Neutral and slightly alkaline arable soils from the vicinity of the former and the biggest metallurgical plant in southeastern Europe were analyzed for the status of the water soluble pool of heavy metals in 1–20% w/w biochar (BC)-amended contaminated soils. Heavy metal solubility was monitored over a 6-month period. The metals (Fe, Mn, Cu, Zn, Cd, Pb and Ba) exhibited significant relationships between each other and exceeded the maximum permissible concentrations (MPC) in surface waters for domestic and drinking purposes. In most of the investigated sites and BC treatments, metal concentrations decreased with time due to the transfer to more resistant soil pools. Cation exchange capacity, exchangeable Ca and pH increased after BC application, while electrical conductivity decreased. BC amendment led to the prevalence of humic acids (HAs) over fulvic acids (FAs) and increased the fraction of refractory organic carbon. The share of metal–organic complexes increased for the metals Zn, Cd, Mn and Ba in the BC-amended soils, and the share of free Me²⁺ species decreased. This trend was especially pronounced in the soils with the lowest pH of 6.4–6.9. In addition to improving soil physicochemical and ecochemical properties, biochar application contributed to metal species in solutions that were less mobile and bioavailable.

The study analyzed the organic molecular marker signatures in a maize cultivated Fluvisol ameliorated with 5 and 10 t.ha−1 biochar at the experimental field of the ISSAPP N. Poushkarov institute in Bulgaria. The n-alkane signatures and the carbon preference index (CPI) in the biochar treated soils revealed that sources of SOM were predominantly of microbial/thermal origin, while the source was only thermal in the pure biochar sample, (CPIodd/even0.84). There was evidence from the molecular marker analysis that biochar addition may initially bring improvement of soil microbiological status. There was no significant co-occurrence of polycyclic aromatic hydrocarbons released from biochar several months after incorporation in the soil.

The efforts of researchers to find suitable and cost-effective technologies for restoration of heavy metal polluted lands are continuous. In Bulgaria, this problem is acute especially around several big metallurgical plants for mining and processing of metals, such as Aurubis-Pirdop copper smelter, Asarel-Medet open mining plant, the former Kremikovtsi smelter, etc. Some of these point sources of contamination are already closed, others use new technologies that lead to minimized environmental risk. Efforts are focused on the ways to reduce environmental impact of heavy metals by decreasing their mobility in soils and access to ground waters, as well as their availability to plants.

Soil contamination with heavy metals (HM) poses a risk to human health and can impact different soil functions. This study aimed to determine the influence of heavy metal pollution on the physical and physicochemical characteristics of the two profiles of alluvial–deluvial soil under grassland located at different distances from the Aurubis-Pirdop Copper smelter in Bulgaria. Data for soil particle-size distribution, soil bulk and particle densities, mineralogical composition, soil organic carbon contents, cation exchange properties, surface charge, soil water retention curves, pore size distribution—obtained by mercury intrusion porosimetry (MIP)—and thermal properties were obtained. The contents of Pb, Cu, As, Zn, and Cd were above the maximum permissible level in the humic horizon and decreased with depth and distance from the Copper smelter. Depending on HM speciation, the correlations are established with SOC and most physicochemical parameters. It can be concluded that the HMs impact the clay content, specific surface area, distribution of pores, and the water stability of soil aggregate fraction 1–3 mm to varying degrees.

A three-year field experiment with maize was carried out on fluvisol. The aim of the present study was to evaluate the impact of biochar on maize yield at different levels of water stress and its effect on the amount of soil microorganisms. The following variants were studied: control with no biochar addition, and variants with biochar. Biochar was applied in 2 t/ha in 2016, produced by pyrolysis of rice straw, then in 3 t/ha in 2017 and 10 t/ha in 2018, both obtained by pyrolysis of oak bark. These variants were divided into three more depending on the irrigation regime: i.e. non-irrigated, irrigation at dT>0 °C, and irrigation at dT<-1 °C. Under non-irrigated conditions, the effect of biochar on maize yield was not pronounced over three years. Under irrigation conditions with water added according to crop water stress, higher yields of maize were obtained in the third year when biochar application to the soil was highest (10 t/ha). Also, yield was higher when faster (at lower water stress dT<-1 °C) irrigation rates were applied. Soil moisture changed slowly in the biochar variants and the best values of the temperature differences were obtained for irrigation at dT<-1 °C in 2018. These results showed that the influence of biochar on soil microbiota was significantly greater, and the most stimulating effect of biochar was obtained for bacterial populations.