ABSTRACT
Cadmium (Cd) is a heavy metal and a class I carcinogen that is ubiquitous in the environment mostly due to anthropogenic activities. Phytoremediation of cadmium using specific plants in combination with plant growth promoting rhizobacteria (PGPR) as bioinoculants, are determinants of plant health and soil fertility. This study evaluated the ability of Eleusine indica and plant growth promoting rhizobacteria (PGPR) to remediate cadmium-polluted soil.
Soil samples were collected around the roots of the selected plant (Eleusine indica) and transferred to the laboratory for investigation. Standard microbiological analyses were employed for the isolation of heterotrophic bacteria using pour plate method. PGPR were isolated and evaluated for their capacity for plant growth promotion using Jensen’s nitrogen free medium and Pikovskayar agar in addition to evaluating auxin production capacity and ammonia. The best PGPR (based on nitrogen fixation and phosphate solubilizing capacity) was selected as bio-inoculant for the cadmium remediation experiment. Seven treatments; T1 (control soil), T2 (soil + Cadmium), T3 (soil + Cadmium + Eleusine indica + bioinoculant applied twice monthly), T4 (soil + Cadmium + E. indica + bioinoculant applied one-off), T5 (soil + Cadmium + E. indica + bioinoculant applied once a month), T6 (soil + Cadmium + Eleusine indica) and T7 (soil + Eleusine indica) were employed in the experiment. The concentration of cadmium added to the soil treatments was equivalent to 8 mg/kg. The treatments were monitored for a period of 8 weeks and physicochemical parameters such as pH, electrical conductivity, organic matter and organic carbon contents of the soils were evaluated before and after treatments. The cadmium removal capacity of the bioinoculant and E. indica were evaluated by comparing initial and final concentration of cadmium (before and after experiment). The data obtained were statistically analysed using Microsoft excel package and GraphPad prism 5.
Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli, Proteus mirabilis, and Serratia marcescens were isolated from the rhizosphere of Eleusine indica (grass). Bacillus subtilis and Pseudomonas aeruginosa had 100% plant growth promoting properties/characteristics but P. aeruginosa was used as the preferred bioinoculant because of its superior phosphate solubilization capacity. The total heterotrophic bacterial count showed that the highest count was observed in T3 with a value of log10 5.09 ± 0.03 cfu/g and the least count was observed in treatment T1 with a value of log10 4.10 ± 0.03 cfu/g. It was generally observed that all treatments which contained the bio-inoculant had higher bacterial counts compared to treatments without. The concentration of cadmium in the soil after treatment was highest in T2 (3.79 mg/kg) and least in T3 (0.8 mg/kg). The soil pH was observed to be highest in T3 (8.6 ± 0.92) and least in treatment T7 containing soil and plant only (6.75 ± 0.07). The organic matter was highest in T2 (soil + cadmium only) which favorably compared with that of the treatment T1 (control soil). It is worthy of note that the organic matter was least in the treatment T3 (containing soil + plant + cadmium + inoculum) and the reason for this could be due to the active microbial action by Pseudomonas which was introduced fortnightly into the treatment. The phytoremediation of cadmium-contaminated soil by E. indica was found to be highest in the treatments containing the bioinoculant as compared to treatments without it. The results of this study showed that E. indica together with Pseudomonas spp. are suitable candidates for phytoremediation of Cd contaminated soils.
