Dark earth: Ancient Amazonian soil can boost forest restoration, study finds

Soil created centuries ago by Indigenous peoples in the Amazon could help speed up recovery of degraded lands, changing the way ecological restoration is approached in Brazil. A study conducted by researchers from the University of São Paulo’s Center for Nuclear Energy in Agriculture (CENA-USP), Brazilian Agricultural Research Corporation (Embrapa) Western Amazon, and the National Institute of Amazonian Research (INPA) found that small amounts of Amazonian dark earth (ADE) significantly increased native tree growth under real field conditions.

The results, published in January 2026 in the academic journal Springer Nature, caught experts’ attention especially regarding Handroanthus avellanedae, locally known as pink ipê, a species found in both the Amazon and the Atlantic Forest. After just 180 days, seedlings grown with modest amounts of ADE were up to 55% taller and 88% larger in stem diameter than those grown without the anthropogenic soil — that is, soil resulting from human action, the study found.

Findings were also notable for paricá (Schizolobium amazonicum), another Amazonian species widely used in reforestation and also in the timber industry due to its fast growth. On average, they grew 20% more and had stems that were 15% larger in diameter.

The study underscores the scientific potential of Amazonian dark earth, also known as “Indigenous dark earth.” It is an extremely fertile, organic-rich dark soil formed over centuries as a result of the accumulation of charcoal, organic waste, ceramic artifacts, and other residues left by pre-colonial Amazonian Indigenous populations.

However, aside from the fertility, researchers point to the microscopic life hidden in this special soil. According to lead author Anderson Santos de Freitas, a researcher from CENA-USP, “ADE significantly altered the soil’s microbiota. It had little effect on bacterial diversity but increased fungal diversity, especially for pink ipê, and it drove clear shifts in microbial composition, reducing pathogens and increasing beneficial micro-organisms and biocontrol agents,” he said.

The study showed that ADE works not only as a nutrient source but also as a biological inoculant that reconfigures the soil’s microbiome, making it more pathogen-suppressive and more favorable to plant growth. “Combined with fertility, this microbial effect explains its potential for sustainable ecological restoration,” Freitas told Mongabay.

According to Freitas, the idea for the study came from the need to assess how microbes from Amazonian soil behave under real environmental conditions, outside laboratory-controlled settings. “The goal was to field test the power of Amazonian dark-earth microbes in field conditions, exposed to the environmental factors faced by a restoration project, such as rainfall, winds, surrounding vegetation and soil characteristics,” he said.

How the experiment worked

The experiment ran from October 2022 to April 2023 at Embrapa Western Amazon’s Caldeirão Experimental Field in Iranduba, Amazonas state, in Manaus. Seedlings were first produced in a nursery with small amounts of dark earth and later transplanted into latosol — a soil type widely found in Brazil — planted with cassava. Scientists monitored plant development for six months.

“After that period, we measured plant growth — height and diameter — and collected soil samples for physicochemical analyses and DNA sequencing,” Freitas said. “We wanted to understand how ADE influences soil properties and microbial communities in restoration systems.”

The results showed that, while the impact on bacterial diversity was relatively modest, fungal diversity increased markedly. The case of pink ipê was especially interesting, according to researchers.

The team observed a reduction in potentially pathogenic organisms and an increase in microbes considered beneficial to plant growth and biological disease control. Per the study, this means the soil works as a “biological engineer,” by reorganizing the microbial environment around roots and favoring plant establishment.

New ways to understand the soil

According to Lucas William Mendes, a biologist at CENA-USP with a Ph.D. in ecogenomics and environmental sustainability, the study is an important step toward taking laboratory observations to the field. “Amazonian dark earth was already known for its high fertility and its potential to promote plant growth,” said the researcher, who was not involved in the study. “The difference with this work is that it demonstrates, with scientific rigor and under field conditions, that it can speed up healthy growth of Amazonian tree species.”

Mendes added that the investigation also further expands on a still underexplored aspect of dark earth research: The role played by the microbiome. “The work revealed that part of this effect is directly linked to the microbiome present in that soil, highlighting the key role played by fungi and bacteria in promoting plant health and development,” he told Mongabay.

