How 2,000-year-old soil could be a lifeline for the Amazon rainforest

Amazonia dark earth is chock full of nutrients and stable organic matter that can boost plant growth.
Amazonian dark earth (ADE) is a nutrient rich dark soil that could play a role in reforestation.
Amazonian dark earth (ADE) is a nutrient rich dark soil that could play a role in reforestation. Luís Felipe Guandalin Zagatto

Nicknamed the “Earth’s lungs” for its dense oxygen producing forests, the Amazon can absorb 132 billion tons of the planet’s carbon. However, more than 30,000 square miles of the Amazon have been lost since the 1970s. Deforestation, clearing land for agriculture, and climate change fueled wildfires have severely taken its toll on the region, where about 10,000 acres of forest (almost the size of California) has been destroyed every day since 1988. 

However, there is still time to save it—and now scientists may have a “secret weapon” that could not only help reforest the Amazon, but other depleted forests around the world. And it comes from soil deep in the region’s past.

[Related: Brazil’s presidential election is a win for the Amazon—and the planet.]

From roughly 450 BCE and 950 CE, the people living along today’s Amazonia transformed the originally poor soil over many human generations. The soils were enriched with charcoal from low-intensity fires for cooking and burning refuse, animal bones, broken pottery, compost, and manure. The fertile result of these processes is Amazonian dark earth (ADE), or terra preta. The exceptionally fertile black soil is rich in nutrients and stable organic matter derived from charcoal. According to a study published May 5 in the journal Frontiers in Soil Science, it now may help reforest the same area where it was created. 

“Here we show that the use of ADEs can enhance the growth of pasture and trees due  to their high levels of nutrients, as well as to the presence of beneficial bacteria and archaea in the soil microbial community,” co-author Luís Felipe Zagatto, a graduate student at the Center for Nuclear Energy in Agriculture at São Paulo University in Brazil said in a statement. “This means that knowledge of the ‘ingredients’ that make ADEs so very fertile could be applied to help speed up ecological restoration projects.”

The team’s primary aim was to study how ADEs, or ultimately soils with a microbiome that has been artificially composed to imitate them, could boost reforestation. To do this, they conducted controlled experiments in a lab to mimic the ecological succession that happens in the soil when pasture in deforested areas is actively restored to its forest state. 

They sampled ADE from the Caldeirão Experimental Research Station in the Brazilian state of Amazonas. The control soil in the experiments was from the Luiz de Queiróz Superior School of Agriculture in the state of São Paulo. They filled 36 pots with about 6.6 pounds of soil inside a greenhouse with an average temperature of 94ºF to anticipate global warming beyond current average temperatures in Amazonia (between 71 and 82ºF).

One third of the pots only received the control soil, while another third received a 4 to 1 mixture of the control soil and ADE, and the final third has 100 percent ADE. They planted seeds of palisade grass, a common forage for Brazilian livestock, to imitate pasture. The seedlings were allowed to grow for 60 days before the grass was cut so that only the roots remained in the soil. 

Each of the three soils were then replanted with tree seeds of either a colonizing species called Ambay pumpwood, Peltophorum dubium, or with cedro blanco.

[Related: The Amazon is on the brink of a climate change tipping point.]

The seeds were allowed to germinate and then grow for 90 days and then the team measured their height, dry mass, and extension of the roots. They also quantified the changes in the soil’s pH, microbial diversity, texture, and concentration of organic matter–potassium, calcium, magnesium, aluminum, sulfur, boron, copper, iron, and zinc–over the course of the experiment. 

At the beginning, ADEs showed greater amounts of nutrients than control soil, roughly 30 times more phosphorus and three to five times more of each of the other measured nutrients, except manganese. The ADE also had a higher pH and had more sand and silt in it, but less clay. 

Following the experiment, the control soils contained less nutrients than they had at the start, which reflects take-up by the plants. However, the 100 percent ADE soils remained richer than control soils, while nutrient levels were intermediate in the 20 percent ADE soils.

The 20 percent and 100 percent ADE soils also supported a greater biodiversity of both  bacteria and archaea than control soils.

“Microbes transform chemical soil particles into nutrients that can be taken up by plants. Our data showed that ADE contains microorganisms that are better at this transformation of soils, thus providing more resources for plant development,” co-author and University of São Paulo molecular biologist Anderson Santos de Freitas said in a statement.  “For example, ADE soils contained more beneficial taxa of the bacterial families Paenibacillaceae, Planococcaceae, Micromonosporaceae, and Hyphomicroblaceae.”

Additionally, adding ADE to soil improved the growth and development of plants. The dry mass of palisade grass was increased 3.4 times in the 20 percent ADE soil and 8.1 times in 100 percent ADE compared to in control soil. 

These results were enough to convince the team that ADE can boost plant growth, but it does come with some caution. 

“ADE has taken thousands of years to accumulate and would take an equal time to regenerate in nature if used,” co-author and  University of São Paulo molecular biologist Siu Mui Tsai said in a statement. “Our recommendations aren’t to utilize ADE itself, but rather to copy its characteristics, particularly its microorganisms, for use in future ecological restoration projects.”