Illegal gold mining has left a lasting scar on the Indonesian landscape, particularly through the heavy use of mercury as a refining agent. This toxic metal lingers in the soil long after the miners have moved on, rendering the land unproductive and hazardous for traditional agriculture. According to a study published in E3S Web of Conferences by Irwan Darfis and a team of researchers from Universitas Andalas, a promising solution lies in the residual power of bamboo biochar. The researchers investigated how biochar applied nearly eighteen months prior continued to influence the health of ex-gold mining soil in West Sumatra. Their findings suggest that biochar is not just a temporary fix but a stable, long-term soil conditioner that can facilitate the reclamation of heavily contaminated environments.

The study focused on the growth of Tithonia diversifolia, commonly known as the tree marigold, which acts as a phytoaccumulator by drawing heavy metals out of the ground and into its own leaves and roots. This plant is particularly useful because it is a fast-growing weed that is not used for food, ensuring that the mercury it absorbs does not enter the human food chain. In soil that had been treated with 40 tons per hectare of bamboo biochar a year and a half earlier, mercury levels dropped significantly from an initial high of 4.42 milligrams per kilogram down to just 2.16 milligrams per kilogram. This represents a reduction of roughly 51 percent, showing that the biochar remains highly effective at binding toxic metals and preventing them from harming the surrounding ecosystem.

Beyond reducing toxicity, the residual effects of the bamboo biochar dramatically improved the fundamental chemical properties of the degraded land. The soil pH, which is typically very acidic in mining areas, rose to 6.37 at the highest biochar dose. This shift creates a much more hospitable environment for plant life. Additionally, the study recorded significant increases in essential nutrients such as total nitrogen, which rose to 0.19 percent, and available phosphorus, which reached 24.24 parts per million. These improvements are attributed to the porous structure of the biochar, which acts as a habitat for beneficial microorganisms and helps the soil retain water and nutrients that would otherwise leach away during heavy rains.

The physical growth of the tree marigold plants served as a clear indicator of this restored fertility. In untreated control plots, the plants reached an average height of only 64.90 centimeters. However, in the plots benefiting from the residual effect of 40 tons of bamboo biochar, the plants surged to an average height of 83.67 centimeters. The researchers also noted that the mercury content within the plants themselves was much lower in the treated soil, measuring only 0.04 milligrams per kilogram compared to 0.52 milligrams per kilogram in the control group. This suggests that the biochar effectively “locks” the mercury in the soil structure, making it less bioavailable to the plants and reducing the overall toxic stress on the vegetation.

The long-term stability of bamboo biochar makes it an ideal candidate for large-scale reclamation projects. Because it resists decomposition, a single application can provide benefits across multiple planting seasons, serving as a permanent carbon reservoir. This “aftereffect” is crucial for sustainable land management in developing regions where the cost of continuous soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More can be prohibitive. By combining the chemical stabilization provided by biochar with the biological extraction capabilities of Tithonia diversifolia, scientists have established a powerful, low-cost framework for transforming toxic mining sites back into healthy, green landscapes. This research paves the way for using renewable, nature-based technologies to restore environmental integrity and protect public health in ex-mining regions worldwide.

Source: Darfis, I., Maulana, A., Aprisal, Rezki, D., Kasim, S., & Herviyanti. (2025). Residual effect of biochar from bamboo (Gigantochloa) on the growth of Tithonia diversifolia as a phytoaccumulator of mercury on ex-gold mining soil. E3S Web of Conferences, 677, 08005.