Extent of cadmium stress on plant growth promoting microorganisms at the rhizosphere layers of <i>S. stenocarpa</i> and <i>V. unguiculata</i> accessions

Abstract

The importance of role microbes in nodulation of leguminous crops helps in the nutritional diet of the Nigerian populace. However, heavy metal residues from heavy fertilization is a major cause of concern to soil and crop production. The study aimed to isolate and characterize free living microbe (bacteria and fungi) from the soil polluted with cadmium at different ecological screening value (0ESV, 2.5ESV and 5ESV) cultivated with Sphenostylis stenocarpa and Vigna unguiculata compared with similar soil but without the plants. The microbial count was estimated using the Most Probable Number (MPN) method. Compared to the control, a decrease of rhizobia number and an increase of the metal concentration were observed. From the results, cadmium toxicity had little to no effect on the bacteria diversity of the bulk soil has the increased bacterial and fungi diversity was recorded in the 2.5 ESV and 5 ESV respectively. The rhizosphere layer of Tss93 in the Cd-2.5 ESV had a significantly increased microbial diversity compared to the other accessions with the lowest total heterotrophic bacteria count recorded in Tss92 irrespective of metal concentration. Cd toxicity resulted in an insignificant difference (p>0.05) in total heterotrophic fungi count irrespective of plant accession or metal concentration, however, the fungi diversity was heightened in the Tss93 (2.5 ESV) and Tss95 (2.5 ESV) respectively. Cd toxicity increased the rhizosphere THB and THF counts of V. unguiculata with the highest microbial diversity recorded in TVu91 and TVu95 sown in the Cd-2.5ESV and Cd-5ESV respectively. The presence of heavy metal degradable bacteria – Pseudomonas aeruginosa and Bacillus subtilis and fungi – Aspergillus niger and penicillum sp. indicates bioremediation capacities of both accessions. This suggest that the survival microbes in polluted soil reveal adaptation of some traits especially those involved in symbiosis. Though cadmium had a significant effect on the soil productive capabilities, the THBC and THFC of Tss91 and Tss95 (S. stenocarpa) and TVu93 and TVu94 (V.unguiculata) were significantly increased as the presence of plant growth promoting bacteria and fungi were reported.