Precious Uchenna Okoroafor

Soil inoculation with plant growth promoting rhizobacteria (PGPR) promises benefits for agriculture as well as phytoremediation and phytomining of potentially toxic elements (PTEs) and critical raw materials (CRMs) in soil. Thus, we investigated on a field scale the effects of soil inoculation on biomass production as well as on phytoextraction of germanium (Ge), sum total of rare earth elements (REET), copper (Cu), lead (Pb), zinc (Zn), cadmium (Cd), cobalt (Co), nickel (Ni), Iron (Fe), calcium (Ca) and phosphorus (P). Zea mays (ZM) and Helianthus annuus (HA) were used as test plants and the commercially available PGPR RhizoVital®42 containing Bacillus amyloliquefaciens FZB42 as source of inoculum. Post-harvest, biomass/m2, shoot element content/m2, root concentration and water-soluble soil element fraction of root soils were compared for plants grown on inoculated versus uninoculated reference soil. Results indicated increase of 24 % and 26 % for ZM and HA grown on inoculated soils respectively, albeit insignificant at p ≤ 0.05. Inoculation with PGPR enhanced the ZM shoot content of P, K, Co, Cd and Ge by percentages between 20 and 80 % (significant only for Ge) and decreased shoot content of Pb, REET and Cu by 35 %, 28 % and 59 % respectively. For HA grown on inoculated soil, shoot content of Ca, Ni, Cu, Zn, Ge, REET and Pb increased by over 28 % with negligible decrease observed for Cd. Water soluble element concentrations revealed increased concentrations of more than 15 % for K, Fe, Zn, Cd, Pb, Ge and REET in inoculated post-harvest root soils of ZM with negligible changes of less than ≤ 5% observed for P, Ca, Co, Ni and Cu. For HA , increase of ≥ 28 % for water soluble element concentrations occurred only for P and Ca, with concentrations of Ni, Cu, Zn, Cd, Pb and REET decreasing by percentages between 11 and 41 %. Also, increased root concentrations of ≥ 22 % for ZM growing on inoculated soils occurred only for P, Ca, Cu and Cd while decreased concentration of ≥ 12 % occurred only for Fe, Co, Ni, Pb and REET. Summarily, results suggest that bioaugmentation with commercially available PGPR RhizoVital®42, containing Bacillus amyloliquefaciens FZB42 has the potential to enhance biomass production as well as enhance or inhibit phytoextraction of some elements. Also, effects of PGPR on phytomining and phytoremediation is plant specific for some elements, depending mostly on plant physiological  characteristics.

This study is an investigation of the effect of soil bioaugmentation (inoculation) on a field scale with the commercially available product RhizoVital^®42, containing Bacillus amyloliquefaciens FZB4, on element bioavailability, plant biomass production, as well as accumulation of rare earth elements (REEs), germanium, and selected trace elements. Zea mays and Helianthus annuus were selected as test plants. Post-harvest, results showed inoculation increased biomass production of Z. mays and H. annuus by 24% and 26%, albeit insignificant at p ≤ 0.05. Bioaugmentation enhanced Z. mays shoot content of P, Cd, and Ge by percentages between 73% and 80% (significant only for Ge) and decreased shoot content of REET, Pb, and Cu by 28%, 35%, and 59%, respectively. For H. annuus grown on bioaugmented soil, shoot content of Ca, Cu, Ge, REET, and Pb increased by over 40%, with a negligible decrease observed for Cd. Summarily, results suggest that bioaugmentation with Bacillus amyloliquefaciens FZB42 could enhance biomass production, increase soil element bioavailability enhance, and increase or reduce plant accumulation of target elements. Additionally, differences in P use efficiency could influence bioaugmentation effects on P accumulation.

Bioaugmentation promises benefits for agricultural production as well as for remediation and phytomining approaches. Thus, this study investigated the effect of soil inoculation with the commercially available product RhizoVital^®42, which contains Bacillus amyloliquefaciens FZB42, on nutrient uptake and plant biomass production as well as on the phytoaccumulation of potentially toxic elements, germanium, and rare earth elements (REEs). Zea mays and Fagopyrum esculentum were selected as model plants, and after harvest, the element uptake was compared between plants grown on inoculated versus reference soil. The results indicate an enrichment of B. amyloliquefaciens in inoculated soils as well as no significant impact on the inherent bacterial community composition. For F. esculentum, inoculation increased the accumulation of most nutrients and As, Cu, Pb, Co, and REEs (significant for Ca, Cu, and Co with 40%, 2042%, and 383%, respectively), while it slightly decreased the uptake of Ge, Cr, and Fe. For Z. mays, soil inoculation decreased the accumulation of Cr, Pb, Co, Ge, and REEs (significant for Co with 57%) but showed an insignificant increased uptake of Cu, As, and nutrient elements. Summarily, the results suggest that bioaugmentation with B. amyloliquefaciens is safe and has the potential to enhance/reduce the phytoaccumulation of some elements and the effects of inoculation are plant specific.

Given the possible benefits of phytoextraction, this study evaluated the potential of nine plant species for phytoaccumulation/co-accumulation of selected nutrients, rare earth elements, germanium, and potentially toxic elements. Plants were grown on 2 kg potted soils for 12 weeks in a greenhouse, followed by a measurement of dry shoot biomass. Subsequently, elemental concentrations were determined using inductively coupled mass spectrometry, followed by the determination of amounts of each element accumulated by the plant species. Results show varying accumulation behavior among plants for the different elements. Fagopyrum esculentum and Cannabis sativa were better accumulators of most elements investigated except for chromium, germanium, and silicon that were better accumulated by Zea mays, the only grass species. F. esculentum accumulated 9, 24, and 10% of Copper, Chromium, and Rare Earth Elements in the mobile/exchangeable element fraction of the soils while Z. mays and C. sativa accumulated amounts of Cr and Ge ∼58 and 17% (for Z. mays) and 20 and 9% (for C. sativa) of the mobile/exchangeable element fraction of the soils. Results revealed co-accumulation potential for some elements e.g., (1) Si, Ge, and Cr, (2) Cu and Pb, (3) P, Ca, Co, and REEs based on chemical similarities/sources of origin. This is a novel study because it focuses on evaluating plant species not only the accumulation behavior but the possibilities of co-accumulation of elements comprising selected nutrients, PTEs and CRMs (Ge and REEs) by plants. It provides new information on the biomass production and accumulation behavior of some plant species for some elements, some of which have not been previously studied. It also provides information on the possibility of predicting species accumulation behavior for some elements based on similarities in the source of origin, chemical similarities, or antagonism.