COVER STORY Green Energy’s Environmental Impacts A New Race For Solar

Energy generation, like all forms of human activity, is not free from environmental impacts on animal, vegetable, and mineral resources. Greenhouse gas emissions from burning fossil fuels aside, energy producers are already held accountable for oil spills, scrutinized for habitat destruction stemming from hydroelectric projects and mountaintop coal mining, and under pressure to prevent bird injuries and deaths from power-line electrocutions and wind turbine collisions.

Planned solar energy farms that would occupy hundreds of acres of pristine desert land present a new set of environmental challenges.

Scientists don’t yet fully know how these large solar installations will affect desert ecology. But several recent research studies aimed at gauging the effects of human activity on soil biology and chemistry are revealing information that those in industry and government can use to make decisions that may minimize environmental disruption.

THIN BUT FERTILE [+]Enlarge Credit: Courtesy of Ferran Garcia-Pichel

For example, earlier this year microbiologist Ferran Garcia-Pichel of Arizona State University and colleagues reported the results of a sampling survey of carbon dioxide-fixing and nitrogen-fixing photosynthetic bacteria (cyanobacteria) living in the desert soils of the western U.S. (Science 2013, DOI: 10.1126/science.1236404). The researchers found that two particular cyanobacteria species dominate the arid land and neatly split the territory between them depending on temperature. Microcoleus steenstrupii is the lead species in the warmer southern deserts, and M. vaginatus is more prevalent in cooler northern deserts.

“Cyanobacteria are very important in global carbon and nitrogen cycles but are of special importance in dryland regions, where they can cover up to 70% or more of the soil surface,” says ecologist Jayne Belnap, a biocrust specialist with the U.S. Geological Survey’s Southwest Biological Science Center in Moab, Utah. “In deserts, they are the only source of carbon in the spaces between plants and are often the dominant source of nitrogen.”

Cyanobacteria, along with lichens and mosses, contribute nearly 8% of global land biomass production and nearly half of global land nitrogen fixation, according to a study by a research team in Germany (Nat. Geosci. 2012, DOI:10.1038/ngeo1486). For reference, the amount of carbon taken up by these organisms is about the same as the amount of carbon released annually into the atmosphere from biomass and fossil-fuel burning.

Garcia-Pichel believes the pattern of cyanobacteria temperature segregation occurs worldwide and that it will not be easy for M. vaginatus to evolve quickly enough to tolerate the higher temperatures expected from global warming. “By using our data with current climate models, we can predict that in 50 years the cyanobacterium that fares better in warmer temperatures will push the cold-loving one off the map,” he says.

His team studied biological soil crusts, or biocrusts, which are millimeter- to centimeter-thick communities of microorganisms that form on top of soils in arid regions. Hundreds of different microbe species live in just a pinch of these desert biocrusts and play a key but often overlooked role in desert ecology. The biocrusts aid water retention, prevent erosion, and trap dust containing mineral nutrients. They also contribute to soil fertility by converting carbon dioxide and nitrogen from the air into sugars, amino acids, and other nutrient biomolecules for themselves, other bacteria, and plants.

“We know very little about how increasing temperatures will affect individual species,” Belnap says. But knowing there is a temperature dependence “gives us every reason to suspect that warmer temperatures will affect the role these organisms play in carbon and nitrogen cycles.”

Garcia-Pichel and colleagues sequenced the genomes of the bacteria and used computer models to determine the temperature-dependent geographical distribution. The team confirmed the temperature dependence by growing microbes under controlled conditions in the lab. Scientists are increasingly using this approach to study soil microbes in various habitats.

For example, Noah Fierer of the University of Colorado, Boulder, and coworkers combined genetic analysis of microbes with species distribution models to study soil samples collected at undisturbed tallgrass prairie sites in the U.S. Midwest (Science 2013, DOI: 10.1126/science.1243768). Pristine tallgrass prairies once covered nearly 10% of the U.S., but plowing, irrigation, fertilizer, and pesticides have endangered these ecosystems. Fierer’s team found that one phylum of bacteria, the Verrucomicrobia, once dominated such soils and provided nutrients to native grasses. But these bacteria have now all but disappeared from cultivated soils, Fierer and colleagues note.

“No soil is unaffected by human actions,” comment soil biologist Mary C. Scholes of the University of the Witwatersrand, in South Africa, and ecologist Robert J. Scholes of South Africa’s Council for Scientific & Industrial Research, in a perspective on the Fierer research published in Science. “The fertility attributes of a soil cannot be separated from the purpose for which the land is used, nor from the unintended consequences of this usage.”