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Public Roads - January/February 2015

January/February 2015
Issue No:
Vol. 78 No. 4
Publication Number:
Table of Contents

Slowing Climate Change One Highway At A Time

by Doug Romig, Bill Dunn, Amy Estelle, and Greg Heitmann

New Mexico is leveraging rights-of-way to potentially reduce the amount of greenhouse gas emissions entering the atmosphere.


Grassy highway rights-of-way, like this one along rural State Highway 120 outside Wagon Mound, NM, may have the potential to sequester carbon.


Global climate change is arguably one of the foremost environmental challenges of our time. It is driven by increasing amounts of carbon dioxide (CO2) and other greenhouse gases (GHGs) emitted into the atmosphere, which trap heat and lead to a rise in the earth's surface temperatures. Predicted impacts of climate change include prolonged droughts, rising sea levels, and increased severity of storms, which could all be costly to society.

The main human activity that emits CO2 is the combustion of fossil fuels, including gasoline and diesel fuel, for energy use in the transportation sector. Therefore, the transportation industry is working to find ways to slow its contribution to climate change and protect vital natural resources. One method that has potential is using highway rights-of-way (ROWs) for carbon sequestration.

Highway ROWs are important contributors to road safety. They provide clear lines of sight and corridors for vehicles to travel safely. They also contribute to maintaining the integrity of paved surfaces by facilitating stormwater drainage through vegetated buffers. And if research currently being conducted along New Mexico's State highways is successful, ROWs also might help slow global climate change. Here's the story of this innovative new role for ROWs.

The Science of Carbon Sequestration

Solving the problem of global climate change is relatively straightforward--at least scientifically: reduce the amount of carbon emitted into the atmosphere and increase the amount removed. The process of removing CO2 from the atmosphere is called carbon sequestration. Both the land and the ocean naturally sequester atmospheric carbon and are major reservoirs of carbon. Collectively they absorb approximately half of the carbon emissions produced by humans each year while the other half enters the atmosphere, causing increased CO2 concentrations that contribute to climate change.

One way to absorb carbon from the atmosphere naturally is through photosynthesis, the process whereby plants capture CO2 using energy from sunlight, and incorporate carbon into their roots, stems, trunks, and leaves. Ultimately, the plant dies and much of the carbon stored in its tissues is returned to the atmosphere through decomposition. Some of the plant carbon is consumed by animals (such as termites, earthworms, and rabbits) and microorganisms (such as bacteria, protozoa, and fungi) inhabiting the soil and they then transform it into soil organic matter.

Soil organic matter, otherwise known as humus, is the well-decomposed residues of plants and soil animals. It exists as large, complex carbon compounds that resist additional degradation. Soil carbon also exists, to a lesser degree, as inorganic carbonate minerals like calcite and gypsum. Soils are the largest land-based reservoir for carbon, accounting for nearly 80 percent of the carbon found in terrestrial ecosystems. In fact, an estimated 2,500 gigatons of carbon are currently stored in soils. That is more than three times the amount in the atmosphere, and about 100 times the amount of carbon produced by human activities each year.

However, the rate at which plants transfer carbon to the soil generally is slow and a function of climate (temperature and precipitation). Soil carbon tends to accumulate faster in cooler, wetter climates simply because plants grow faster and the rate of decomposition is slower. To increase the amount of carbon sequestered by soils, changes in land management are necessary.

FHWA's Pilot Project

Beginning in 2008, the Federal Highway Administration's (FHWA) Office of Planning, Environment, and Realty and the Volpe National Transportation Center conducted a Carbon Sequestration Pilot Project. The project sought to "assess whether a roadside carbon sequestration effort through modified maintenance and management practices is appropriate and feasible for State departments of transportation . . . when balanced against ecological and economic uncertainties."

The FHWA project team estimated the amount of carbon that could be stored using native vegetation management on lands within the National Highway System. The team also considered the potential revenue that could be generated through the sale of carbon credits in a cap-and-trade market (a market-based approach that provides economic incentives for achieving reductions in GHG emissions) if normal vegetation management operations were modified to facilitate carbon sequestration in ROW soils and vegetation.

