Amazonian dark earth could bring life back to decimated forestland
Even though the Amazon is still a powerhouse of plant and animal life, about 18% of it has been cut down since the 1970s. Seeking ways to help get some of that lost forestland back, researchers have turned to a rich soil created by the activities of the Amerindians thousands of years ago.
About 2,000 years ago, as the Amerindian people were living their lives in what is now Amazonia, they inadvertently created an incredibly rich soil. Known as Amazonian dark earth (ADE) or terra preta, this soil was formed as the charcoal from campfires combined with other bits of daily life like animal bones, broken pottery pieces, compost and manure. Not only does the soil contain these ancient components of human life, but it has been found to harbor a rich microbiome as well.
Curious to know if ADE could help restore some of the Amazon's rich rainforests, researchers at São Paulo University in Brazil undertook a simulation. They filled containers with three different types of soil: as a control, they used soil from Amazonian cropland, then they created a mix of the cropland soil with 20% ADE in one container, and placed 100% pure ADE in another.
Next, they sowed grass in each of the containers. Once the grass reached maturity, they cut it down, leaving the roots in place, and planted a variety of trees in the containers. This process simulated what would happen naturally when cropland is left untended and converts first to grassland, and then to forest.
The researchers found that after the trees had grown for 90 days, all soils had fewer nutrients because they had been taken up by the plants, but the ADE soils retained more than the control soil. Both ADE soils also had a greater biodiversity of bacteria and other microbes than the control.
"Microbes transform chemical soil particles into nutrients that can be taken up by plants," said joint lead author Anderson Santos de Freitas. "Our data showed that ADE contains microorganisms that are better at this transformation of soils, thus providing more resources for plant development. For example, ADE soils contained more beneficial taxa of the bacterial families Paenibacillaceae, Planococcaceae, Micromonosporaceae, and Hyphomicroblaceae."
Most impressive is the fact that when grown in this rich microbial soil, the harvested dry mass of the grass was 3.4 times greater in the 20% ADE mixture and a whopping 8.1 times greater in the 100% ADE soil. The trees also fared better. The 20% mixture yielded trees that grew 2.1 and 5.2 times taller by species than in the control soil, and an impressive 6.3 times taller in the 100% ADE soil. One tree species known as Abay pumpwood wouldn't even grow in the control medium, but grew just fine in both ADE soils.
The research team cautions against using actual ADE in reforestation strategies, but says that creating soil rich with the same microbes found in ADE could yield much the same effects, thereby helping to encourage depleted cropland to grow new forests.
"ADE has taken thousands of years to accumulate and would take an equal time to regenerate in nature if used," said senior study author Siu Mui Tsai. "Our recommendations aren’t to utilize ADE itself, but rather to copy its characteristics, particularly its microorganisms, for use in future ecological restoration projects."
The research has been published in the journal, Frontiers in Soil Science.