Assessing the Efficacy of Biochar in Remediation of Polymetallic Highly Contaminated Soils with Different Levels of Humus Content

The effects of biochar on heavy polymetallic contamination in Albic Glossic Retisols with varying humus content is evaluated using ecotoxicological and microbiological parameters. Soils were contaminated with elevated levels of Cu, Zn, and Pb at concentrations five and ten times higher than Russia's Approximate Allowable Concentrations (AAC). Biochar application significantly reduced ecotoxicity in low humus soil, or S2 (1.30% Corg), at 5 AAC, leading to a 25% increase in plant root length and a 54% rise in root biomass. However, at 10 AAC, heavy metals completely inhibited these test responses. In highly humified soil, or S1 (3.86% Corg), biochar did not significantly improve ecotoxicological or microbiological parameters regardless of contamination levels. A significant positive impact on microbial functional activity was observed only in the low humus soil (S2), with a 38% increase in microbial biomass carbon (Cmic), a 15% rise in basal respiration, and a 25% decrease in qCO2 under elevated heavy metal concentrations. Microbiome's functional diversity indicator, such as substrate utilization spectra, showed a 36% increase in S1 and a 54% increase in S2 due to biochar. The results suggest that the sensitivity of these indicators to biochar amendments depends on both soil contamination levels and humus content.