FHWA determined that more than 5 million acres (2 million hectares) of ROW are managed along the 163,000 miles (262,000 kilometers) of paved and unpaved roadways nationwide. The soils and vegetation on these lands currently store an estimated 100 million tons (91 million metric tons, MT) of carbon and sequester carbon at an estimated rate of 4 million tons (3.6 million MT) per year (1.17 tons per acre per year, or 2.62 MT per hectare per year). While carbon storage occurs naturally as part of the carbon cycle, ROW management practices have the potential to incrementally increase or accelerate carbon sequestration.

The project team also estimated that the ROW on the National Highway System could potentially sequester up to seven times more carbon than it currently stores (between 468 and 750 million tons, or 425 and 680 million MT) through a combination of natural absorption and land management practices.

For more information on FHWA's Carbon Sequestration Pilot Project, visit

Carbon Trading

Carbon trading markets, otherwise known as carbon registries, began on a country-by-country basis as early as 2003. Then in 2005, the Kyoto Protocol, a treaty in which industrialized countries agreed to reduce their collective emissions of GHGs, went into effect. European companies that emit GHGs are mandated to participate in carbon markets, but participation by U.S. companies in the Nation's carbon market remains voluntary.

Each registry establishes its own rules regarding carbon trading. In order to secure a tradable carbon credit, a carbon registry establishes a rigorous protocol to confirm that a particular activity provides a real, additional, and verifiable reduction in atmospheric carbon. For example, protocols related to land uses, such as grazing and forestry, prescribe land management activities and GHG monitoring strategies that protect or restore lands and preserve or increase the amount of carbon sequestered in soil and vegetation. The registry assigns a value to the protocol based on how much it offsets GHG emissions relative to a "business as usual" baseline.


Carbon cycles among the atmosphere, plants, and soils through the processes of photosynthesis, respiration, and decomposition. Worldwide, soils store 2,500 gigatons of carbon or 80 percent of the carbon stored in terrestrial ecosystems. Improving the soil's ability to sequester carbon may help slow down global climate change.


This team member is taking a soil sample at a ROW site in New Mexico.


However, no carbon registry currently offers a protocol for generating carbon credits for ROW vegetation management. New Mexico's research will support development of such a protocol.

Managing ROW Vegetation

In 2011, the New Mexico Department of Transportation (NMDOT) began the first of a two-phase research study building on the results of FHWA's pilot project. The study's goal is to evaluate the potential to increase soil carbon sequestration and storage by managing ROW vegetation.

FHWA selected NMDOT for the study because the State has many miles of rural roads and thus many acres of ROW to manage. It also has diverse forests and grasslands that have the potential to sequester soil carbon. In addition, New Mexico was, at the time of selection, a member of a voluntary emissions trading program to meet statewide carbon reduction goals.

"New Mexico has a history of solving problems," says J. Don Martinez, the division administrator of the FHWA New Mexico Division Office. "[And] within New Mexico, we have numerous and diverse climate zones that could potentially translate across many regions of the country."

Rick Wessel, an anthropologist and archeologist with the Environmental Design Bureau of NMDOT, was the agency's advocate. "We saw the potential to offset rising maintenance costs and shrinking operational budgets with revenue from vegetation management that also mitigated climate change," he says. "It was a win-win opportunity. We also thought the project could assist New Mexico in meeting its emissions reduction goals. . . ."

In the first phase of the project, the goals were to determine the number of acres available for carbon sequestration, the amount of soil carbon currently in ROW soils, the environmental characteristics that affect carbon sequestration and soil carbon in these systems, and which vegetation management practices may lead to an increase in soil carbon.

What Is Out There Now?

So how do researchers go about estimating the amount of carbon in soils along approximately 4,780 miles (7,700 kilometers) of State highways that crisscross the fifth largest State in the country? From a scientific point of view, the assessment of soil carbon must contend with multiple levels of natural variability, including different soils and plant communities, varied precipitation and temperature regimes, and elevations between 2,500 and 13,000 feet (762 and 3,962 meters). Furthermore, because the soils in a ROW are manipulated during road construction, they may not resemble the native soils adjacent to the road.

To account for this variability, the researchers measured soil and vegetation at randomly selected sites in three environments: (1) high mountains with pine and fir forests; (2) lower mountains and foothills with shrub and woodland species; and (3) prairies dominated by short, sod-forming grasses. To account for the variability within ROWs, the research team developed a conceptual site model with four zones, each with a unique history of disturbance, management, local topography, and potential soil and vegetation characteristics. The zones are: (1) managed zone, adjacent to the road and subject to frequent maintenance activities and runoff from the road; (2) inflection zone, the swale where stormwater accumulates and drains; (3) transition zone, where the soils and native plant community were initially disturbed during road construction, but have returned to natural vegetation; and (4) natural zone, areas within the ROW not disturbed by road construction and in relatively pristine condition.

