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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.
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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.
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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.
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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 acid—or any other organic matter component—and
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. |
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How Does Glomalin Work?
It is glomalin that gives soil its tilth—a 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 nutrients—particularly
phosphorus—to 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.
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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 resist crusting.
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.
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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 century—hyphae 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 levels—more than 100 mg/g—in 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 soils—or 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 the experiments."
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 in all.
"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 issues—such 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 matter—things 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. |