The article quotes Sir Albert Howard, a pioneer researcher in what we now call organic farming. In another one of his books, An Agricultural Testament (published in 1943), Sir Albert relates his observation that work-oxen on his experimental farm in India that had a diet of ‘organic’ crops did not contract Foot and Mouth Disease even when exposed, nose to nose, to infected animals on neighboring farms. This certainly raises the question: What effect does current food production practices in this country have on human health?

New research: synthetic nitrogen destroys soil carbon, undermines soil health

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.

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 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.

Idea Germinates Anew

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:

In other words, synthetic nitrogen degrades soil.

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.

The Dirt on Nitrogen, Soil, and Carbon

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 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, 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."


The Morrow Plots at the University of Illinois

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 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:

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.)