As co-leader of the new DOE Center for Research on Enhancing Carbon Sequestration in Terrestrial Ecosystems (CSITE), Gary Jacobs of ORNL's Environmental Sciences Division (ESD) explains some of the concepts that the center's 28 scientists are studying and some of the questions they seek to answer.
Earth's Vegetation and Soil:
Natural Scrubber for Carbon Emissions?
An Interview with ORNL's Gary Jacobs
CSITE co-leaders Blaine Metting (left) and Gary Jacobs inspect a field of Queen Anne's lace (Daucus carota) at the Fermi National Accelerator Laboratory's National Environmental Research Park. How can forests, pastures, croplands, and soils reduce carbon dioxide levels in the atmosphere in an era of increasing industrialization, and how can CSITE facilitate this process?
Mac Post of ESD, one of our experts on the carbon cycle, says that several lines of evidence indicate that the terrestrial biosphere, most likely the Northern Hemisphere, could be taking up a net amount of nearly 2 billion tons of carbon per year. Some of this uptake is perhaps due to regrowth of forests harvested previously this century in North America and Europe. Another factor may be the enhanced growth of natural vegetation resulting from rising atmospheric carbon dioxide concentrations stimulating photosynthesis. This Northern Hemisphere sink is larger than the estimated 0.5 to 1.5 billion tons of carbon emitted to the atmosphere through conversion of natural ecosystems to agriculture, primarily in tropical regions. Thus, the earth's vegetation and soil could act as a huge natural scrubber for carbon dioxide emissions from industrial sources and land-use changes.
Terrestrial ecosystems remove atmospheric carbon dioxide by plant photosynthesis during the day, which results in plant growth (roots and shoots) and increases in microbial biomass in the soil. Plants release some of the stored carbon back into the atmosphere through respiration. When a plant sheds leaves and roots die, this organic material decays, but some of it can be protected physically and chemically as dead organic matter in soils, which can be stable for up to thousands of years. The decomposition of soil carbon by soil microbes releases carbon dioxide to the atmosphere. This decomposition also mineralizes organic matter, which makes nutrients available for plant growth. The total amount of carbon stored in an ecosystem reflects the long-term balance between plant production and respiration and soil decomposition.
CSITE seeks to demonstrate through research that forests, pastures, cropland, other vegetation, and their associated soils can be managed and manipulated to sequester even more carbon from the atmosphere. People can help reduce carbon dioxide releases to the atmosphere and enhance carbon sequestration by protecting and adding to ecosystems that store carbon. For example, we can preserve forests instead of burning them to clear land for farms. We can grow more trees. We can reduce soil erosion. Some agricultural and forestry management techniques are already helping to sequester additional carbon. The focus of our center, however, is to do research to determine the most effective, most acceptable ways to manipulate and manage ecosystems to increase carbon storage in above-ground biomass and below-ground roots and soil. For example, we will look at different approaches to fertilizing and cultivating forest plantations and crops. R&D is needed to understand, measure, implement, and assess these strategies.
What is the scope of research for CSITE?
We will try to discover how changes in land use and land management affect the ability of vegetation and soil to sequester carbon. To measure and predict these changes, we will rely on various tools, ranging from remote sensing to simulation modeling. We seek to understand how microbial activity, soil aggregation, and other processes at the molecular level control carbon sequestration in vegetation and the soil. We will also do several assessments: We will determine scientifically the national potential for sequestering carbon in terrestrial ecosystems. We will evaluate the actual net effect on greenhouse warming potential of practices that enhance carbon sequestration in terrestrial ecosystems. This assessment will take into account the greenhouse gas costs of improving plant productivity, such as the increased carbon dioxide and nitrogen oxide emissions associated with fertilizer production and machinery operation. We will develop a quantitative understanding of the environmental impacts of increasing carbon sequestration in terrestrial ecosystems and create improved tools for predicting impacts. Finally, we will analyze soil carbon sequestration to determine its economic and social impacts, especially possible pressures on land use and production in the agricultural and forest sectors.
Why is ORNL particularly well qualified to study carbon sequestration in terrestrial ecosystems?
CSITE's proposal was successful partially because we formed a national partnership with some of the best researchers and institutions in the field. The future success of CSITE depends on whether our collaborative team can address the most pressing scientific challenges. As for ORNL's specific qualifications, several ESD researchers — Mac Post, Jeff Amthor, Gregg Marland, Stan Wullschleger, and Bob Luxmoore, for example — have considerable experience modeling, monitoring, and conducting large experiments on forests and other ecosystems, with an emphasis on understanding the carbon cycle and impacts of global change on ecosystems. Rich Norby, Paul Hanson, and others have studied the effects on forest growth of increasing carbon dioxide concentrations and changing inputs of water. Janet Cushman and Lynn Wright have managed a national program for developing better ways to raise faster-growing biomass crops for energy production. ESD staff also played a key role in writing a chapter on soils and vegetation for DOE's report, Carbon Sequestration: State of the Science, which was compiled, edited, and published by ORNL staff. (This chapter is the source of much of the material in this interview.) ESD also is home to several key global climate change data centers, such as the Center for Carbon Dioxide Information and Analysis, a NASA Distributed Active Archive Center, and the Atmospheric Radiation Measurements Program data center. Studying the ecological effects of global change has historically been one of the Lab's strengths.
