[permaculture] Can Soil Microbes Slow Climate Change? - Scientific American
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Tue Mar 26 21:33:03 EDT 2019
Can Soil Microbes Slow Climate Change? - Scientific American
Can Soil Microbes Slow Climate Change?
One scientist has tantalizing results, but others are not convinced
- By John J. Berger
<https://www.scientificamerican.com/author/john-j-berger/> on March 26,
With global carbon emissions hitting an all-time high in 2018, the world is
on a trajectory that climate experts believe will lead to catastrophic
warming by 2100 or before. Some of those experts say that to combat the
threat, it is now imperative for society to use carbon farming
techniques that extract carbon dioxide from the air and store it in soils.
Because so much exposed soil across the planet is used for farming, the
critical question is whether scientists can find ways to store more carbon
while also increasing agricultural yields.
David Johnson of New Mexico State University thinks they can. The recipe,
he says, is to tip the soil’s fungal-to-bacterial ratio strongly toward the
fungi. He has shown how that can be done. Yet it is not clear if techniques
can be scaled up economically on large commercial farms everywhere.
Johnson, a trim 67-year-old microbiologist who is as comfortable using the
latest metagenomics technology as he is shoveling cow manure into a
composter, thinks society can only maximize carbon storage, increase soil’s
water-holding capacity and grow plentiful crops if it restores the soil
microbiome. “We currently have very degraded soils physically, chemically,
but mostly biologically,” he says. “Microbes restore this balance.”
Johnson conducts precise soil-biology experiments into how to increase the
capacity of agricultural systems to absorb carbon from the atmosphere. In a
recently completed four-and-a-half-year field trial, Johnson planted
fast-growing cover crops and applied a microbe-rich solution derived from a
vermiculture (worm) compost produced in a low-tech composter of his own
design. The bacteria, fungi and protozoa fed a soil food web of nematodes,
microarthropods and other beneficial organisms.
Through photosynthesis, the cover crops pulled CO2 from the air, sank roots
deep into the earth, and towered over the land. The results were
unusual—and highly controversial. Johnson reported a net annual increase of
almost 11 metric tons of soil carbon per hectare on his cropland. That’s
equivalent to removing about 16 metric tons of carbon dioxide per acre from
the atmosphere annually—roughly 10 times the increase that other scientists
have reported in many different soils and climates.
Johnson ascribes these improvements, along with large increases in crop
yields, to improved soil health stemming from the application of the
microbes from his vermiculture, leading to an increase in the soil’s
Professor Rattan Lal of Ohio State University, widely regarded as a leading
authority on soil carbon sequestration, says he was “intrigued” by
Johnson’s outcome. “I want to understand why he’s getting such exceptional
results.” Lal thinks that further, larger-scale trials are needed to
validate Johnson’s work, of course.
Johnson is also conducting meticulous laboratory studies. They focus on the
correlations among fungal-to-bacterial ratios and soil health, fertility
and crop productivity. He reports finding increases in fungal-to-bacterial
ratio, plus large increases in soil carbon and other nutrients as a result
of his management practices.
In all this work, Johnson maintains that as the ratio of fungi to bacteria
increases, the soil biome becomes more efficient in utilizing carbon and
other nutrients and that the soil therefore releases less CO2 to the
atmosphere. The jury is still out, however. Although peer-reviewed soil
science literature contains some confirmation, other findings in submerged,
forested and subarctic soils—admittedly different circumstances—failed to
confirm the relation.
Keith Paustian, a professor of soil and crop sciences at Colorado State
University, says he has seen some “quite high rates of carbon accrual” in
degraded croplands that were converted to productive perennial grass
systems. But he has not seen strong evidence that the same outcome can be
produced by adding microbes.
Johnson asserts that if his approach were used across agriculture
internationally, the entire world’s carbon output from 2016 could be stored
on just 22 percent of the globe’s arable land. He says that would provide
net benefits of $500 to $600 per acre rather than net costs, if credits are
provided for carbon capture and related benefits are counted, such as
reduced irrigation and increased soil fertility.
To arrive at his global carbon-capture numbers, Johnson projected results
from cropland plots of three to 75 acres of various soil types in five
states. That is still a fairly limited sample. Henry Janzen, a research
scientist at Lethbridge Research and Development Center in Alberta and a
professor at the University of Manitoba, cautions that such a projection is
risky. “Every ecosystem is unique,” he says. “A practice that elicits soil
carbon gain at one site may not be effective at another. And always, the
rate of carbon gain will depend on a host of interactive factors, including
soil properties, previous management practices, climatic conditions and the
vagaries of human whims.”
Janzen also points out that soils do not absorb carbon indefinitely. After
some years or decades, they inevitably approach a new steady state. For
that reason, he says, soil carbon sequestration is rarely seen as a
long-term solution to increased atmospheric carbon dioxide concentrations.
Johnson acknowledges those factors but says managing soil to improve the
health of its microbial life can provide strong carbon gains before the
soil’s capacity levels off. He is in the process of scaling up his
experiments to try to replicate his results on even larger plots in
different geographies with a variety of cover and commodity crops, “to
assess the impact for the rest of the world.”
A New Paradigm?
Johnson’s work is based on a somewhat different paradigm from that of most
conventional soil scientists. They often seek to boost agricultural
productivity in traditional ways by adding fertilizer and using pesticides
and herbicides as needed. This approach is anathema to Johnson. He decries
almost every conventional farming practice—plowing, bare fallowing, and the
application of herbicides, insecticides and fungicides. All these, he says,
“assault soil microbiota.” He claims that glyphosate (sold in commercial
products such as Roundup) will kill *Aspergillus* fungal species in soil.
*Aspergillus* is often regarded as a marker of fungal presence and is
important in carbon and nitrogen cycling.
As for fertilizer, Johnson believes he has demonstrated that microbially
inoculated soil enriched with tilled cover crops naturally accumulates more
than enough nitrogen for vigorous plant growth. (Nitrogen is the limiting
nutrient in most agricultural situations.) In one of his plots where he
reports having increased net primary productivity five times, the soil
accumulated 770 pounds of nitrogen per acre per year.
Much of this fixation is done by free-living nitrogen-fixing bacteria.
Because a normal crop only requires about 180 pounds of nitrogen per acre,
Johnson says it would be unnecessary to add artificial fertilizer to a
system like this.
As with all of Johnson’s work to date, this result has appeared only in the
form of reports and other “grey literature.” Harold van Es, professor of
soil and water management at Cornell University’s School of Integrative
Plant Science, is one of Johnson’s severest critics.
“In science, we strongly believe that research should be subjected to peer
evaluation,” van Es says. “His ideas should not be at all presented as
The fungal-to-bacterial ratio is indeed important, van Es says. “But there
are many ways to increase that ratio,” not just Johnson’s approach.
“Reducing tillage has similar effects and this has been much more widely
Although Johnson has irked some soil scientists and even aroused some ire,
as climate change intensifies in speed and fury, many scientists believe it
is important to leave no stone unturned in the search for ways to limit
carbon emissions quickly. Perhaps the soil’s microbiome can be a powerful
Rights & Permissions
ABOUT THE AUTHOR(S)
John J. Berger
John J. Berger is an environmental science and policy specialist who has
written numerous articles and books about the environment and climate
change. He is the author of *Climate Peril, The Intelligent Reader’s Guide
to the Climate Crisis* <https://www.powells.com/book/-9780985909239>.
- Crisis in the Cryosphere, Part 2
- Crisis in the Cryosphere, Part 1
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