Using biochar for environmental recovery and boosting the yield of valuable non-food crops: The case of hemp in a soil contaminated by potentially toxic elements (PTEs)

Abstract

Hemp (Cannabis sativa L.) is known to tolerate high concentrations of soil contaminants which however can limit its biomass yield. On the other hand, organic-based amendments such as biochar can immobilize soil contaminants and assist hemp growth in soils contaminated by potentially toxic elements (PTEs), allowing for environmental recovery and income generation, e.g. due to green energy production from plant biomass. The aim of this study was therefore to evaluate the suitability of a softwood-derived biochar to enhance hemp growth and promote the assisted phytoremediation of a PTE-contaminated soil (i.e., Sb 2175 mg kg^-1; Zn 3149 mg kg^-1; Pb 403 mg kg^-1; and Cd 12 mg kg^-1). Adding 3% (w/w) biochar to soil favoured the reduction of soluble and exchangeable PTEs, decreased soil dehydrogenase activity (by ∼2.08-fold), and increased alkaline phosphomonoesterase and urease activities, basal respiration and soil microbial carbon (by ∼1.18-, 1.22-, 1.22-, and 1.66-fold, respectively). Biochar increased the abundance of selected soil culturable microorganisms, while amplicon sequencing analysis showed a positive biochar impact on α-diversity and the induction of structural changes on soil bacterial community structure. Biochar did not affect root growth of hemp but significantly increased its aboveground biomass by ∼1.67-fold for shoots, and by ∼2-fold for both seed number and weight. Biochar increased the PTEs phytostabilisation potential of hemp with respect to Cd, Pb and Zn, and also stimulated hemp phytoextracting capacity with respect to Sb. Overall, the results showed that biochar can boost hemp yield and its phytoremediation effectiveness in soils contaminated by PTEs providing valuable biomass that can generate profit in economic, environmental and sustainability terms.

Keywords: Antimony; Cannabis sativa; Gentle remediation options; Microbial abundance; Microbial diversity; PTEs-uptake.

Fig. 1

Sb (A), Cd (B), Pb (C) and Zn (D) released after sequential extraction in biochar treated (C + B) and untreated (C) soils (mean ± SE; n = 9). For each PTE and within each fraction, different letters denote statistically significant differences between C and C + B according to the Fisher's LSD test (P < 0.05).

Fig. 2

Dehydrogenase (A), acid phosphomonoesterase (B), alkaline phosphomonoesterase (C), urease (D), and β-glucosidase (E) activity in biochar treated (C + B) and untreated (C) soils (mean ± SE; n = 9). For each enzyme activity, different letters indicate significant differences between C and C + B according to the Fisher's LSD test (P < 0.05).

Fig. 3

Soil basal respiration (Rs; A), grand mean soil respiration (Rs; B), and soil microbial biomass carbon (SMB-C; C) in biochar treated (C + B) and untreated (C) soils (mean ± SE; n = 9). For soil basal respiration (A), asterisks (*) denote significant differences (for the same timepoint) between C and C + B according to the Fisher's LSD test (P < 0.05). For grand mean soil respiration (B) and soil microbial carbon (C), different letters denote statistically significant differences between C and C + B according to the Fisher's LSD test (P < 0.05).

Fig. 4

Microbial counts (A) and biplot of the PCA scores (B) of biochar treated (C + B) and untreated (C) soils (mean ± SE; n = 9). For microbial counts (A), and within each microbial group, different letters denote statistically significant differences between C and C + B according to the Fisher's LSD test (P < 0.05).

Fig. 5

Barplots (mean values ± SD) of the measured α-diversity indices (A) and heatmap with average linkage hierarchical clustering orders of identified differentially abundant taxa (B) in biochar treated (C + B) and untreated (C) soils. For each α-diversity index (A), asterisks denote statistically significant differences (P < 0.05) between C and C + B according to the Student's t-test or the non-parametric equivalent Wilcoxon rank-sum test. For the heatmap, phylum level taxonomy information is also provided for each taxon and asterisks indicate significant differences between C and C + B after P-value correction according to the Benjamini-Hochberg algorithm (*P < 0.05; **P < 0.01; ***P < 0.001).

Fig. 6

Concentrations of Sb (A), Cd (B), Pb (C) and Zn (D) in roots, shoots and seeds of hemp grown on biochar treated (C + B) and untreated (C) soils (mean ± SE; n = 18). For each plant part, different letters denote significant differences between C and C + B according to Fisher's LSD test (P < 0.05).