The recently released motion picture The Lost City of Z tells the true story of Lieutenant Colonel Percival Harrison Fawcett’s 1925 “lost” expedition to the Mato Grasso region of Brazil’s Amazon rainforest. Inspired by a 1753 manuscript detailing the discovery of an abandoned Greco-Roman-style city swallowed up by the jungle, Fawcett and his team set off to rediscover the city he dubbed “Z.” Fawcett never returned, and many adventurers since have lost their lives attempting to solve his disappearance.
A decade ago, journalist David Gran set off on an expedition to retrace Fawcett’s journey through the jungle. His account of the Xingu Reserve appeared in The New Yorker in 2005. The follow-up book—optioned by Hollywood and now in theaters—details the evidence Gran collected about Fawcett’s possible fate.
In the Amazon, Gran crossed paths with archaeologist Michael Heckenberger, who was mapping an extensive network of ancient earthen mounds, canals, and roads—all unearthed as the forest was clear-cut.
The mounds served as both villages and raised agricultural land. A central plaza with thatched huts on its perimeter was ringed by a series of concentric earthen walls. Between the walls, a natural moat flooded during the rainy season. When the water receded, the trapped fish could be scooped up with wicker baskets. Crops were grown on top of the outer walls.
This produced enough food to support populations of 5,000 or more. The towns connected to each other via a series of canals, bridges, and raised roads. More than three million people lived in these settlements prior to European colonization. Preliminary carbon dating placed them at 900 years old; more recent tests have pushed that date back even further.
Scientific consensus has long held that Amazonian soil was incapable of supporting large agrarian settlements due to nutrient depletion caused by daily rain and the proliferation of plants, insects, and fungi that compete for every scrap of food that hits the forest floor. Ancient cities got around this with ingenious geoengineering—mounds and moats—and, more importantly, by ameliorating the soil with terra preta.
Portuguese for “black earth,” terra preta—or biochar—is a mixture of activated charcoal, crushed pottery, bone, and manure and urine for nitrogen. The charcoal is highly porous, absorbing water, minerals, and nutrients. Its negative surface charge attracts positively charged ions of calcium, potassium, and magnesium buffering the soil’s acidity into a neutral range.
The charcoal’s high porosity also makes it a home for beneficial microorganisms, such as mycorrhizals—a fungus that builds underground networks for transporting nutrients to plants. These are exchanged for sugars in the plant’s roots (“mycorrhizal” refers to this relationship between fungus and host).
Biochar also provides support for nitrogen-fixing microbes, reducing the need for chemical fertilizer. Compared to current methods used to farm the Amazonian River Basin, terra preta produces crop yields 77 to 800 percent beyond that of slash-and-burn or BigAg techniques. While terra preta does not drastically improve the yield for all soil types and climates, it is particularly useful on highly acidic, nutrient-poor soil.
The discovery of terra preta has become a benchmark for archaeologists to locate other pre-colonial settlements. The terra preta is still biologically active, even after 500 years without human intervention. An increase in plant density and diversity directly above these deposits is noticeable, especially when compared with non-black-earth areas only meters away. The distinctions are so stark, they can be seen via satellite.
In some areas of the Mato Grasso, terra preta is mined in shallow pits and sold as potting soil or as topsoil to farms in other areas. The sites remain self-sustaining if several inches of “black earth” is left behind. As the excavated sites fill in naturally, the biologically active components convert the accumulating organic matter above into a dark agricultural substrate, which can be extracted and sold yet again.
Beyond the rainforest, novel applications for biochar are under development. It is a natural choice for carbon sequestration, as most organic waste (agricultural, animal, and human) can be used as feedstock.
Compared to other methods of combating greenhouse gas—and thus combating climate change—the cost of biochar is extremely low. At most, preparation and application per ton/hectare are only $40 USD. In environments where crop yields increase due to improved soil quality, the price often drops into the negative range.
The beauty of biochar for carbon sequestration is that it does not require a change in land use or specialized equipment. “Negative emission technologies,” such as reforestation and biofuel, require repurposing farmland.
Though high-tech pyrolysis centers would be more energy efficient, individual earthen kilns or char pits are often a better fit for remote locations.
David Glenn is a sentient being with a bipedal Simian body-type. His current location is on the North American continent a few hundred meters or so from the world's largest freshwater lake.