Healthy soil and sustainable growing

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'The Soil is Bleeding'
New research: synthetic nitrogen destroys soil carbon, undermines soil health
http://www.grist.org/article/2010-02-23-new-research-synthetic-nitrogen-destroys-soil-carbon-undermines-/
by Tom Philpott 23 Feb 2010 9:47 AM

Just precisely what does all of that nitrogen ferilizer do to the soil?
"Fertilizer is good for the father and bad for the sons."
--Dutch saying

For all of its ecological baggage, synthetic nitrogen does one good deed for 
the environment: it helps build carbon in soil. At least, that's what 
scientists have assumed for decades.

If that were true, it would count as a major environmental benefit of 
synthetic N use. At a time of climate chaos and ever-growing global 
greenhouse gas emissions, anything that helps vast swaths of farmland sponge 
up carbon would be a stabilizing force. Moreover, carbon-rich soils store 
nutrients and have the potential to remain fertile over time--a boon for 
future generations.

The case for synthetic N as a climate stabilizer goes like this. Dousing farm 
fields with synthetic nitrogen makes plants grow bigger and faster. As plants 
grow, they pull carbon dioxide from the air. Some of the plant is harvested 
as crop, but the rest--the residue--stays in the field and ultimately becomes 
soil. In this way, some of the carbon gobbled up by those N-enhanced plants 
stays in the ground and out of the atmosphere.

Well, that logic has come under fierce challenge from a team of University of 
Illinois researchers led by professors Richard Mulvaney, Saeed Khan, and Tim 
Ellsworth. In two recent papers (see here and here) the trio argues that the 
net effect of synthetic nitrogen use is to reduce soil's organic matter 
content. Why? Because, they posit, nitrogen fertilizer stimulates soil 
microbes, which feast on organic matter. Over time, the impact of this 
enhanced microbial appetite outweighs the benefits of more crop residues.

And their analysis gets more alarming. Synthetic nitrogen use, they argue, 
creates a kind of treadmill effect. As organic matter dissipates, soil's 
ability to store organic nitrogen declines. A large amount of nitrogen then 
leaches away, fouling ground water in the form of nitrates, and entering the 
atmosphere as nitrous oxide (N2O), a greenhouse gas with some 300 times the 
heat-trapping power of carbon dioxide. In turn, with its ability to store 
organic nitrogen compromised, only one thing can help heavily fertilized 
farmland keep cranking out monster yields: more additions of synthetic N.

The loss of organic matter has other ill effects, the researchers say. 
Injured soil becomes prone to compaction, which makes it vulnerable to runoff 
and erosion and limits the growth of stabilizing plant roots. Worse yet, soil 
has a harder time holding water, making it ever more reliant on irrigation. 
As water becomes scarcer, this consequence of widespread synthetic N use will 
become more and more challenging.

In short, "the soil is bleeding," Mulvaney told me in an interview.

If the Illinois team is correct, synthetic nitrogen's effect on carbon 
sequestration swings from being an important ecological advantage to perhaps 
its gravest liability. Not only would nitrogen fertilizer be contributing to 
climate change in a way not previously taken into account, but it would also 
be undermining the long-term productivity of the soil.

While their research bucks decades of received wisdom, the Illinois 
researchers know they aren't breaking new ground here. "The fact is, the 
message we're delivering in our papers really is a rediscovery of a message 
that appeared in the '20s and '30s," Mulvaney says. In their latest paper, 
"Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for 
Sustainable Cereal Production," which appeared last year in the Journal of 
Environmental Quality, the researchers point to two pre-war academic papers 
that, according to Mulvaney, "state clearly and simply that synthetic 
nitrogen fertilizers were promoting the loss of soil carbon and organic 
nitrogen."

That idea also appears prominently in The Soil and Health (1947), a founding 
text of modern organic agriculture. In that book, the British agronomist Sir 
Albert Howard stated the case clearly:

    The use of artificial manure, particularly [synthetic nitrogen] ... does 
untold harm. The presence of additional combined nitrogen in an easily 
assimilable form stimulates the growth of fungi and other organisms which, in 
the search for organic matter needed for energy and for building up microbial 
tissue, use up first the reserve of soil humus and then the more resistant 
organic matter which cements soil particles.

In other words, synthetic nitrogen degrades soil.

