Study reveals that nitrogen fertilizers deplete soil orga...( The common practice of adding nitrog...)

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 recommendation
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