Why is soil management important for carbon sequestration?
Soils are estimated to contain about 75% of all terrestrial carbon. Because 25 billion tons of soils are lost through wind and water erosion each year, there is an incentive to prevent erosion not only to benefit agriculture but also to increase carbon sequestration. One solution is to produce and protect soils high in carbon-containing organic matter because they have better texture and are better able to absorb nutrients, retain water, and resist erosion. Soil organic matter processes are a particular emphasis of ESD's Chuck Garten and our Argonne National Laboratory collaborators, Julie Jastrow and Mike Miller.
Jeff Amthor and others estimate that some 40 to 60 billion tons of carbon have been lost from soils since the great agricultural expansions of the 1800s. Removal of natural perennial vegetation and cultivation of the land have caused declines of soil organic matter by 50 to 60% in the top 20 centimeters of soil and 20 to 30% in the top meter of soil. This decline is due largely to a decrease in the formation of new organic matter below the ground and the loss of natural mechanisms that protect soil organic carbon from decomposition and oxidation. Cultivated soil is exposed to the air, so during decomposition by soil microbes, the soil organic matter is oxidized, and the carbon is released to the atmosphere as carbon dioxide.
Which changes in farming practices enable soils to store more carbon?
Cesar Izaurralde and Norm Rosenberg, two of our PNNL partners, are experts in agricultural systems. They point out that soil carbon can be increased by reduced-till agriculture, in which the soil is barely disturbed before crops are planted, and by the practice of returning crop residues to soil to reduce wind erosion. The U.S. Conservation Reserve Program (CRP) of the US Department of Agriculture, which since 1985 has been paying farmers to retire land from cultivation for up to 15 years and plant it in grass to stabilize it, is also increasing soil carbon storage. Some evidence suggests that levels of soil organic carbon have doubled over the past 20 years in the upper 18 centimeters of soil placed in the CRP. In addition, erosion of the land enrolled in the CRP has decreased 21%. All of these practices reflect mainly the "recovery" of soil carbon previously lost because of earlier cultivation.
Which changes in forestry practices would make plants and soils more efficiently remove carbon dioxide from the atmosphere?
Forests in the United States are being managed to produce harvestable fiber and maintain cover, increase water storage, and retain litter. One major challenge is to slow the rate of deforestation. If this trend could be reversed and if reforestation occurs, some modeling studies suggest that, globally, forests could sequester from 200 to 500 billion tons of carbon by 2090. These values are large and controversial, and estimating the potential for carbon sequestration is one of the research challenges. A big challenge is to determine how to manage forest nutrients to achieve both profitable productivity and net carbon storage. Strategies are needed to address both fertilization and incorporation of forest residue into soils.
How can more carbon be stored in soils and plants?
More carbon can be stored below ground by increasing the depth of soil carbon, boosting the density of carbon in the soil, and decreasing the rate at which soil carbon decomposes. CSITE will be focusing initially on the latter two. Harvey Bolton of PNNL, Jizhong Zhou of ESD, and Mike Miller of ANL will be looking at microbiological processes that could be manipulated to reduce decomposition rates of soil organic matter. ESD's John McCarthy will be investigating molecular-scale interactions among clay particles and soil organic matter in search of a better way to protect the organic matter. We hope that other research programs will provide complementary results. For example, advances in biochemical research may produce a "smart fertilizer" that increases a soil's organic content and ability to retain water, protects its organic matter, and improves its texture so it can hold more carbon. Another important R&D area would be the development of new ways to produce fertilizer that use less energy and reduce carbon emissions.
More carbon can be sequestered in vegetation, possibly even by genetically engineering plants to increase their carbon retention. Plants could be engineered to produce cellular structures more resistant to decomposition, increasing the lifetime of soil organic matter and thus sequestering more carbon in soils. We need to find ways to make carbon accumulate faster, increase the vegetation's carbon density, and use biomass carbon in long-lived structural materials and industrial products.
What are the other benefits of storing more carbon in vegetation and soils?
Creating conditions for higher plant productivity and accumulation of soil organic matter will not only sequester more carbon but also restore degraded ecosystems worldwide. Carbon sequestration strategies would improve soil and water quality, decrease nutrient loss, reduce soil erosion, improve wildlife habitats, increase water conservation, and produce additional biomass for energy and other products. Understanding how to increase soil carbon stocks in agricultural lands may be critical to the future sustainability of food production.
How will you know if carbon sequestration is increasing in a terrestrial ecosystem?
A critical question is whether new sensors will be required or if process knowledge — rules of thumb — will be sufficient to estimate changes in carbon sequestration based on the implementation of observable land management practices. Developing measurement and sensing techniques to verify increased carbon sequestration in terrestrial ecosystems and to monitor its effects will be challenging. Detecting changes in terrestrial carbon concentrations at large scales will not be easy. An important R&D goal identified by DOE in its roadmap report is to develop in situ, nondestructive, below-ground sensors to quantify rates and limits of carbon accumulation over various times and land areas. To determine whether increases have occurred in aboveground biomass, new advances in satellite-based-remote sensing will be required. ORNL could certainly play a role in developing some of the needed sensor technology.
