Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon
Glomalin, extracted from
undisturbed Nebraska soil and
A sticky protein seems to be the unsung hero of soil carbon storage.
Until its discovery in 1996 by ARS
soil scientist Sara F. Wright, this soil "super glue" was
mistaken for an unidentifiable constituent of soil organic matter. Rather,
it permeates organic matter, binding it to silt, sand, and clay particles.
Not only does glomalin contain 30 to 40 percent carbon, but it also
forms clumps of soil granules called aggregates. These add structure
to soil and keep other stored soil carbon from escaping.
As a glycoprotein, glomalin stores carbon in both its protein and carbohydrate (glucose or sugar) subunits. Wright, who is with the Sustainable Agricultural Systems Laboratory in Beltsville, Maryland, thinks the glomalin molecule is a clump of small glycoproteins with iron and other ions attached. She found that glomalin contains from 1 to 9 percent tightly bound iron.
A microscopic view of an
arbuscular mycorrhizal fungus
growing on a corn root. The
round bodies are spores, and
the threadlike filaments are
hyphae. The substance coating
them is glomalin, revealed by
a green dye tagged to an
antibody against glomalin.
Glomalin is causing a complete reexamination of what makes up soil organic matter. It is increasingly being included in studies of carbon storage and soil quality. In fact, the U.S. Department of Energy, as part of its interest in carbon storage as an offset to rising atmospheric carbon dioxide (CO2) levels, partially funded a recent study by lab technician Kristine A. Nichols, a colleague of Wright's. Nichols reported on the study as part of her doctoral dissertation in soil science at the University of Maryland.
That study showed that glomalin accounts for 27 percent of the carbon in soil and is a major component of soil organic matter. Nichols, Wright, and E. Kudjo Dzantor, a soil scientist at the University of Maryland-College Park, found that glomalin weighs 2 to 24 times more than humic acid, a product of decaying plants that up to now was thought to be the main contributor to soil carbon. But humic acid contributes only about 8 percent of the carbon. Another team recently used carbon dating to estimate that glomalin lasts 7 to 42 years, depending on conditions.
For the study, the scientists compared different chemical extraction
techniques using eight different soils from Colorado, Georgia, Maryland,
and Nebraska. They found that current assays greatly underestimate the
amount of glomalin present in soils. By comparing weights of extracted
organic matter fractions (glomalin, humic acid, fulvic acid, and particulate
organic matter), Nichols found four times more glomalin than humic acid.
She also found that the extraction method she and Wright use underestimates
glomalin in certain soils where it is more tightly bound than usual.
In her Beltsville laboratory,
soil scientist Sara Wright
examines a soil aggregate
coated with glomalin, a soil
protein she identified in 1996.
In a companion study, Nichols, Wright, and Dzantor teamed up with ARS chemist Walter F. Schmidt to examine organic matter extracted from the same soils under a nuclear magnetic resonance (NMR) imager. They found that glomalin's structure differs from that of humic acidor any other organic matter componentand has unique structural units.
In a current study in Costa Rica, partly funded by the National Science
Foundation, Wright is using glomalin levels and root growth to measure
the amount of carbon stored in soils beneath tropical forests. She is
finding lower levels of glomalin than expected and a much shorter lifespan.
"We think it's because of the higher temperatures and moisture
in tropical soils," she explains. These factors break down glomalin.
Forests, croplands, and grasslands around the world are thought to be valuable for offsetting carbon dioxide emissions from industry and vehicles. In fact, some private markets have already started offering carbon credits for sale by owners of such land. Industry could buy the credits as offsets for their emissions. The expectation is that these credits would be traded just as pollution credits are currently traded worldwide.
Soil scientist Sara Wright
(foreground) and technician
Kristine Nichols use nuclear
magnetic resonance to examine
the molecular structure of
extracted soil organic matter
How Does Glomalin Work?
It is glomalin that gives soil its tiltha subtle texture that
enables experienced farmers and gardeners to judge great soil by feeling
the smooth granules as they flow through their fingers.
Arbuscular mycorrhizal fungi, found living on plant roots around the
world, appear to be the only producers of glomalin. Wright named glomalin
after Glomales, the taxonomic order that arbuscular mycorrhizal fungi
belong to. The fungi use carbon from the plant to grow and make glomalin.
In return, the fungi's hairlike filaments, called hyphae, extend the
reach of plant roots. Hyphae function as pipes to funnel more water
and nutrientsparticularly phosphorusto the plants.
"We've seen glomalin on the outside of the hyphae, and we believe this is how the hyphae seal themselves so they can carry water and nutrients. It may also be what gives them the rigidity they need to span the air spaces between soil particles," says Wright.
Technician Kristine Nichols
checks the progress of corn
plants growing in containers
specially designed for
As a plant grows, the fungi move down the root and form new hyphae
to colonize the growing roots. When hyphae higher up on the roots stop
transporting nutrients, their protective glomalin sloughs
off into the soil. There it attaches to particles of minerals (sand,
silt, and clay) and organic matter, forming clumps. This type of soil
structure is stable enough to resist wind and water erosion, but porous
enough to let air, water, and roots move through it. It also harbors
more beneficial microbes, holds more water, and helps the soil surface
Scientists think hyphae have a lifespan of days to weeks. The much longer lifespan of glomalin suggests that the current technique of weighing hyphae samples to estimate fungal carbon storage grossly underestimates the amount of soil carbon stored. In fact, Wright and colleagues found that glomalin contributes much more nitrogen and carbon to the soil than do hyphae or other soil microbes.
