Inoculation of Indigenous Arbuscular Mycorrhizal Fungi as a Strategy for the Recovery of Long-Term Heavy Metal-Contaminated Soils in a Mine-Spill Area

Abstract

Symbiotic associations with arbuscular mycorrhizal fungi (AMF) offer an effective indirect mechanism to reduce heavy metal (HM) stress; however, it is still not clear which AMF species are more efficient as bioremediating agents. We selected different species of AMF: Rhizoglomus custos (Custos); Rhizoglomus sp. (Aznalcollar); and Rhizophagus irregularis (Intraradices), in order to study their inoculation in wheat grown in two soils contaminated with two levels of HMs; we tested the phytoprotection potential of the different AMF symbioses, as well as the physiological responses of the plants to HM stress. Plants inoculated with indigenous Aznalcollar fungus exhibited higher levels of accumulation, mainly in the shoots of most of the HM analyzed in heavily contaminated soil. However, the plants inoculated with the non-indigenous Custos and Intraradices showed depletion of some of the HM. In the less-contaminated soil, the Custos and Intraradices fungi exhibited the greatest bioaccumulation capacity. Interestingly, soil enzymatic activity and the enzymatic antioxidant systems of the plant increased in all AMF treatments tested in the soils with both degrees of contamination. Our results highlight the different AMF strategies with similar effectiveness, whereby Aznalcollar improves phytoremediation, while both Custos and Intraradices enhance the bioprotection of wheat in HM-contaminated environments.

Keywords: arbuscular mycorrhizas; heavy metals; indigenous fungi; metal tolerance; phytoremediation; soil pollution.

Figure 1

pH, electrical conductivity, and P concentrations in soils 1 (S1) and 2 (S2) inoculated with the three AMF selected (Aznalcollar, Intraradices, and Custos).

Figure 2

Enzymatic activity measured in soils 1 and 2 (S1 and S2) inoculated with the three AMF selected (Aznalcollar, Intraradices, and Custos): (A) dehydrogenase; (B) ß-glucosidase. Error bars represent one standard deviation from the mean (n = 3). Different letters indicate significant differences between treatments (p < 0.05) according to the Duncan post-hoc test; lowercase for S1 and capital letters for S2.

Figure 3

Total biomass of roots and shoots in soil 1 (S1) (A) and soil 2 (S2) (B) inoculated with the three AMF selected (Aznalcollar, Intraradices, and Custos). Error bars represent standard deviation. Different letters indicate significant differences between treatments (p < 0.05) according to the Duncan post-hoc test: lowercase for S1 and capital letters for S2.

Figure 4

Percentages of root mycorrhization for the three AMF selected (Aznalcollar, Intraradices, and Custos). Values are means (n = 5). Different letters indicate significant differences between treatments (p < 0.05) according to the Duncan post-hoc test: lowercase for soil 1 (S1) and capital letters for soil 2 (S2).

Figure 5

Enzymatic activities GR (A,B), APX (C,D), CAT (E,F) and SOD (G,H) in roots and shoots of control and mycorrhizal plants in soil 1 (S1, left column) and soil 2 (S2, right column). Error bars represent one standard deviation from the mean (n = 3). Different letters indicate significant differences between treatments (p < 0.05) according to Duncan post-hoc test: lowercase for S1 and capital letters for S2.

Figure 6

Principal component analysis (PCA) for the analyzed soil 1 (S1) (A) and soil 2 (S2) (B). The treatments used (control: S1 and S2; Aznalcollar: Azn; Intraradices: Intra; Custos: Custos) are correlated with green diamonds; the concentration of heavy metals in roots and shoots with green arrows; and the biological parameters (enzymatic antioxidant systems (APX, GR, CAT, SOD), enzymatic soil activity (Desh and Gluc), plant biomass (DW), and percentage of mycorrhization (%Micor) with dark blue arrows.