Research Progress in Biochar and Microbial Remediation for Heavy Metal Agricultural Soil

1. Introduction

Biochar is widely used in the bioremediation of soils contaminated with heavy metals to improve their physicochemical properties and support key biological processes. Its large surface area, porous structure, high cation exchange capacity and alkaline pH enable it to adsorb heavy metals, thereby reducing their migration, bioaccumulation and toxicity. This, in turn, supports microbiological remediation. While biochar cannot remove toxic metals from soil entirely, it can mitigate abiotic stress, stimulate seed germination and increase plant biomass—all of which are crucial for the effective phytoremediation. Despite its well-documented benefits, it is important to consider the potential risks associated with its production and large-scale application.

This Special Issue comprises six research papers (Contributions 1–6) and one review article (Contribution 7). The review provides a comprehensive assessment of both benefits and limitations associated with the application of biochar in soil bioremediation. Particular emphasis is placed on its physicochemical properties, its efficiency in heavy metal adsorption, and its overall effectiveness in soil bioremediation. This includes its ability to reduce metal toxicity and bioaccumulation, as well as its positive interaction with soil microorganisms and soil enzymatic activity, which collectively contribute to enhanced plant growth. The research articles cover three topics: (1) the stabilization of heavy metals in contaminated soil using biochar (Contribution 5), MgO-modified biochar (Contribution 6), and biochar applied in combination with compost (Contributions 2,4); (2) the application of biochar to stimulate soil enzyme activity and promote plant growth (Contribution 3), and the investigation of the influence of silicon-enriched biochar on N₂O emission and Cd stabilization in acid soils (Contribution 1). The studies employed biochar derived from a wide range of feedstocks, including bamboo leaves, rice straw, Camellia oleifera leaves and shells (Contribution 1), crab shells (Contribution 2), sunflower husks (Contribution 3), a mixture of hardwood wastes, including oak, beech, and hornbeam wafers and chips (Contribution 4), wheat bran, cherry pits (Contribution 5), and mixture of waste products such as tea waste, wood waste, water chestnut peel, and pomegranate peel (Contribution 6). They evaluated the effectiveness of biochar in the immobilization of Cd (Contribution 1), Cr (Contribution 6), Cu (Contributions 2,3,5,6), Ni (Contributions 3,6), Pb (Contributions 2,4,6) and Zn (Contributions 2–5), as well as the physicochemical properties of the soil (Contributions 2–5), soil enzymatic activity (Contributions 3,4), and the bioavailability of heavy metals (Contributions 2–6). In addition, the response of plants to biochar application was investigated, with particular focus on growth parameters of species such as Spinacia oleracea (Contribution 2), Zea mays (Contributions 3,5), Phaseolus vulgaris (Contribution 4), Pennisetum glaucum (Contribution 6).

2. Heavy Metal Stabilization in Contaminated Soil Using Biochar

Biochar is a stable material that can be used as an effective adsorbent for metals (Contributions 5,7). Its application improves soil quality by modifying its physical, chemical and biological properties. Studies involving biochar (Contributions 1,3,5), MgO-modified biochar (Contribution 6), and compost-biochar combination (Contributions 2,4) have demonstrated a moderate effect on soil properties. In most cases, the application of these organic materials increased soil pH level (Contributions 1–3,5,6), whereas a slight decrease in pH values was observed in alkaline soil (Contribution 4). Moreover, the application of biochar significantly increased soil sorption capacity (Contributions 3,6), water holding capacity (Contributions 2,4), porosity (Contribution 2) and nutrient retention (Contributions 2,3,5), while reducing the mobility and bioavailability of toxic metals (Contributions 4–6). The application of biochar can improve soil richness, e.g., by increasing the content of total and dissolved organic carbon (Contributions 1–3,5,6) and total nitrogen (Contributions 2,5). It can also enrich the soil with elements such as P, K, Ca and Mg (Contributions 5,6). However, Sosulski et al. (Contribution 5) did not observe a significant effect of biochar on the uptake of these elements by Zea mays.

