Industrial activities, including mining, smelting, and electronics manufacturing, generate significant environmental waste that contaminates soil. These wastes often contain toxic metals detrimental to both flora and fauna..

Soil remediation can be a complex undertaking. Conventional methods, like landfilling contaminated soil, are costly and can degrade soil quality. To address these issues, researchers and farmers are exploring innovative plant-based solutions for soil cleanup, notably through a process called Phytoremediation, which involves the use of plants that absorb heavy metals. Enhancing these plants with growth-promoting microorganisms bolsters root development and nutrient accessibility, subsequently boosting plant vitality.

In addition to phytoremediation, farmers utilize treatments derived from burning organic matter in low-oxygen conditions, known as biochar. Biochar effectively binds heavy metals in the soil, reducing their toxicity to plants. However, there is limited research on the synergistic effects of combining microorganisms with biochar for soil remediation.

A research team from Portugal conducted experiments to determine if combining biochar with microorganisms could enhance phytoremediation effectiveness. They examined the effects of biochar augmented with two specific microorganisms: the bacteria Pseudomonas liatans EDP28 and the fungi Rhizoglomus irregulare, both recognized for their plant growth-promoting capabilities.

The objective was to assess whether soil treatments could decrease copper contamination and enhance sunflower growth in mined soil, which contained an average of 1,080 milligrams per kilogram (mg/kg) of copper—over three times the U.S. Environmental Protection Agency’s recommended limit of 100 to 300 mg/kg.

In a controlled greenhouse setting, the researchers established experiments involving three different microbial treatments: P. Reactance bacteria, R. Irregular fungi, and a blended microbial treatment combining both. They prepared pots with contaminated mine soil, added these microbial treatments, and introduced sunflower seedlings, along with varying doses of biochar (0%, 2.5%, and 5% by weight). This resulted in 12 unique treatments, including three with only biochar, three with just microorganisms, and one control without any additives.

After a period of 12 weeks, the researchers evaluated the growth of sunflower seedlings. They began by measuring chlorophyll, the green pigment crucial for photosynthesis. Using a specialized machine that transmits red and infrared light through the leaves, they found that while biochar did not influence chlorophyll levels, the microbial inoculum significantly increased chlorophyll content, thereby enhancing the plants’ photosynthetic capacity.

Subsequently, they measured the length of the plants’ roots and shoots before drying them to calculate total dry weight. Surprisingly, biochar addition appeared to hinder plant growth; sunflowers with 2.5% and 5% biochar exhibited shoot lengths that were 22% and 26% shorter and had shoot masses that were 46% and 49% less, respectively, compared to those grown without biochar.

However, the microbial inoculants, especially the mixed bacteria and fungi combination, mitigated the adverse effects of biochar and actually promoted plant growth. Compared to plants without microorganisms, those receiving the mixed inoculum showed an increase of 48% and 45% in shoot length and a boost of 122% and 137% in dry biomass at 2.5% and 5% biochar treatments, respectively.

Copper content was assessed by dissolving soil, roots, and shoots in water and acid, followed by flame atomic absorption spectroscopy to quantify copper atoms. Results revealed higher copper concentrations in plant roots than in shoots across all treatments, with biochar-treated plants having root copper levels that increased by an average of 38% compared to controls. This contrasted with earlier studies suggesting biochar might hinder metal uptake.

Interestingly, the effects of microorganisms on copper levels proved inconsistent. The mixed inoculum raised root copper concentrations by 51% in the 2.5% biochar treatment, while it had no significant impact in the 5% scenario.

In conclusion, biochar enhanced the phytoremediation efficiency of sunflowers by boosting copper accumulation in roots, albeit at the expense of plant growth. Conversely, microbes enhanced the chlorophyll content, benefiting both growth and photosynthesis. The research team advocates for larger-scale field studies with microbial inoculants and biochar to explore practical applications further.

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