That conclusion has been current in organic-farming circles since Sir 
Albert's time. In an essay in the important 2002 anthology Fatal Harvest 
Reader, the California organic farmer Jason McKenney puts it like this:

    Fertilizer application begins the destruction of soil biodiversity by 
diminishing the role of nitrogen-fixing bacteria and amplifying the role of 
everything that feeds on nitrogen. These feeders then speed up the 
decomposition of organic matter and humus. As organic matter decreases, the 
physical structure of soil changes. With less pore space and less of their 
sponge-like qualities, soils are less efficient at storing water and air. 
More irrigation is needed. Water leeches through soils, draining away 
nutrients that no longer have an effective substrate on which to cling. With 
less available oxygen the growth of soil microbiology slows, and the 
intricate ecosystem of biological exchanges breaks down.

Although those ideas flourished in organic-ag circles, they withered to dust 
among soil scientists at the big research universities. Mulvaney told me that 
in his academic training -- he holds a PhD in soil fertility and chemistry 
from the University of Illinois, where he is now a professor in the 
Department of Natural Resources and Environmental Sciences -- he was never 
exposed to the idea that synthetic nitrogen degrades soil. "It was completely 
overlooked," he says. "I had never heard of it, personally, until we dug into 
the literature."

What sets the Illinois scientists apart from other critics of synthetic 
nitrogen is their provenance. Sir Albert's denouncement sits in a dusty old 
tome that's pretty obscure even within the organic-agriculture world; Jason 
McKenney is an organic farmer who operates near Berkeley--considered la-la 
land by mainstream soil scientists. Both can be -- and, indeed have been -- 
ignored by policymakers and large-scale farmers. By contrast, Mulvaney and 
his colleagues are living, credentialed scientists working at the premier 
research university in one of the nation's most prodigious 
corn-producing--and nitrogen-consuming --states.

Abandon all hope, all fertilizer execs who enter here. The Dirt on Nitrogen, 
Soil, and Carbon

To come to their conclusions, the researchers studied data from the Morrow 
plots on the University of Illinois' Urbana-Champaign campus, which comprise 
the "the world's oldest experimental site under continuous corn" cultivation. 
The Morrow plots were first planted in 1876.

Mulvaney and his collaborators analyzed annual soil-test data in test plots 
that were planted with three crop rotations: continuous corn, corn-soy, and 
corn-oats-hay. Some of the plots received moderate amounts of fertilizer 
application; some received high amounts; and some received no fertilizer at 
all. The crops in question, particularly corn, generate tremendous amounts of 
residue. Picture a Midwestern field in high summer, packed with towering corn 
plants. Only the cobs are harvested; the rest of the plant is left in the 
field. If synthetic nitrogen use really does promote carbon sequestration, 
you'd expect these fields to show clear gains in soil organic carbon over 
time.

Instead, the researchers found, all three systems showed a "net decline 
occurred in soil [carbon] despite increasingly massive residue [carbon] 
incorporation." (They published their findings, "The Myth of Nitrogen 
Fertilization for Soil Carbon Sequestration," in the Journal of Environmental 
Quality in 2007.) In other words, synthetic nitrogen broke down organic 
matter faster than plant residue could create it.

A particularly stark set of graphs traces soil organic carbon (SOC) in the 
surface layer of soil in the Morrow plots from 1904 to 2005. SOC rises 
steadily over the first several decades, when the fields were fertilized with 
livestock manure. After 1967, when synthetic nitrogen became the fertilizer 
of choice, SOC steadily drops.

In their other major paper, "Synthetic Nitrogen Fertilizers Deplete Soil 
Nitrogen: A Global Dilemma for Sustainable Cereal Production" (2009), the 
authors looked at nitrogen retention in the soil. Given that the test plots 
received annual lashings of synthetic nitrogen, conventional ag science would 
predict a buildup of nitrogen. Sure, some nitrogen would be removed with the 
harvesting of crops, and some would be lost to runoff. But healthy, fertile 
soil should be capable of storing nitrogen.

In fact, the researchers found just the opposite. "Instead of accumulating," 
they wrote, "soil nitrogen declined significantly in every subplot sampled." 
The only explanation, they conclude, is that the loss of organic matter 
depleted the soil's ability to store nitrogen. The practice of 
year-after-year fertilization had pushed the Morrow plots onto the chemical 
treadmill: unable to efficiently store nitrogen, they became reliant on the 
next fix.