Rising CO2 Boosts Glomalin, Too
In an earlier study, Wright and scientists from the University of California
at Riverside and Stanford University showed that higher CO2
levels in the atmosphere stimulate the fungi to produce more glomalin.
They did a 3-year study on semiarid shrub land and a 6-year study on
grasslands in San Diego County, California, using outdoor chambers with
controlled CO2 levels. When CO2
reached 670 parts per million (ppm)the level predicted by mid
to late centuryhyphae grew three times as long and produced five
times as much glomalin as fungi on plants growing with today's ambient
level of 370 ppm.
Longer hyphae help plants reach more water and nutrients, which could
help plants face drought in a warmer climate. The increase in glomalin
production helps soil build defenses against degradation and erosion
and boosts its productivity.
Wright says all these benefits can also come from good tillage and
soil management techniques, instead of from higher atmospheric CO2.
"You're in the driver's seat when you use techniques proven to do the same thing as the higher CO2 that might be causing global warming. You can still raise glomalin levels, improve soil structure, and increase carbon storage without the risks of the unknowns in global climate change," she says.
Putting Glomalin to Work
Wright found that glomalin is very manageable. She is studying glomalin
levels under different farming and ranching practices. Levels were maintained
or raised by no-till, cover crops, reduced phosphorus inputs, and the
sparing use of crops that don't have arbuscular mycorrhizal fungi on
their roots. Those include members of the Brassicaceae family, like
cabbage and cauliflower, and the mustard family, like canola and crambe.
"When you grow those crops, it's like a fallow period, because
glomalin production stops," says Wright. "You need to rotate
them with crops that have glomalin-producing fungi."
In a 4-year study at the Henry A. Wallace Beltsville (Maryland) Agricultural
Research Center, Wright found that glomalin levels rose each year after
no-till was started. No-till refers to a modern conservation practice
that uses equipment to plant seeds with no prior plowing. This practice
was developed to protect soil from erosion by keeping fields covered
with crop residue.
Glomalin went from 1.3 milligrams per gram of soil (mg/g) after the
first year to 1.7 mg/g after the third. A nearby field that was plowed
and planted each year had only 0.7 mg/g. In comparison, the soil under
a 15-year-old buffer strip of grass had 2.7 mg/g.
Wright found glomalin levels up to 15 mg/g elsewhere in the Mid-Atlantic region. But she found the highest levelsmore than 100 mg/gin Hawaiian soils, with Japanese soils a close second. "We don't know why we found the highest levels in Hawaii's tropical soils. We usually find lower levels in other tropical areas, because it breaks down faster at higher temperature and moisture levels," Wright says. "We can only guess that the Hawaiian soils lack some organism that is breaking down glomalin in other tropical soilsor that high soil levels of iron are protecting glomalin."
It's Persistent and It's Everywhere!
The toughness of the molecule was one of the things that struck Wright
most in her discovery of glomalin. She says it's the reason glomalin
eluded scientific detection for so long.
"It requires an unusual effort to dislodge glomalin for study:
a bath in citrate combined with heating at 250 °F for at least an
hour," Wright says. "No other soil glue found to date required
anything as drastic as this."
"We've learned that the sodium hydroxide used to separate out
humic acid in soil misses most of the glomalin. So, most of it was thrown
away with the insoluble humus and minerals in soil," she says.
"The little bit of glomalin left in the humic acid was thought
to be nothing more than unknown foreign substances that contaminated
Once Wright found a way to capture glomalin, her next big surprise
was how much of it there was in some soils and how widespread it was.
She tested samples of soils from around the world and found glomalin
"Anything present in these amounts has to be considered in any
studies of plant-soil interactions," Wright says. "There may
be implications beyond the carbon storage and soil quality issuessuch
as whether the large amounts of iron in glomalin mean that it could
be protecting plants from pathogens."
Her recent work with Nichols has shown that glomalin levels are even
higher in some soils than previously estimated.
"Glomalin is unique among soil components for its strength and
stability," Wright says. Other soil components that contain carbon
and nitrogen, as glomalin does, don't last very long. Microbes quickly
break them down into byproducts. And proteins from plants are degraded
very quickly in soil.
"We need to learn a lot more about this molecule, though, if we
are to manage glomalin wisely. Our next step is to identify the chemical
makeup of each of its parts, including the protein core, the sugar carbohydrates,
and the attached iron and other possible ions." Nichols is starting
to work on just that.
"Once we know what sugars and proteins are there," says Nichols,
we will use NMR and other techniques to create a three-dimensional image
of the molecule. We can then find the most likely sites to look for
iron or other attached ions.
"Researchers have studied organic matter for a long time and know
its benefits to soil. But we're just starting to learn which components
of organic matter are responsible for these benefits. That's the exciting
part of glomalin research. We've found a major component that we think
definitely has a strong role in the benefits attributed to organic matterthings
like soil stability, nutrient accessibility, and nutrient cycling."
As carbon gets assigned a dollar value in a carbon commodity market,
it may give literal meaning to the expression that good soil is black
gold. And glomalin could be viewed as its golden seal.By Don
Comis, Agricultural Research Service Information Staff.
This research is part of Soil Resource Management, an ARS National
Program (#202) described on the World Wide Web at http://www.nps.ars.usda.gov.
Sara F. Wright and Kristine A. Nichols are with the USDA-ARS Sustainable Agricultural Systems Laboratory, Bldg. 001, 10300 Baltimore Ave., Beltsville, MD 20705; phone (301) 504-8156 [Wright], (301) 504-6977 [Nichols], fax (301) 504-8370.
"Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon" was published in the September 2002 issue of Agricultural Research magazine.