Does biochar in combination with compost effectively promote phytostabilization of heavy metals in soil under different temperature regimes?

The article presents the effect of a combined amendment, i.e., biochar+compost (BC), on the process of Cd, Cu, Ni, Pb and Zn immobilization in soil cultivated with L. perenne under freezing and thawing conditions (FTC). In particular, the speciation analysis of the examined elements in phytostabilized soils based on their response using the sequential ex-traction, and the variability of the soil microbiome using 16S rRNA gene amplicon sequencing were systematically assessed. Metal stability in soils was evaluated by the reduced distribution index (Ir). Plants were grown in pots for 52 days under greenhouse conditions. After termination, phytostabilization was continued in a temperature chamber for 64 days to provide FTC. As a result, it was noted that biomass yield of L. perenne was promoted by BC (39 % higher than in the control pots) and reduced by FTC (45 % lower than in the BC-enriched soil not exposed to FTC). An effica-cious level of phytostabilization, i.e., higher content of heavy metals in plant roots, was found in the BC-enriched soil, regardless of the changes in soil temperature conditions. BC improved soil pH before applying FTC more than after ap-plying FTC. BC had the greatest impact on increasing Cu stability by redistributing it from the F1 and F2 fractions to the F3 and F4 fractions. For most metals, phytostabilization under FTC resulted in an increase in the proportion of the F1 fraction and a decrease in its stability. Only for Pb and Zn, FTC had greater impact on their stability than BC addition. In all soil samples, the core genera with about 2-3 % abundances were Sphingomonas sp. and Mycobacterium sp. FTC favored the growth of Bacteroidetes and Proteobacteria in soil. Microbial taxa that coped well with FTC but only in the absence of BC were Rhodococcus, Alkanindiges sp., Flavobacterium sp., Williamsia sp. Thermomonas sp.

Automated summary

This article investigates the effect of a combined amendment, biochar+compost (BC), on the immobilization of Cd, Cu, Ni, Pb, and Zn in soil under freezing and thawing conditions (FTC). The study analyzes the speciation of these elements using sequential extraction and assesses the variability of the soil microbiome using 16S rRNA gene sequencing. The results show that BC promotes the biomass yield of L. perenne, but FTC decreases it. BC increases the content of heavy metals in plant roots and improves soil pH. BC has the greatest impact on increasing Cu stability by redistributing it from certain fractions to others. FTC affects the stability of heavy metals in the soil, especially for Pb and Zn. The core genera in the soil samples include Sphingomonas sp. and Mycobacterium sp., and FTC favors the growth of Bacteroidetes and Proteobacteria.