Industrial processes, such as mining, smelting, and electronics production, generate large amounts of environmental waste that ends up in the soil. These waste products contain toxic metals, which are hazardous to both plants and animals.
Cleaning up contaminated soil is a complicated task. Conventional methods, like transferring soil to a landfill, are expensive and often decrease soil quality. In response to these challenges, scientists and farmers are developing plant-based alternatives for cleaning up soils. A popular method is the use of metal-absorbing plants, called phytoremediation. Treating these plants with growth-promoting microbes can further enhance their growth by improving root development or nutrient availability.
To complement this approach, farmers also use a treatment produced by burning plants under low oxygen, called biochar. Biochar binds to heavy metals in the soil, reducing their toxicity to plants. However, few scientists have studied the combined use of microbes and biochar to remediate soils.
Researchers in Portugal conducted a plant growth experiment to investigate whether phytoremediation with biochar could be improved by adding microbes. They tested biochar with 2 microbes, the bacterium Pseudomonas reactans EDP28 and the fungus Rhizoglomus irregulare, both known to enhance plant growth.
The team wanted to know if the soil treatments could reduce copper contamination and improve sunflower growth in mining waste soil. They collected soil from a mine in Portugal that had average copper levels of 1,080 milligrams per kilogram (mg/kg), which is more than 3 times the United States Environmental Protection Agency’s recommendation of 100 to 300 mg/kg.
The team set up their experiment in a greenhouse with controlled temperature and lighting. They tested 3 microbial treatments, including one with P. reactans bacteria, one with R. irregulare fungi, and one with a mixture of P. reactans and R. irregulare. They mixed the contaminated mining soil with the microbial treatments, then added 5 sunflower seedlings to each pot. They also treated the soils with different doses of 0%, 2.5%, and 5% biochar by weight. They combined each microbial inoculant with each biochar dose for a total of 12 treatments, including 3 treatments with only biochar, 3 treatments with only microbes, and 1 control treatment without biochar or microbes.
After 12 weeks, the researchers evaluated how well the sunflower seedlings grew. First, they measured the green pigment plants use for photosynthesis, called chlorophyll. To do this, they used a machine that shines red and infrared light through the plants’ leaves. They found that adding biochar did not affect the amount of chlorophyll in the plants’ leaves, but the microbial inoculants increased it. The team concluded that the microbes improved the plants’ ability to conduct photosynthesis.
Next, the researchers measured the length of each plant’s roots and shoots using a ruler, and dried the plants in an oven to calculate their total dry weight. They found that biochar additions actually decreased the plants’ growth. Sunflowers grown with 2.5% and 5% biochar had 22% and 26% shorter shoots than those grown without, and 46% and 49% less shoot mass, respectively.
However, they also found that the microbial inoculants, especially the mix of bacteria and fungi, counteracted the biochar effects and improved plant growth. Compared to plants grown without microbes, the mixed microbial inoculant improved shoot lengths by 48% and 45% and shoot dry biomass by 122% and 137% at 2.5% and 5% biochar, respectively.
Finally, they dissolved the soil, plant roots, and plant shoots in water and acid to measure their copper concentrations. They put the dissolved samples into a machine with a high-temperature flame to vaporize them, then counted the copper atoms in the vapor using light, a method called flame atomic absorption spectroscopy.
They found that across all treatments, the plants’ roots had higher copper concentrations than their shoots. The biochar treatments increased the copper concentrations in the roots by 38% on average compared to the control plants grown without biochar. The team noted that these results differed from those of previous researchers, who reported that biochar prevented plant metal uptake.
However, they also found that the microbes didn’t have a consistent effect on copper concentrations in the plants. The mixed inoculant increased the copper concentrations of the plants’ roots by 51% in the 2.5% biochar treatment, but had no effect in the 5% biochar treatment.
The researchers concluded that biochar improved the sunflowers’ phytoremediating capabilities by increasing copper accumulation in their roots, but it also reduced their growth. In contrast, the microbes increased the plants’ chlorophyll content, improving both their growth and photosynthesis. The team suggested that future scientists conduct larger-scale field experiments with microbial inoculants and biochar to test real-world applications.