The researchers found similar data from other test plots. "Such evidence is 
common in the scientific literature but has seldom been acknowledged, perhaps 
because N fertilizer practices have been predicated largely on short-term 
economic gain rather than long-term sustainability," they write, citing some 
two dozen other studies which mirrored the patterns of the Morrow plots.

The most recent bit of evidence for the Mulvaney team's nitrogen thesis comes 
from a team of researchers at Iowa State University and the USDA. In a 2009 
paper (PDF), this group looked at data from two long-term experimental sites 
in Iowa. And they, too, found that soil carbon had declined after decades of 
synthetic nitrogen applications. They write: "Increases in decay rates with N 
fertilization apparently offset gains in carbon inputs to the soil in such a 
way that soil C sequestration was virtually nil in 78% of the systems 
studied, despite up to 48 years of N additions."


morrowFertile ground for research: the Morrow Plots at the University of 
Illinois.Photo:brianholsclaw Slinging Dirt
Mulvaney and Khan laughed when I asked them what sort of response their work 
was getting in the soil-science world. "You can bet the fertilizer industry 
is aware of our work, and they aren't too pleased," Mulvaney said. "It's all 
about sales, and our conclusions aren't real good for sales."

As for the soil-science community, Mulvaney said with a chuckle, "the 
response is still building." There has been negative word-of-mouth reaction, 
he added, but so far, only two responses have been published: a remarkable 
fact, given that the first paper came out in 2007.

Both published responses fall into the those-data-don't-say-what-you-say-they 
category. The first, published as a letter to the editor (PDF) in the Journal 
of Environmental Quality, came from D. Keith Reid, a soil fertility 
specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. 
Reid writes that the Mulvaney team's conclusion about synthetic nitrogen and 
soil carbon is "sensational" and "would be incredibly important if it was 
true."

Reid acknowledges the drop in soil organic carbon, but argues that it was 
caused not by synthetic nitrogen itself, but rather by the difference in 
composition between manure and synthetic nitrogen. Manure is a mix of 
slow-release organic nitrogen and organic matter; synthetic nitrogen 
fertilizer is pure, readily available nitrogen. "It is much more likely that 
the decline in SOC is due to the change in the form of fertilizer than to the 
rate of fertilizer applied," Reid writes.

Then he makes a startling concession:

    From the evidence presented in this paper, it would be fair to conclude 
that modern annual crop management systems are associated with declines in 
SOC concentrations and that increased residue inputs from high nitrogen 
applications do not mitigate this decline as much as we might hope.

In other words, modern farming -- i.e., the kind practiced on nearly all 
farmland in the United States -- destroys soil carbon. (The Mulvaney team's 
response to Reid's critique can be found in the above-linked document.)

The second second critique (PDF) came from a team led by D.S. Powlson at the 
Department of Soil Science and Centre for Soils and Ecosystem Function at the 
Rothamsted Research Station in the United Kingdom. Powlson and colleagues 
attack the Mulvaney team's contention that synthetic nitrogen depletes the 
soil's ability to store nitrogen.

"We propose that the conclusion drawn by Mulvaney et al. (2009), that 
inorganic N fertilizer causes a decline in soil organic N concentration, is 
false and not supported by the data from the Morrow Plots or from numerous 
studies worldwide," they write.

Then they, too, make a major concession: "the observation of significant soil 
C and N declines in subsoil layers is interesting and deserves further 
consideration." That is, they don't challenge Mulvaney team's contention that 
synthetic nitrogen destroys organic carbon in the subsoil.

In their response (PDF), Mulvaney and his colleagues mount a vigorous defense 
of their methodology. And then they conclude:

    In the modern era of intensified agriculture, soils are generally managed 
as a commodity to maximize short-term economic gain. Unfortunately, this 
concept entirely ignores the consequences for a vast array of biotic and 
abiotic soil processes that aff ect air and water quality and most important, 
the soil itself.

So who's right? For now, we know that the Illinois team has presented a 
robust cache of evidence that turns 50 years of conventional soil science on 
its head--and an analysis that conventional soil scientists acknowledge is 
"sensational" and "incredibly important" if true. We also know that their 
analysis is consistent with the founding principles of organic agriculture: 
that properly applied manure and nitrogen-fixing cover crops, not synthetic 
nitrogen, are key to long-term soil health and fertility.

The subject demands more study and fierce debate. But if Mulvaney and his 
team are correct, the future health of our farmland hinges on a dramatic 
shift away from reliance on synthetic nitrogen fertilizer.