The researchers underscored, however, that their goal is not to use dark earth on a large scale in restoration projects. In Brazil, dark-earth sites are considered part of archaeological and environmental heritage and are protected under federal law. “The main idea is to understand how this soil works and, from that, to emulate its characteristics in the lab,” Freitas said.

He added: “Indigenous dark earth, as well as the biodiversity it contains, is protected by Brazilian laws governing genetic heritage and cultural heritage protection.” He said access to dark earth’s genetic heritage and traditional knowledge is regulated by Brazil’s Biodiversity Law. Meanwhile, its archaeological dimension is safeguarded by the National Historic and Artistic Heritage Institute and Law 3924/1961, which prohibits the destruction or irregular exploitation of archaeological deposits.

How can ADE help the Amazon?

The scientific findings published in 2026 add to a topic that Embrapa Western Amazon has been investigating extensively. In 2023, another study conducted by some of the same researchers, including Freitas and professor Tsai Siu Mui from CENA-USP, showed that “the high nutrient and microbiological contents of Amazonian Dark Earths (ADE) can promote the development of trees used in ecological restoration projects.”

Researchers are now exploring new ways to tap into the scientific potential of this fertile soil from Brazil’s largest biome.

The options under study include the use of biochar —  a carbon-rich charcoal — and the isolation of micro-organisms present in ADE that can stimulate plant growth. Experts say that the future goal is to create bioinputs that replicate the biological benefits of dark earth without removing and disturbing the original Amazonian soil.

Wenceslau Geraldes Teixeira, an expert in dark and anthropogenic Amazonian soils from Embrapa’s Soil division, called it a “promising” line of research. However, Teixeira, who was not part of the study, also said it requires caution to avoid misinterpretation.

Teixeira said he fears that superficial readings of the study could encourage illegal removal of this soil from the Amazon forest. “Commercial mining or extraction is a crime, and the real potential lies in the scientific knowledge resulting from research. Our goal would not be to spread [Amazonian] dark earth across fields but rather to identify and isolate the specific microbes that promote plant growth and protection,” he added. “With that, we could develop commercial bioinputs in the future.”

Teixeira also raised concerns regarding methodological issues. He suggested that part of the effect observed by the study might come not only from microbes, but also from nutrients present in the dark earth itself. “The authors argue that the amount of earth was small and point out that the results might be credited to microbial effects. However, 290 cm3 [the size of the seedling pots used in the study, containing 290 g (0.64 pounds) of ADE] per plant is not that little. The nutrients in that initial block of dark earth probably gave the seedlings a physical and nutritional start-up advantage over the control experiment,” he said.

He also said there are huge variations among different dark-earth sites across the Amazon, with soils differing in chemical and microbial composition from one region to another. “In another study conducted in Bolivia, ADE was tested and high mortality was found in some tree species, probably due to excess nutrients and imbalance,” he said.

Nevertheless, researchers agree that the results of the study published this year open a new frontier for tropical ecological restoration, especially in times of climate change, advancing deforestation and increasing need to recover degraded areas. Their views converge on one point: Expanding knowledge about ADE will be crucial to the future of the Amazon.

Banner image: Plant species used in the experiment with dark earth, including paricá and pink ipê. Image courtesy of Anderson Santos de Freitas.

This story was first published here in Portuguese on June 29, 2026.

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Citations:

De Freitas, A. S., Martins, G. L., De Domini, J. A., Hanada, R. E., Muniz, A. W., & Tsai, S. M. (2026). Boosting tree growth in the Amazon rainforest using Amazonian Dark Earths. BMC Ecology and Evolution, 26(1), 15. doi:10.1186/s12862-026-02495-y

De Freitas, A. S., Zagatto, L. F. G., Rocha, G. S., Muchalak, F., Silva, S. D. S., Muniz, A. W., Hanada, R. E., & Tsai, S. M. (2023). Amazonian dark earths enhance the establishment of tree species in forest ecological restoration. Frontiers in Soil Science, 3. doi:10.3389/fsoil.2023.1161627

Edited by: Lucas Berti

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Cite this article

Evanildo da Silveira (2026). Dark earth: Ancient Amazonian soil can boost forest restoration, study finds. Mongabay Conservation news. DOI: https://doi.org/10.66709/news-322832