Deforestation Robs Amazon Soil of Life

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A tree freshly cut from the Amazon forest in Anapu, Para, Brazil.
Paulo Fridman/Corbis

Standing where the Amazon rainforest became pasture, it is easy to recognize the absence of trees. What you might not notice is the drop in biodiversity under your feet, where a crucial part of every ecosystem lives.

“We knew that if you cut the trees, the animals will disappear, but we didn’t know what happened to the microbes,” said Jorge Rodrigues, assistant professor of biology at University of Texas at Arlington and leader of the research team. “We showed for the first time, in the case of the Amazon, you would see losses of microbe species when moving land use from the forest to the pasture.”  Rodrigues published that finding recently in the journal Proceedings of the National Academy of Science (PNAS).

One reason to be concerned about the health of microorganism populations is that they help the entire ecosystem function.

“The microbes are the ones that keep everything cycling,” said soil scientist Teri Balser of the University of Florida, who was not involved with the research project.

These tiny organisms are crucial for tasks like decomposing matter into plant food, particularly in tropical soils like those in the Amazon rainforest.

Tropical soils are not as fertile and have less organic matter mixed in with the mineral components of soil. Since there aren’t a lot of extra nutrients in tropical soil, Balser said plants are more dependent on microorganisms to recycle undecomposed material back into a plant-ready form.

However, Balser said even though decreased microbial diversity may have consequences for the land, the actual significance of homogenizing microorganism populations is not well understood.

“We really don’t know how much it matters when you lose microbial diversity,” she said. It is a lingering question for soil microbiologists.

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“We’re working on such a different scale than plants and animals that it’s really hard to know how important a change in microbes is,” she said. “A cubic meter of ground for a microbe is the equivalent of the whole planet for a plant.” That difference in scale makes interpreting changes in microbial diversity challenging.

Even without knowing exactly how an ecosystem functions with a homogenized microorganism population, Rodrigues said another reason to be concerned about losing diversity is the loss of material with possible value for medicine.

“Lots of antibiotics are made by bacteria,” Rodrigues said. “We might be losing a lot of potential antibiotics, or biotechnological advances later on.”

A diversity of bacteria and other microorganisms helps land respond to stress according to Rodrigues. Decreasing the types of microscopic life in Amazonian soil may decrease the land’s ability to adapt to change he said.

The next question Rodrigues and the research team are asking is, does microbial diversity rebound if a pasture becomes forest again? He wants to know if the deforested Amazon pasture is left to grow trees again, whether the missing microorganisms will re-colonize the soil.

“We still have time for that, for the secondary forest come back,” he said.