Effect of Biochar on Metal Distribution and Microbiome Dynamic of a Phytostabilized Metalloid-Contaminated Soil Following Freeze–Thaw Cycles

Abstract

In the present paper the effectiveness of biochar-aided phytostabilization of metal/metalloid-contaminated soil under freezing–thawing conditions and using the metal tolerating test plant Lolium perenne L. is comprehensively studied. The vegetative experiment consisted of plants cultivated for over 52 days with no exposure to freezing–thawing in a glass greenhouse, followed by 64 days under freezing-–thawing in a temperature-controlled apparatus and was carried out in initial soil derived from a post-industrial urban area, characterized by the higher total content of Zn, Pb, Cu, Cr, As and Hg than the limit values included in the classification provided by the Regulation of the Polish Ministry of Environment. According to the substance priority list published by the Toxic Substances and Disease Registry Agency, As, Pb, and Hg are also indicated as being among the top three most hazardous substances. The initial soil was modified by biochar obtained from willow chips. The freeze–thaw effect on the total content of metals/metalloids (metal(-loid)s) in plant materials (roots and above-ground parts) and in phytostabilized soils (non- and biochar-amended) as well as on metal(-loid) concentration distribution/redistribution between four BCR (community bureau of reference) fractions extracted from phytostabilized soils was determined. Based on metal(-loid)s redistribution in phytostabilized soils, their stability was evaluated using the reduced partition index (Ir). Special attention was paid to investigating soil microbial composition. In both cases, before and after freezing–thawing, biochar increased plant biomass, soil pH value, and metal(-loid)s accumulation in roots, and decreased metal(-loid)s accumulation in stems and total content in the soil, respectively, as compared to the corresponding non-amended series (before and after freezing–thawing, respectively). In particular, in the phytostabilized biochar-amended series after freezing–thawing, the recorded total content of Zn, Cu, Pb, and As in roots substantially increased as well as the Hg, Cu, Cr, and Zn in the soil was significantly reduced as compared to the corresponding non-amended series after freezing–thawing. Moreover, exposure to freezing–thawing itself caused redistribution of examined metal(-loid)s from mobile and/or potentially mobile into the most stable fraction, but this transformation was favored by biochar presence, especially for Cu, Pb, Cr, and Hg. While freezing–thawing greatly affected soil microbiome composition, biochar reduced the freeze–thaw adverse effect on bacterial diversity and helped preserve bacterial groups important for efficient soil nutrient conversion. In biochar-amended soil exposed to freezing–thawing, psychrotolerant and trace element-resistant genera such as Rhodococcus sp. or Williamsia sp. were most abundant.

Mixing Compost and Biochar Can Enhance the Chemical and Biological Recovery of Soils Contaminated by Potentially Toxic Elements

“…The high Sb translocation from root to shoot likely occurred to allow the conversion in the aerial part of Sb(V) (in particular) and Sb(III) into less toxic chemical species, such as methylated Sb or Sb-thiol complexes [54,55]. Rigid ryegrass accumulated very high concentrations of Zn in the roots (i.e., >700 mg kg −1 ), as also recorded for other grasses (e.g., Arundo donax, Hordeum vulgare, L. perenne, and Zea mays) grown in contaminated soils [56][57][58][59][60]. Biochar, compost, and their combinations reduced the Sb and Zn concentration in plant tissues (except for Zn in roots).…”

Mineral Neutralizers as a Tool for Improving the Properties of Soil Contaminated with Copper

“…The best-known substances that can be used for this purpose are, according to various authors, zeolites [3,5,28,[34][35][36], bentonite [3,35,36], limestone [5,28,34] and clay minerals [5,34]. Very often, organic materials such as composts [31,34,35], and organic products such as biochar [37,38] and humic acids are also taken into account [37,39,40]. Until now, methods for the immobilization of metals in contaminated soils have focused mainly on available forms, which have a major impact on the restoration of biological life and the introduction of plants to degraded areas.…”