One scientist has tantalizing
results, but others are not convinced.
This article was in Scientific American online but is worthy of being more widely dispersed into the wider agricultural and soil science community. Worth a read!
By John J. Berger on March
26, 2019
Can Soil Microbes Slow
Climate Change?
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 shovelling 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 fungal-to-bacterial ratio.
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.
EXTRAORDINARY CLAIMS
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—ploughing, 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 scientific facts.”
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 documented.”
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 tool.
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.
Recent Articles
Crisis in the Cryosphere,
Part 2
Crisis in the Cryosphere,
Part 1
Published online here on Blogger with acknowledgments to the author and Scientific American online
