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 classic view based on analyses of soil samples
from the Morrow Plots - America's oldest experimental field - that date back
to before the current practice began.
The consequences of their findings are that (corn) farmers can reduce
their fertilizer inputs, obtain higher yields by doing so over the longer
term, and save money. In the case of biofuel production, this means the
energy balance of the fuel would increase, as less nitrogen fertilizers are
used. Nitrogen fertilizer production is an energy intensive process and
accounts for much of the loss in the net energy of biofuels.

The research, also drawing upon data from other long-term trials
throughout the world, was conducted by 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 University of
Illinois Morrow Plots in order to check how management practices have
affected soil properties. 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. - Saeed Khan, soil scientists, University of Illinois

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 they 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.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.

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, the scientists 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. 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. - Tim Ellsworth, soil scientist,
University of Illinois

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.


[biopact.com]