For two reasons, the project focused on areas in the State that receive at least 14 inches (35 centimeters) of precipitation annually. First, research has shown that, especially during periods of drought, more arid soils become net emission sources of CO2 rather than carbon sinks (a reservoir for storing carbon). Second, aridic soils are naturally low in soil carbon and have slow sequestration rates, which make it difficult to measure changes in soil carbon over time. In fact, many carbon registries do not offer emission offsets from drier regions that have limited capacity to sequester soil carbon.

The research team collected more than 850 soil and 350 vegetation samples from 117 sites across New Mexico during October and November 2011. With soil samples from each ROW zone, researchers measured key characteristics such as total soil carbon, nitrogen, and texture. They visually assessed the vegetation by estimating the cover of grass, flowering plants, dead plant material, rock, and bare ground in sampling frames centrally placed in each ROW zone. Lastly, they collected all plant material (live and dead) from selected frames and weighed it to determine the aboveground biomass.

Researchers also measured the ROW width at each sample site. The data showed an average width of approximately 50 feet (15 meters) on each side of the paved roadway from the edge of the pavement to a property boundary, usually a fenceline. The NMDOT ROWs encompass 55,730 acres (22,557 hectares) of prairies, shrublands, and forests, equivalent to the land area of a large western cattle ranch.

From this dataset, the research team estimated that New Mexico State highway ROWs have a total of nearly 1.1 million tons (1 million MT) of soil carbon or 19 tons of soil carbon per acre (42 MT per hectare). As expected for New Mexico's semiarid climate, this amount of soil carbon is relatively low compared to other ecosystems such as temperate grasslands (116 tons per acre, 259 MT per hectare) or temperate forests (47 tons per acre, 106 MT per hectare). Researchers measured the highest carbon density at high elevation forested sites (25 tons per acre, 57 MT per hectare).

However, because most New Mexico highways are located in shortgrass prairies in the eastern part of the State, the prairie biome accounted for 65 percent of the total soil carbon in State ROWs. The highest densities of soil carbon occurred in the managed zones of both high elevation and prairie grassland sites where vegetation production benefits from water running off the pavement. Natural zones of forested sites at high elevations also had high soil carbon, due in part to increased amounts of snow and rain.

Driving Carbon Into New Mexico Soils

Using the data collected, researchers developed statistical models to search for environmental parameters associated with differences in soil carbon. For New Mexico, the models identified as the most important factors related to soil carbon were annual precipitation, grass production, dead plant material or litter, and clay content. The models also identified nitrogen levels in the soil to be directly related to the amount of carbon.


Raising the height of mowers like these has the potential to increase carbon sequestration.


These findings were on par with current knowledge about the transfer of carbon from the atmosphere to the soil in southwestern plant communities. Specifically, warm season grasses, such as blue grama, flourish during the summer monsoonal rains that fall from July through September in New Mexico. During the growing season, blue grama grasses capture atmospheric carbon and store it in their fine roots as carbohydrates, which provide energy to keep the plant alive during the dormant season. A third of the grass roots die in the process, and these dead root tissues are transformed into organic matter by soil insects, bacteria, and fungi. Soils with higher nitrogen content also tend to produce more grass when sufficient soil moisture is available. In addition, clay content enhances the ability of soil to hold water for plants to utilize during the growing season.

The models also provided a means to predict how vegetation management might increase soil carbon. In particular, the models suggested that a small increase in grass production in the managed zone could result in a fairly substantial increase in soil carbon.

The next step in the research project was to determine how to employ the findings to increase grass growth, and in turn, soil carbon.

Land Management To Increase Soil Carbon

Researchers investigated several treatments for land management to determine if they could potentially lead to an increase in grass productivity. Three showed the most promise: increasing mowing heights, seeding legumes (members of the pea plant family) to fix soil nitrogen in established ROW grasslands, and soil imprinting. Most important, NMDOT could implement these treatments on a large scale without interfering with required highway maintenance activities.

