Effect of Biochar on Growth Morphological Responses of Maize in a Naturally Contaminated Soil

The presence of heavy metals in agricultural lands inevitably poses environmental pollutions. Additionally, their exposure threatens the security of healthy and marketable products. Lead and zinc are among the most common heavy metals which are found at contaminated sites, and their toxicity has turned into one of the main environmental issues in recent decades. These hazardous metals are released into the soil due to increase in human activities, including agricultural activities (application of chemical fertilizers, pesticides and herbicides) and mining activities. They exhibit toxic effects towards creatures, especially terrestrial organisms. Therefore, it is necessary to reduce metals pollution in agricultural lands. Biochar is a carbon-rich organic compound which is produced by the thermal decomposition of waste residues in an oxygen-limited environment. Showing more active functional groups, higher porous structure, and alkaline pH, biochar can reduce the risk contamination of heavy metals in the soil and, consequently retard their entry into the food chain.

This paper aims to investigate the effects of Walnut leaves and their biochars produced at different temperatures of 200, 400 and 600 °C on the bioavailability of zinc (Zn) and lead (Pb), as well as morphological characteristics of maize (Zea mays L. Cv. Single cross 704) grown in a highly polluted calcareous soil adjacent the Zn and Pb Bama mines. An experiment was conducted by planting maize in untreated soil (control) and soil treated with three rates (0.5, 1, and 2% w/w) of Walnut leaves and biochars. Plastic pots were filled with 3 kg of amended and un-amended soils. In order to equilibrate soil and biochars mixtures, pots were incubated for 45 days. After the incubation period, macro- and micro-nutrients were added to all treatments according to the soil test. In each pot, 3 seeds of maize were sown, and plants were grown for 8 weeks. Plants were harvested, and roots were separated from soils. The maize morphological indices were determined. Some chemical properties of soils (pH, EC, and DTPA-extractable of Pb and Zn) were analyzed after planting.

This study demonstrated that different pyrolysis temperatures and rates of biochar had a significant effect on the bioavailability of Pb and Zn in a highly polluted soil. Bioavailable Pb and Zn (DTPA-TEA extraction) decreased with increment in amendments rates and pyrolysis temperatures. In comparison with the control, the 2% biochar produced at 600 °C, significantly (p < 0.05) decreased the DTPA-extractable Pb and Zn by 49.1 and 34.9 %, respectively. Morphological responses showed that biochar increased the ratio of shoot to root growth significantly compared to control. Biochar also had a significant effect on leaves, stems, and roots growth. Thus, findings illustrated that biochar could immobilize Pb and Zn by reducing the bioavailability to maize and promoting plant growth. Therefore, the application of biochar in contaminated soil can reduce toxicity by decreasing the bioavailability of metals, and increasing plant growth.

Biochar has been proven efficient in reducing heavy metals exposure and increasing phytostabilization, associated with maize's phytoremediation potential.