The N2 Dilemma: Is America Fertilizing Disaster? A Grist Special Series , ‘The Soil is Bleeding’By Tom Philpott
23 Feb 2010 9:47 AM, www.grist.org
New research: synthetic nitrogen destroys soil carbon, undermines soil health
“Fertilizer is good for the father and bad for the sons.
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.
An Old 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:
The use of artificial manure, particularly