The standard mowing height in New Mexico is approximately 6 inches (15 centimeters), which generally removes most of the live tissue of grasses. If the mowing occurs during the growing season, grasses suddenly have significantly less leaf area for photosynthesis. In response, the plants divert the energy they capture into replacing leaf tissue instead of storing carbon in their roots. In some instances, grasses may even draw reserves from their roots to grow new leaves, thus decreasing the overall amount of plant carbon that could become organic matter. By raising the mower height to leave more leaf tissue, the rate of photosynthesis does not significantly decrease, and carbohydrates continue to be sent to the roots, which eventually leads to additional carbon sequestration.

Legumes have a symbiotic relationship with certain bacteria in their roots that enables them to convert atmospheric nitrogen into a form that is usable by plants. Increases in soil nitrogen can stimulate plant growth and increase root production, thereby potentially leading to an increase in soil carbon.


Interseeding native legumes, like the white prairie clover being seeded by this machine (above), can increase available soil nitrogen.

Soil imprinters are machines that mechanically create an array of offset indentations or divots approximately 4 inches (10 centimeters) deep on the soil surface. The uneven soil surface captures water and enables it to infiltrate the soil rather than run off, leading to an increase in the amount of water available to plants. The divots also may capture surface litter and potentially increase soil fertility. The combined increase in soil moisture and nutrients could, again, lead to more plant growth and higher rates of carbon sequestration.

Testing and Beyond

In the summer of 2013, NMDOT began the second phase of the project, testing the three treatments at eight sites in the State. The sites represent the ranges of precipitation and soil carbon found across New Mexico.


Another ROW vegetation management practice that could increase carbon sequestration is imprinting. Shown is a machine that is imprinting the soil, which enables the soil to capture more water and nutrients.


Over the next 2 years, the research team will measure soil carbon at the beginning and end of the test period; aboveground productivity of ROW vegetation; and the exchange in carbon among the atmosphere, vegetation, and soil using a state-of-the-art flux chamber. The equipment measures the rate of CO2 flow in and out of the soil, soil moisture, and temperature. Together, these data, and the ecosystem models built from them, should provide a clearer picture of how ROWs can become reservoirs to store atmospheric carbon.

All in all, NMDOT has made progress in addressing its research objectives. If the findings from the second phase indicate carbon storage in ROWs is worth pursuing, the next step will be to ensure that any revisions to policies for ROW vegetation management are aligned with the primary objective of NMDOT--to provide a safe, reliable, and efficient transportation system. To ensure that this balance is achieved, the research team includes field maintenance staff who can confirm that recommended changes in roadside management are both practical and achievable. In addition, revisions to ROW vegetation management to sequester carbon will have to be accomplished within NMDOT's budgetary constraints.


This state-of-the-art flux chamber measures the exchange of CO2 among the atmosphere, plants, and soil, which helps researchers understand the dynamics of CO2 in the ROW ecosystem and evaluate the potential of these lands to sequester soil carbon.


"There seems to be endless debate pertaining to climate change and a link to greenhouse gases--gases that include carbon dioxide," says FHWA's Martinez. "NMDOT and the New Mexico Division Office, in conjunction with FHWA [headquarters], made a decision to look for solutions, and through scientific research, assess the use of NMDOT rights-of-way as a potential storage area for carbon. Depending on our findings, we are hopeful to have protocols in place that scrub carbon from the air and trap it in the soils of highway rights-of-way. From there we would like to be able to share our research nationally and internationally and see if others can apply it."

Doug Romig is senior soil scientist with Golder Associates Inc. in Albuquerque, NM. He has extensive experience in land restoration and natural resource management throughout the southwestern United States. He holds B.S. degrees in soil science and range management from New Mexico State University and an M.S. in soil science from the University of Wisconsin-Madison.

Bill Dunn, Ph.D., is owner/ecologist of Big Picture Conservation LLC, an environmental consulting firm in Albuquerque, NM. He holds a Ph.D. in landscape ecology from the University of New Mexico. Previously, he was a wildlife biologist for New Mexico Department of Game and Fish.

Amy Estelle, Ph.D., is the engineering coordinator for NMDOT's Research Bureau and project manager for the Carbon Sequestration Pilot Project. She holds a B.S. in microbiology, an M.S. in natural resource management and administration, and a Ph.D. in American studies with emphasis on environment, technology, and culture.

Greg Heitmann is an environmental engineer with FHWA's New Mexico Division Office. He holds a B.S. in electrical engineering from South Dakota State University and has 25 years of experience working in the transportation field.

For more information, contact Doug Romig at