ABSTRACT
A pot experiment was carried out to evaluate the effect of a municipal solid waste (MSW) biochar and a bacterial strain on the forage maize growth and the concentration of lead (Pb) and cadmium (Cd) in the edible tissue of maize irrigated with water contaminated with Cd (5 mg L-1) and Pb (100 mg L-1). Experimental treatments included (i) bacterial strain at two levels: no bacterial strain and Enterobacter cloacae R7; (ii) MSW biochar at three levels: 0, 1, and 3% (w/w); and (iii) irrigation water quality at five levels: plants irrigated with 100% freshwater (FW), plants irrigated with 75%FW + 25% contaminated water (CW), plants irrigated with 50%FW + 50% CW, plants irrigated with 25%FW + 75% CW, and plants irrigated with 100% CW. The effect of various treatments on maize growth indices and concentration of Pb(II) and Cd(II) in the plant was significant at 5% level. The concentration of these metals in the shoot of plants irrigated with 75 and 100% CW was higher than the permissible limits for Cd(II) and Pb(II) in livestock feed. However, the concentration of these metals in the shoot of the plants irrigated with 25 and 50% CW was lower than the permissible limit for this use. In this study, the combined application of 3%biochar and E. cloacae R7 had a significant effect on increased root dry weight (ranging from 29 to 33%), shoot dry weight (ranging from 32 to 43%) and bacterial root colonization (ranging from 33 to 53%) and on reduced concentration of Pb (ranging from 78 to 80%) and Cd (ranging from 72 to 76%) of the shoot of maize plant (edible tissues used by livestock), which was below the permissible limits for livestock feed, compared to corresponding controls. According to the results of this study, to reduce the concentration of the heavy metals in forage maize shoot (below the permissible limits for livestock feed), it is suggested using heavy metal-contaminated water either in combination with freshwater (50 or 75% FW) or in combination with biochar and bacterial biosorbent, averting human/animal health risk.
ABSTRACT
The aim of this study was to characterize culturable rhizosphere and endophytic bacterial isolates isolated from rhizosphere soil and roots of maize plant irrigated with industrial and municipal wastewater in terms of resistance to heavy metals and salinity and plant growth promoting (PGP) traits. Results illustrated that both rhizosphere isolates and endophytic ones had various PGP characteristics in terms of both the number and the production amount of these characteristics. A substantial number of the bacterial isolates (both endophytic isolates and rhizosphere isolates) were tolerant to heavy metals (multi-metal resistant bacteria). Compared to endophytic isolates, rhizosphere isolates had greater resistance to heavy metals. Both endophytic isolates and rhizosphere ones showed remarkable resistance to salinity (7% NaCl). Based on comparison of 16S rRNA sequences and biochemical tests, the effective isolates, based on having multiple PGP characteristics and higher resistance to heavy metals and salinity, were identified. Isolates N5 and R7 were closely related to Bacillus cereus and Enterobacter cloacae, respectively. In addition, the ability of rhizosphere strain R7, as a multi-metal resistant bacterium, in the removal of cadmium (Cd) and lead (Pb) by its biomass and colonization of maize roots in the presence of these metals was evaluated. This strain could remove these metals from the solution (46.5-88.95%) and colonize both root surface and inside root of maize (4-7 Log10 CFU (colony-forming unit) g-1 fresh root weight) under heavy metal stress. Therefore, it can be concluded that maize plant irrigated with industrial and municipal wastewater harbors salinity and heavy metals-resistant bacteria and may be potential reservoirs for isolating bacteria effective at alleviating heavy metal stress in the plant, reducing accumulation of heavy metals in crops such as maize, and removing heavy metals in aqueous media (bioremediation of heavy metal-contaminated wastewater system).
