A new study by scientists from the University of Illinois contradicts the generally accepted theory that nitrogen fertilizers benefit the soil by building up organic carbon.
As part of their study, the team analyzed samples for organic carbon in the soil to identify changes that have occurred since the onset of synthetic nitrogen fertilization in 1955.
Findings revealed that after nearly five decades of massive inputs of residue carbon ranging from 90 to 124 tons per acre, all of the residue carbon had disappeared.
There had been a net decrease in soil organic carbon that averaged 4.9 tons per acre and regardless of the crop rotation, the decline became much greater with the higher nitrogen rate, said Saeed Khan from the UoI.
"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.
Co-researcher Tim Ellsworth said they were quite surprised to find 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.
The Morrow Plots are America's oldest experimental field.
Study co-author Charlie Boast, also from the University of Illinois, said that findings were consistent with previous studies showing a consistent decline in organic carbon in fertilized soil.
"In numerous publications spanning more than 100 years and a wide variety of cropping and tillage practices, we found consistent evidence of an organic carbon decline for fertilized soils throughout the world and including much of the Corn Belt besides Illinois," said Boast.
Study co-author Richard Mulvaney said the findings had 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," he said.
"The rates have been progressively inflated over the years by yield increases from agricultural advances such as better varieties and higher populations," added Khan.
The study, "The Myth of Nitrogen Fertilization for Soil Carbon Sequestration" appears in the November/December 2007 issue of the Journal of Environmental Quality.