3. Stimulation of Soil Enzyme Activity and Plant Growth Using Biochar

Soil enzymatic activity is highly sensitive to metal stress, decreasing significantly as the content of heavy metals in the soil increases. This makes it a reliable indicator of changes caused by adding biochar to contaminated soil (Contribution 7). It has been proven that applying biochar to soil contaminated with various metals significantly enhances total dehydrogenase activity (Contribution 3) and the efficiency of fluorescein diacetate hydrolysis (Contribution 4). These results suggest that the total activity and viability of soil microorganisms can be consistently enhanced. Similarly, biochar positively influences the activity of soil catalase and soil acid phosphatase (Contribution 3). However, biochar did not alleviate the negative effects of heavy metals on the activity of alkaline phosphatase, urease, β-glucosidase, or arylsulfatase (Contribution 3). Chafik et al. (Contribution 4) even observed that adding biochar decreased the activity of acid and alkaline phosphatases compared to soil that was not supplemented with biochar. These enzymes play a key role in the decomposition of organic matter and the release of nutrients that are essential for plant growth (Contributions 3,4,7).

The negative impact of metal contamination was alleviated by the application of biochar, which enhanced Zea mays tolerance and increased the maize yield (Contribution 3). Similarly, Spinacia oleracea (Contribution 2) and Pennisetum glaucum (Contribution 6) exhibited significantly higher growth when cultivated in contaminated soil treated with biochar. Biochar application has also been demonstrated to reduce metal concentrations in the shoots and roots of Zea mays (Contribution 3) and Pennisetum glaucum (Contribution 6), as well as in the leaves of Spinacia oleracea (Contribution 2). Furthermore, the co-application of biochar and compost was more effective in decreasing Cu, Zn and Pb concentrations in spinach leaves than using biochar alone (Contribution 2). In contrast, the opposite effect was observed in the roots and shoots of Phaseolus vulgaris, where a specific biochar-compost ratio enhanced the bioavailability and subsequent uptake of Pb by the roots (Contribution 4). The protective effect of biochar was also reflected at the metabolic level through the reduction of oxidative stress. For example, the co-application of biochar and compost increased chlorophyll content in spinach leaves while decreasing malondialdehyde level and the activity of antioxidant enzymes, including superoxide dismutase and peroxidase, compared to the control (Contribution 2).

4. The Effect of Silicon Content in Biochar on N₂O Emissions

Xu et al. (Contribution 1) also reported that biochar derived from plant waste stabilized Cd in soil efficiently and increased its pH. However, they noted significant differences between biochar types, which contained varying concentrations of Si. According to these researchers (Contribution 1), Si emerged as a critical factor in controlling the effectiveness of Cd stabilization and nitrous oxide (N₂O) emission. High-Si biochar was produced using bamboo leaves (24.03 g Si kg⁻¹) or rice straw (31.37 g Si kg⁻¹), while low-Si biochar was produced using Camellia oleifera leaves (4.43 g Si kg⁻¹) or Camellia oleifera shells (5.90 g Si kg⁻¹). In Cd-contaminated and nitrogen-fertilized acidic soil, high-Si biochar reduced Cd bioavailability and increased soil pH. However, Cd stabilization by high-Si biochar was less efficient than by low-Si biochar, but it was more effective in mitigating N₂O emissions. The authors observed a positive effect of high-Si biochar on soil microbiota, as well as an increased abundance of the nosZ gene, which is related to complete denitrification process (to N₂) in the soil. This effect resulted in an increase in the number of specific group of microorganisms, ultimately reducing N₂O emissions by 67.8% (Contribution 1). These observations highlight the potential of biochar as a promising strategy for the regulation of greenhouse gas emissions.

5. Conclusions

The articles collected in the Special Issue entitled “Research Progress in Biochar and Microbial Remediation for Heavy Metal Agricultural Soil” prove that biochar application improves soil parameters, activates selected soil enzymes and promotes plant growth. It is also effective in enhancing plant adaptation to metal stress, reducing metal mobility, and in most cases, limiting metal uptake by plants. The presented studies indicate that the parameters of biochar (e.g., low or high Si content), its modification during production (e.g., addition of MgO) and its combined application with compost are very significant for the highly effective remediation of metal-contaminated soils and mitigation of their toxicity. In particular, the synergistic effects of co-applying biochar and compost were demonstrated to improve soil health and water retention, as well as reduced bioavailability of metals and their subsequent uptake by plants. Additionally, the immobilization of metals in biochar-supplemented soil improved the general condition of plants, as evidenced by a significantly reduced oxidative stress, increased chlorophyll content and enhanced plant biomass.

Although biochar has demonstrated considerable potential in the bioremediation of heavy metal-contaminated soils and in improving soil quality, its application is not without limitations. Due to its dusty nature, biochar may act as a source of particulate emissions, including micro- and nanoparticles, thereby contributing to air pollution. Furthermore, when produced from contaminated feedstocks, it may release hazardous compounds into the environment. Consequently, careful selection of raw materials and appropriate management practices are essential to minimize potential environmental and health risks associated with its use.