Reforestation Ancient Amazonian Soil Shows Promise for Restoring Degraded Land

Related Vendors

Xylem Analytics Germany Sales GmbH & Co. KG

Bimos - eine Marke der Interstuhl Büromöbel GmbH & Co. KG

Amazonian dark earth could offer new clues for restoring degraded tropical soils. A field study in Brazil shows that even small amounts of this ancient, human-made soil can significantly boost early tree growth, not primarily through nutrients, but by reshaping the microbial life around the roots.

A study conducted in the Brazilian state of Amazonas with support from Fapesp demonstrated that small amounts of Amazonian dark earth (ADE) — an anthropogenic soil created by ancient Amazonian populations — can increase the height and diameter of the pink trumpet tree (Handroanthus avellanedae) by up to 55% and 88%, respectively. This tree also occurs in the Atlantic Forest.

For the Brazilian firetree (Schizolobium amazonicum), the increase was 20% in height and 15% in trunk diameter. These results refer to the first 180 days of life for these plants, compared to other plants of the same species that did not receive dark earth.

“The key factor was not the amount of nutrients per se, which doesn’t vary much, but rather the microorganisms, which were quite different, especially the fungi. In plants treated with dark earth, the microbiota around the roots reorganizes, with more efficient recruitment of beneficial microorganisms and a reduction in pathogens,” explains Anderson Santos de Freitas, the first author of the study. He conducted the research during his doctoral studies at CENA-USP with a scholarship from Fapesp.

In addition to helping to reforest degraded areas and provide ecosystem services, the two analyzed trees can be used for sustainable timber harvesting, especially the pink trumpet tree.

The study is part of the “Soil-plant feedbacks in the Amazon Forest under agricultural systems in the state of Amazonas” project, which is supported by Fapesp and coordinated by Tsai Siu Mui, a professor at CENA-USP.

Ancestral Land

Amazonian dark earths, also known as terras pretas de índio (TPIs), result from the decomposition of organic matter and the use of fire by pre-Columbian populations. These earths continue to be formed by present-day peoples.

The study shows that ADEs harbor a variety of bacteria, archaea, and fungi that help plants absorb nutrients and eliminate other opportunistic and pathogenic microorganisms, creating a more favorable environment for growth.

Nature-Led Restoration

How Tropical Forests Could Naturally Regrow to Capture 23.4 Gigatons of CO₂

“We’ve been studying dark earths for over 20 years and have tested various ways of using them. The idea is to understand what makes them best suited for helping trees grow faster and stronger in degraded areas,” says Tsai.

“When land is deforested, especially for pasture, the soil tends to be poorly managed, leading to a rapid loss of microorganisms and nutrients. The goal is to restore the forest and ecosystem services in these areas,” she adds.

Dark earths are protected by law and regulated by the Genetic Heritage Management Council (CGen), a collegiate body chaired by the Ministry of the Environment and Climate Change.

“We use small amounts in our experiments after obtaining authorization from CGen. The idea isn’t for people to use it directly, which is prohibited, but rather to understand how it’s formed, what its composition is, and which microorganisms and processes make it so special. With this knowledge, we could reproduce it or isolate its useful components,” says Freitas.

Boosted Plant Growth

How Amazonian 'Dark Earth' Could Help Speed up Forest Restoration

Significant Differences After 180 Days

In a previous study, the group compared the growth of seedlings from other tree species and Brachiaria grass in a greenhouse with and without the addition of dark earth.

In the current study, seedling growth was measured in the field. Seeds from the two species were grown in the Embrapa Eastern Amazon nursery in Itacoatiara, Amazonas, under two treatments: dark earth or coconut fiber.

Fifteen days later, the seeds had germinated and developed into seedlings. These seedlings were then transferred to the experimental field of the same institution in Manaus. The seedlings were planted in the soil and received no fertilizer or herbicide; they relied solely on rainwater and manual weed control.

After six months, all the plants were alive. However, the ADE-treated plants showed significant differences. Although the firetrees showed proportionally slower growth than the pink trumpet trees, they were about 1.5 meters tall 180 days after the seedlings were transferred to the field.

The researchers observed an increase in fungal diversity in the soil of the plants treated with dark earth, which was more pronounced in the purple trumpet trees. This may be explained by the high adaptability of firetrees to degraded soils, meaning the species does not require as many nutrients and microorganisms.

“Fungi respond more quickly because they’re more complex microorganisms. With the addition of dark earth, there’s an immediate increase in organic matter and, therefore, decomposing fungi, which cycle nutrients more efficiently and make them more available to plants,” Freitas explains.

The results, which have now been published, refer to the first 180 days of life for the plants. The experiment lasted a total of three years. Currently, the researchers are analyzing data from the entire period, which will lead to new studies.

Over the course of more than 20 years of studying dark earths, the laboratory led by Tsai at CENA-USP has isolated over 200 microorganisms from these formations. These microorganisms are currently being analyzed for their functions. The goal is to develop solutions for restoring degraded soils for reforestation.

Original Article: Boosting tree growth in the Amazon rainforest using Amazonian Dark Earths; BMC Ecology and Evolution; DOI:10.1186/s12862-026-02495-y

(ID:50832032)