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Study Reveals that Nitrogen Fertilizers Deplete Soil Organic Carbon

Acres News
October 29, 2007

Urbana - The common practice of adding nitrogen fertilizer is believed to
benefit the soil by building organic carbon, but four University of
Illinois soil scientists dispute this view based on analyses of soil
samples from the Morrow Plots that date back to before the current
practice began.

The research, also drawing upon data from other long-term trials
throughout the world, was conducted by U of I soil scientists Saeed Khan,
Richard Mulvaney, Tim Ellsworth, and Charlie Boast. Their paper "The Myth
of Nitrogen Fertilization for Soil Carbon Sequestration" is published in
the November/December 2007 issue of the Journal of Environmental Quality.
"It is truly fortunate that researchers over the past 100 years have been
diligent in collecting and storing samples from the U of I Morrow Plots in
order to check how management practices have affected soil properties,"
said Khan. The Morrow Plots are America's oldest experimental field. "We
were intrigued that corn growth and yields had been about 20 percent lower
during the past 50 years for the north (continuous corn) than for the
south (corn-oats-hay) end of the Morrow Plots, despite considerably
greater inputs of fertilizer nitrogen and residues."

To understand why yields were lower for plots that received the most
nitrogen, Khan and his colleagues analyzed samples for organic carbon in
the soil to identify changes that have occurred since the onset of
synthetic nitrogen fertilization in 1955. "What we learned is that after
five decades of massive inputs of residue carbon ranging from 90 to 124
tons per acre, all of the residue carbon had disappeared, and there had
been a net decrease in soil organic carbon that averaged 4.9 tons per
acre. Regardless of the crop rotation, the decline became much greater
with the higher nitrogen rate," said Khan.

Mulvaney says that the findings have troubling implications for corn
production due to the widespread use of yield-based nitrogen
recommendations since the 1970s. "The one- size-fits-all approach was
intended to minimize the risk of nitrogen deficiency as insurance for high
yields. Unfortunately, the usual result is over-fertilization because of
the assumption that the fertilizer supplies more nitrogen than the soil.
The opposite is true in most cases, and especially for the highly
productive soils of the Corn Belt that receive the highest nitrogen
rates." Added Khan, "The rates have been progressively inflated over the
years by yield increases from agricultural advances such as better
varieties and higher populations."

Their findings for the Morrow Plots are confirmed in published literature
from field studies that included initial soil organic carbon data. "In
numerous publications spanning more than 100 years and a wide variety of
cropping and tillage practices," said Boast, "we found consistent evidence
of an organic carbon decline for fertilized soils throughout the world and
including much of the Corn Belt besides Illinois."

"We don't question the importance of nitrogen fertilizers for crop
production," said Ellsworth. "But, excessive application rates cut profits
and are bad for soils and the environment. The loss of soil carbon has
many adverse consequences for productivity, one of which is to decrease
water storage. There are also adverse implications for air and water
quality, since carbon dioxide will be released into the air, while
excessive nitrogen contributes to the nitrate pollution problem."
Because soils differ in their capacities to supply nitrogen, Khan and his
colleagues stress the need for soil testing, ideally on a site-specific
basis, as a prerequisite to soil-based nitrogen management that optimizes
fertilizer rates.

In comparing USDA data for Iowa and Illinois, the two states that rank
highest in corn production, they found that from 1994 to 2001, annual
grain yields in Iowa averaged 1.7 billion bushels with 740 thousand tons
of nitrogen, as compared to an average of 1.5 billion bushels produced in
Illinois with 847 thousand tons of nitrogen. The difference, Khan said,
translates into lower fertilizer efficiency that cost Illinois farmers 68
million dollars per year.

Funding for this research was provided in part through a Hatch project,
with additional support generated by the 15N Analysis Service.



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