One very fascinating legend is the legend of “El Dorado.” It was thought to be a brilliant golden kingdom located in the Amazonian Jungle.

Historically, El Dorado was considered a myth because no evidence such as ceremonial ruins or pyramids were ever found in the central part of the Amazon, which suggested that a civilization never existed.

New research shows the true secret of El Dorado is not in its riches or gold, but resides in the fertile soil called Terra Preta.

In this article, we will explore the unravelling secrets of Terra Preta in light of current scientific advances and modern tools.

The history of El Dorado

Francisco Orellana of Spain, the first European to successfully navigate the Amazon River, came to the Amazon around 1540 (Briones, 2012).

During his boat trip along the Rio Negro, he described the wonders found along his way. Extensive villages full of houses with no spaces among them and rich agricultural and fertile soils. Everything Orellana described was a newly discovered, thriving civilization.

About 20 years later, new Spanish conquistadors came to the Amazon looking for the lost golden city of El Dorado hoping to find what Orellana described.

To their surprise, Spanish visitors only found a few settlements, nothing close to what Orellana reported. Because of that, Francisco Orellana was considered a liar, and El Dorado was deemed a complete myth.

Unraveling the Secret of El Dorado

One of the critical questions about El Dorado is how these extensive villages and millions of people could exist in the Amazonian jungle.

Many believe in a concept where complex civilizations can only develop and thrive if intensive agriculture exists.

This concept has raised contrary versions about El Dorado and its existence. On one side, some researchers still support El Dorado as a myth because huge ceremonial or political centers buildings similar to the Mayan pyramids have not been found in the center of the Amazonian jungle.

On the other hand, new scientists from different fields converge to complete the picture and support El Dorado's existence. Paths, regular lines, big pottery, and bone residue have been found along the Amazon River.

Furthermore, the contact with living Amazonian tribes led to more digging into the ancient Amazonian cultures and the extinct settlements. Most of the old words used by current natives are associated with crops planted since pre-colombian times.

Also and more important is the discovery of Terra Preta. Terra Preta is a particularly dark soil considered to be man-made by natives before the Europeans arrived.

What is unique about Terra Preta is its capacity to keep fertile for millennia, therefore, favoring crop productivity without adding extra minerals. Researchers asked about the components of Terra Preta that could explain this extraordinary recovery capacity over the years.

The secrets of Terra Preta

Interestingly, Terra Preta soils have been found in all the places Orellana described. Studies suggest Terra Preta contains twice as much black carbon (70%) than surrounding soils.

This black carbon is charcoal or better known as biochar. It differs from your BBQ charcoal in that biochar is produced from naturally-occurring and human-induced burning without much oxygen, a process called pyrolysis.

Figure 1. Biochar texture and microscope structure of biochar showing the holes where microbes live.

Aside from elevated levels of biochar, Terra Preta is characterized by higher organic C, N, Ca, and P, higher cation exchange capacity, and a higher pH than the surrounding soils.

Nowadays, Brazilian farmers dig 20 cm in Amazonian soils and sell the Terra Preta to other farmers. They allow the soil to rest, and after years they come back and find the soil in similar conditions as the last mining process. Then, the most intriguing property of Terra Preta is its capacity to sustain its fertility even without applying mineral fertilizers over the years.

Although Terra Preta soil seems relatively easy to imitate for current agricultural process, until now it has not been possible. Why? Researchers hypothesized that this regeneration capacity makes Terra Preta a living thing. They focused their attention on the Terra Preta microbiome.

The microbial world of Terra Preta

Surprisingly, and contrary to what many people think, the amount of microbes in soil is relatively low. For instance, less than 1% of the available soil surface area is typically occupied by microorganisms (Wilpiszeski et al 2019).

But when it comes to Terra Preta the story is different. The peculiar features of Terra Preta have motivated the comparison of Terra Preta with surrounding soils.

Tsai et al., (2019) compared Terra Preta soils with adjacent areas from Central Amazon in Brazil. They found acidobacteria is particularly rich in the soil from areas surrounding Terra Preta soils.

At the same time, Terra Preta is rich in Firmicutes and Proteobacteria phyla and low in acidobacteria. That could explain why Terra Preta has a higher pH compared to other soils (Tsai et al., 2008).

