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  • From: "TradingPostPaul" <tradingpost@riseup.net>
  • To: livingontheland@lists.ibiblio.org
  • Subject: [Livingontheland] Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon
  • Date: Thu, 04 Jan 2007 21:13:29 -0700


Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon
A sticky protein seems to be the unsung hero of soil carbon storage.
http://www.ars.usda.gov/is/AR/archive/sep02/soil0902.htm

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.

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 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.
Soil scientist and technician examine extracted soil organic matter
constituents: Click here for full photo caption.

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.

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.

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.


"Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon" was
published in the September 2002 issue of Agricultural Research magazine.







  • [Livingontheland] Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon, TradingPostPaul, 01/04/2007

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