Rooting out whether soil will store or release carbon during warming – Oakridger
“Stop treating soil like dirt.”
That’s the title of a 2019 TED talk in by Asmeret Asefaw Berhe. Besides its value to farmers, gardeners and all of us who eat, soil should be treated with respect for another reason as fossil fuel burning and climate warming persist.
“Soil holds twice as much carbon as the atmosphere,” said Colleen Iversen, distinguished staff scientist at Oak Ridge National Laboratory, in a recent virtual talk to Friends of ORNL.
As a plant ecologist and leader of the Plant-Soil Interactions Group in ORNL’s Environmental Sciences Division, “understanding the cycling of carbon, nutrients, water and energy among the living and nonliving parts of the natural world” is what interests her, she said. She and some colleagues are focused on fine roots of plants entangled with belowground fungi and soil laced with bacteria, clay and rock particles. Enormous amounts of carbon have accumulated in this organic mixture composing much of Earth’s terrestrial surface.
“Understanding what happens to carbon stored in soils for thousands of years is important for thinking about feedbacks of carbon compounds from the soil to the atmosphere that could accelerate the rate of global warming,” Iversen said.
She and her colleagues are gaining insights into below-ground ecosystem responses to rising atmospheric levels of carbon dioxide and the resultant increased warming. They explore what happens by deploying advanced robot cameras to image newborn roots, their rates of growth and times of death (usually less than a year later). The scientists also use stable isotopes of carbon and nitrogen to track the movement of carbon and nutrients throughout plants and their root systems. Plants use carbon to grow their biomass and nitrogen to produce their proteins.
While others admire trees during hikes, Iversen thinks about what’s hidden beneath her feet. “The tiny roots, which are narrower than the cord attaching my headphone to the computer, do the work of acquiring water and nutrients for the plant above to keep it alive. But they don’t always do it alone. Fuzzy mycorrhizal fungi at the tips of some plant roots assist them in taking up nutrients in exchange for carbon that the plants fix by photosynthesis.”
Iversen said one goal of her program’s field research is to root out how interactions between plants and soil might change under different environmental conditions. Another goal is to provide these data to computer scientists improving models on supercomputers, like Summit at ORNL, to predict future climates and their impacts under different scenarios of energy use.
The first major field experiment she worked on, starting in 2004 under the leadership of her adviser Rich Norby, was the Free-Air CO2 Enrichment (FACE) experiment in a temperate, deciduous forest at ORNL; it ran from 1998 to 2009 to determine the photosynthetic response of trees to rising levels of carbon dioxide (CO2).
According to Iversen, “The question was, if there is more CO2 in the air because of fossil fuel burning, would the trees take up the extra greenhouse gas and use it to grow more?”
Two sets of mature forest trees in enclosures were exposed to ambient carbon dioxide levels and two were subjected to the higher CO2 concentrations expected at mid-century. In 1998 the atmospheric CO2 concentration was 380 parts per million; today it is 415 ppm, helping to explain why the past decade was the hottest on record, triggering more frequent and intense extreme events such as heat waves, drought, wildfires, hurricanes and flooding.
“What we found was that elevated carbon dioxide did increase carbon uptake by the plants,” she said. “But instead of the trees growing taller or adding more stem wood and leaves, most of the carbon fixed went below-ground for the production of additional roots that grew deeper as the trees attempted to acquire more nutrients since they had more carbon to grow more biomass. But because more roots were produced, this had implications for long-term storage of carbon in the soil.”
The Biological and Environmental Research program in the Department of Energy’s Office of Science then began funding an experiment in Minnesota in which sets of enclosed forest trees in a type of wetland, called a peatland or bog, would be exposed to both increased CO2 and enhanced warming. The Spruce and Peatland Responses Under Changing Environments (SPRUCE) project has been underway since 2015 under the leadership of ORNL’s Paul Hansen. Iversen is leader of the Belowground Tasks component of the SPRUCE field experiment, which is expected to run for a decade.
“Peatlands are important because they cover only 3% of the global land surface, but they store more than a third of terrestrial carbon,” she said. “At the SPRUCE site the soil carbon is more than three meters deep and has been accumulating for 11,000 years since the glaciers scraped out the landscape that then was filled with aquatic plants. A lot of carbon is stored there because it is cold, wet and acidic so decomposition of organic matter by microorganisms is slow.”
The SPRUCE experiment attempts to determine the impacts of warming up and drying out the bog. In one enclosure, the trees and shrubs were exposed to an additional 9 degrees Celsius of warming and 900 ppm of CO2, a worst-case scenario possible in the next century.
“We were surprised to find perfectly intact shrub roots at a depth of 175 centimeters below the water table level,” she said. “When we measured the level of carbon-14 in them to find out how old they were, we found those roots were dead and perfectly preserved with a carbon age of 5,000 years of more. These roots were not decomposing and releasing carbon.”
The researchers found that warming enhanced the growth of fine shrub roots “but it was not enough to increase carbon accumulation in the bogs.” In fact, warming caused the bog to turn from a carbon sink to a carbon source to the atmosphere in the form of CO2 and methane, contributing to global warming.
On the frozen tundra near the Barrow Environmental Observatory in Alaska, some 150 researchers with four DOE national laboratories (including ORNL) and the University of Alaska at Fairbanks are participating in DOE’s Next-Generation Ecosystem Experiments (NGEE) Arctic project. It is led by ORNL’s Stan Wullschleger, and Iversen is its deputy director.
Alaska is warming two to three times faster than the rest of the world, she said, because Arctic Ocean ice, which reflects sunlight, is melting.
“So, you get dark water that absorbs the sun’s heat, leading to faster warming,” Iversen said.
She said the researchers are trying to understand various phenomena such as greenhouse gas production from the expected thawing of permafrost – soil layers that have been frozen for thousands of years. As rising air temperatures warm the ground, organic matter from dead plants and animals is made available to microorganisms that convert it to CO2 and methane.
They are also examining the increase in wildfires from lightning strikes and the “the march of dark shrubs across the tundra” that reduce the land’s reflection of sunlight while increasing its heat absorption. “We are getting a landscape view that helps us and also computer models predicting what the future might look like,” Iversen said.
Referring to her interest in roots, she noted that “some tundra plant species have five times as much biomass belowground as they do aboveground. Some plants hug the ground to avoid its harsh conditions. The biomass is mostly belowground to take up and store nutrients and carbon to help the plants grow. We found that many plants have different strategies aboveground and below-ground for getting water and nutrients while coping with changes in environmental conditions.”
Ecologists’ data on plant-and-soil responses to increases in greenhouse gases and warming should help scientists get to the root of the climate change problem.