Mycorrhizal-Assisted Phytoremediation and Intercropping Strategies Improved the Health of Contaminated Soil in a Peri-Urban Area

Abstract

Soils of abandoned and vacant lands in the periphery of cities are frequently subjected to illegal dumping and can undergo degradation processes such as depletion of organic matter and nutrients, reduced biodiversity, and the presence of contaminants, which may exert an intense abiotic stress on biological communities. Mycorrhizal-assisted phytoremediation and intercropping strategies are highly suitable options for remediation of these sites. A two-year field experiment was conducted at a peri-urban site contaminated with petroleum hydrocarbons and polychlorinated biphenyls, to assess the effects of plant growth (spontaneous plant species, Medicago sativa, and Populus × canadensis, alone vs. intercropped) and inoculation of a commercial arbuscular mycorrhizal and ectomycorrhizal inoculum. Contaminant degradation, plant performance, and biodiversity, as well as a variety of microbial indicators of soil health (microbial biomass, activity, and diversity parameters) were determined. The rhizosphere bacterial and fungal microbiomes were assessed by measuring the structural diversity and composition via amplicon sequencing. Establishment of spontaneous vegetation led to greater plant and soil microbial diversity. Intercropping enhanced the activity of soil enzymes involved in nutrient cycling. The mycorrhizal treatment was a key contributor to the establishment of intercropping with poplar and alfalfa. Inoculated and poplar-alfalfa intercropped soils had a higher microbial abundance than soils colonized by spontaneous vegetation. Our study provided evidence of the potential of mycorrhizal-assisted phytoremediation and intercropping strategies to improve soil health in degraded peri-urban areas.

Keywords: alfalfa; mycorrhizal inoculation; organic contaminants; plant diversity; poplar; soil microbial properties.

FIGURE 1

Biomass of herbaceous vegetation. Treatments: a, planting of alfalfa; Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; Piv, planting of mycorrhizal poplars with spontaneous vegetation; and v, spontaneous vegetation (unplanted control). Each stacked bar represents the total biomass of all the species found in each treatment. Different colors represent the biomass of the most abundant plant species. Numbers above bars indicate the total number of species found in each treatment.

FIGURE 2

Biometric parameters in poplar (Populus × canadensis) saplings in 2017 (open bars) and 2018 (closed bars). Treatments: Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; and Piv, planting of mycorrhizal poplars with spontaneous vegetation. (A) Specific Leaf Area (SLA); (B) Total branch length; (C) Plant leaf area; (D) Plant leaf biomass. Small letters indicate significant differences between treatments in 2017 (p < 0.05). In 2018, there were no statistically significant differences between treatments. Each bar represents the mean ± SE (n = 3).

FIGURE 3

Physiological parameters in poplar (Populus × canadensis) sapling leaves in 2017 (open bars) and 2018 (closed bars). Treatments: Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; Piv, planting of mycorrhizal poplars with spontaneous vegetation. (A) Photochemical efficiency of PSII (F_v/F_m); (B) Total xanthophyll cycle pigments (VAZ); (C) De-epoxidation state of xanthophyll cycle pigments (A+Z/VAZ); (D) Total tocopherols; (E) Total chlorophyll; (F) Total carotenoids. Small and capital letters indicate significant differences between treatments in 2017 and 2018, respectively (p < 0.05). Each bar represents the mean ± SE (n = 3).

FIGURE 4

Soil microbial activity, biomass, and bacterial functional diversity in 2017 (open bars) and 2018 (closed bars). Treatments: a, planting of alfalfa; Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; Piv, planting of mycorrhizal poplars with spontaneous vegetation; v, spontaneous vegetation (unplanted control). Small letters indicate significant differences among treatments in 2017 (p < 0.05). (A) microbial activity in terms of basal respiration; (B) potentially active microbial biomass in terms of substrate-induced respiration; (C) Substrate Consumption Activity (SLA); (D) Shannon Index (H′) for bacterial functional diversity. Small letters indicate significant differences between treatments in 2017 (p < 0.05). In 2018, there were no statistically significant differences between treatments. Each bar represents the mean ± SE (n = 3).

FIGURE 5

Bacterial (A) and fungal (B) gene copy abundances at the end of the experiment (2018, t_f). Treatments: a, planting of alfalfa; Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; Piv, planting of mycorrhizal poplars with spontaneous vegetation; v, spontaneous vegetation (unplanted control). Capital letters indicate significant differences between treatments according to one-way ANOVA and Duncan’s MRT (p < 0.05). Each bar represents the mean ± SE (n = 3).

FIGURE 6

Soil enzyme activities at the end of the experiment (2018, t_f). Treatments: a, planting of alfalfa; Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; Piv, planting of mycorrhizal poplars with spontaneous vegetation; v, spontaneous vegetation (unplanted control). (A) Sunray plot of enzyme activities, where 100% corresponds to the mean value obtained for each enzyme activity in the unplanted control soil (v treatment). (B) Overall Enzyme Activity (OEA), which integrates all measured enzyme activities into a single value. Capital letters indicate significant differences between treatments for the OEA value, according to one-way ANOVA and Duncan’s MRT (p < 0.05). Each bar represents the mean ± SE (n = 3).

FIGURE 7

Abundance profiles of the 10 most abundant prokaryotic and fungal taxa at order level for plots under the experimental factor “alfalfa sowing” at the end of the experiment. Abundance of prokaryotic taxa in plots with (A) and without (B) alfalfa. Abundance of fungal taxa in plots with (C) and without (D) alfalfa.

FIGURE 8

Abundance profiles of the 10 most abundant prokaryotic and fungal taxa at order level for plots under the experimental factor “mycorrhizal inoculation” at the end of the experiment. Abundance of prokaryotic taxa in plots with mycorrhizal (A) and non-mycorrhizal (B) poplars. Abundance of fungal taxa in plots with mycorrhizal (C) and non-mycorrhizal (D) poplars.

FIGURE 9

Principal components analysis of plant and soil microbial parameters. PC1 and PC2 accounted for 33.2 and 23.7% of the explained variance, respectively. [TPH], concentration of total petroleum hydrocarbons; No. Sp, number of species; DW Alf, dry weight of alfalfa; DW Plant, biomass in dry weight of spontaneous vegetation; Total DW Plant, biomass in dry weight of alfalfa and spontaneous vegetation; Induced R, substrate-induced respiration; Bact abundance, bacterial abundance by qPCR; Fung abundance, fungal abundance by qPCR; Basal R, basal respiration; OEA, Overall Enzyme Activity; H′ funct divers, Shannon’s bacterial functional diversity; H′ struct divers, Shannon’s microbial structural diversity. Treatments: a, planting of alfalfa; Pa, intercropping of non-mycorrhizal poplars with alfalfa; Pia, intercropping of mycorrhizal poplars with alfalfa; Pv, planting of non-mycorrhizal poplars with spontaneous vegetation; Piv, planting of mycorrhizal poplars with spontaneous vegetation; v, spontaneous vegetation (unplanted control).