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.