This higher pH (or alkaline) allows a better plant growth as alkaline pH favors nutrient availability for plants.

Think about water. Water has a pH about 7 (alkaline) and plants need water, so when both nutrients and water are in similar pH plants are happier.

Furthermore, if we compared Terra Preta wih permafrost soils like those found in Alaska, other groups such as Actinobacteria (38.6%), Proteobacteria (18.4%) and Verrucomicrobia (14.3%) are most abundant.

Environment and soil properties help in the establishment of specific microbes like those occurring in Terra Preta and permafrost soils (Luláková et al. 2019).

Figure 2. Comparisons of the soil composition and microbial distribution between Terra Preta and surrounding soils.

Microbes in soil often carry out biochemical processes, which leave them with an imbalance of electrons.

For instance, they could need to discharge electrons (when there is excess) from some processes, while in others, they could need electrons (when there is a deficit). These charge imbalances create issues with microbial activity.

To overcome these electron imbalances, bacteria naturally grow pili (microbial wires), which help them touch other microbes to facilitate the transfer of electrons. These electrons are the product of converting certain mineral nutrients into a usable form.

Unfortunately, not many microbes can produce pili because it is energetically expensive.

This is where biochar appears to make a significant difference. Biochar provides spaces for microbes to stay and allows a more efficient electron transfer than pili.

Although the mechanism by which this occurs is not well understood, the results suggest that biochar promotes interspecies electron exchange via a conduction-based mechanism, in which electrons migrate through the biochar from electron-donating to electron-accepting cells.

The way biochar can accept electrons is because in their surface, biochar has redox-active moieties or RAMs (like oxygen atoms with double bonds).

These oxygen molecules act as electron acceptors, allowing bacteria to release electrons (Figure 3). Biochar use these electrons to perform redox reactions and stabilize their RAMs.

Figure 3. Diagram of the biochar-mediated electron transfer pathways with soil bacteria.

Conclusion

Although the Terra Preta microbiome is still being researched, understanding how an ancient method can lead to high soil fertility is essential. This knowledge could be massively applied worldwide to promote fertile soils in poor regions.

Keywords

Soil microbiome, Terra preta, El Dorado, soil fertility.

References

ALL Power Labs. (2019). A Perspective on Terra Preta and Biochar. ALL Power Labs.

BBC. 2014. The Secret of El Dorado: Terra Preta. https://www.permaculturenews.org/2014/10/23/secret...

Briones, A. M. (2012). The secrets of El Dorado viewed through a microbial perspective. 3(July), 1–6. https://doi.org/10.3389/fmicb.2012.00239

Johnston-monje, D., Lundberg, D. S., Lazarovits, G., Reis, V. M., & Raizada, M. N. (2016). Bacterial populations in juvenile maize rhizospheres originate from both seed and soil. Plant and Soil, 337–355. https://doi.org/10.1007/s11104-016-2826-0

Julia, M., Brossi, D. L., Mendes, L. W., Germano, M. G., & Lima, A. B. (2014). Assessment of Bacterial bph Gene in Amazonian Dark Earth and Their Adjacent Soils. 9(6). https://doi.org/10.1371/journal.pone.0099597

Kim, J., Sparovek, G., Longo, R. M., Jose, W., Melo, D., & Crowley, D. (2007). Bacterial diversity of terra preta and pristine forest soil from the Western Amazon. 39, 684–690. https://doi.org/10.1016/j.soilbio.2006.08.010

Luláková, P., Perez-Mon, C., Šantrůčková, H., Ruethi, J., & Frey, B. (2019). High-alpine permafrost and active-layer soil microbiomes differ in their response to elevated temperatures. Frontiers in Microbiology, 10(APR), 1–16. https://doi.org/10.3389/fmicb.2019.00668

Orozco-ortiz, J. M., Peña-venegas, C. P., Bauke, S. L., Borgemeister, C., Mörchen, R., Lehndorff, E., & Amelung, W. (2021). Terra Preta Properties in Northwestern Amazonia ( Colombia ). 1–16.

Tsai, S. M., & Thies, J. E. (2008). Biodiversity in Amazonian Dark Earth : A Contribution for the Sustainability of Tropical Soils from the Microbial Symbioses. February 2019. https://doi.org/10.1007/978-1-4020-8252-8