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.
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.
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.
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
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.
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.
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
"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
Agricultural Research Service Information Staff.
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)
"Glomalin: Hiding Place for a Third of the World's Stored Soil Carbon"
was published in the
issue of Agricultural Research magazine.
Organic Products that Work!
Our Affordable Gardening Supply Products, which
we call Natures Own Growing System, include
Organic Gardening Kit,
BioMinerals 2.5-4-.2 general
Biosol Forte 7-2-1 Organic Fertilizer,
Planters II Trace Mineral Fertilizer,
Soft Rock Phosphate-50Lb,
Microbe Tea Brewing Kits,
Bio-Peat moss and mushroom compost,
Organic Bar Soaps,
Just Like Sugar Natural Sweetener, and
Spider Elimination Kits
(spider traps). Most of these products are
used to grow
high brix plants which are high in plant nutrients (mineral and
People interested in these products are back
yard gardeners, farmers, farm crop management, flower growers, home
and garden supply companies, house plant growers, landscape
lawn care companies,
plant nursery owners,
urban gardeners, and those who wish to pursue
container gardening methods. Those interested in bettering their
diets and health use our products to grow highly nutritious foods.
The application of these products supports agriculture soil
in general but has many specific applications on this web site for
black leaf fungus on quaking aspen
bulbs and flowers,
fairy rings in lawns,
frost protection on
Asian pear fruit trees,
growing fruit trees,
necrotic ring spots in lawns,
promotion of earthworms in
red hot chili peppers,
turf grass management
of blue grass and all other kinds of grasses,
wine grape, vine and vineyard management,
zucchini and squash of all kinds,
different vegetables and several thousand other specific plants
- virtually everything grown on farms and in gardens aimed at plant
and root enhancement. Numerous pictures are present in a variety of
Contained in these products
are soil life forms that are often found in some compost tea brewing
kits but are much more effective in our
BioVam Mycorrhiza product and our
Microbe Tea Brewing kits. Organisms like, Mycorrhiza fungi (endomycorrhiza
fungi and ectomycorrhiza fungi), nitrogen fixing bacteria, phosphorous
solubilizing bacteria, and bacteria that are beneficial organisms found in our BioVam product. Nitrogen fixing bacteria, cellulose
eating trichoderma fungi, and many beneficial soil organisms are found
in our Microbe Tea Brewing Kits.
Privacy, Return, Refund Policy
© Copyright 1998 -
by T & J Enterprises. ALL RIGHTS RESERVED.