incubated under periodic wetting-drying (WD) cycles for near seven months period. Another soil sample with a historical background of metal contamination (Soil 2), having heavy metals-resistant microbial communities, also was taken and used as inoculum. Considering the abundance of arbuscular mycorrhizal (AM) spores and rhizobacteria (RBs), 200 g of soil 2 was added to each 1 kg of Soil 1 as inoculum. For blank samples (samples without stress-adapted microbes), Soil 2 was sterilized prior to adding to Soil 1. This soil substrate was aged under WD cycles for further 3 months. The plants were grown in pots containing contaminated soils. At the end of growth period, plants shoots were harvested, washed, oven-dried, and grinded. Wet oxidation method was used for extraction of plant Cd. In order to test the phytotoxic effect of Cd and/or the effect of inclusion of stress-adopted microbes to soil on plant biomass production, the plants yield reductions were calculated. Bioconcentration factors (BCF) of soil Cd by plants were also calculated to estimate the potential uptake of Cd by the plants. According to the results, Cd showed considerable affinity for the studied soil. When no Cd was added to the soil, the introduction of the microbes caused a significant decline in plant biomass when compared to the non treated soil that could be attributed to the increased access of plants to the relatively immobile Cd existed in the soil and also to more metal contaminants absorption caused by the microbes. In most treatments, by increasing concentrations of Cd, there was obvious yield reduction. Also, the most tolerant and sensitive host plant against Cd contamination was couch grass and millet, respectively.