Study Reveals that Nitrogen Fertilizers Deplete Soil Organic Carbon

Discussion in 'Organic Lawn Care' started by Gerry Miller, Oct 29, 2007.

  1. Gerry Miller

    Gerry Miller LawnSite Senior Member
    Messages: 504

    Published: Oct. 29, 2007

    Source: Saeed Khan (217-244-7592;

    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.


    News writer: Debra Levey Larson
    phone: 217-244-2880; email:
  2. LindblomRJ

    LindblomRJ LawnSite Silver Member
    Messages: 2,570

    Interesting study. Any idea what the implications would be in a turf application?
  3. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,335

    None. The study suggests shifting from the typical yield-based determination of N requirements to a soil based determination.
  4. muddstopper

    muddstopper LawnSite Silver Member
    Messages: 2,345

    Kiril, I am going to disagree with you about implications in a turf application.
    True these test where based on row crops and crop removal, whereas in a turf situation the residues are mostly left on the lawn. There are more than a few that practice clipping removal. All soils will contain some biology and most of the biology is easily transferred to a non biological site by animals, and even the wind. That biology will feed on the extra N that is applied to the soil and increase in numbers to a unsustainable level without other N inputs. Once the easily available N is consumed by the microbes, the microbes will turn on the soil humus and consume that. The net result is a decrease in soil humus levels and soil carbon. The soil carbon is then released to the atmosphere in the form of CO2 gas. To combat the carbon loss, the microbes need another source of carbon. With turf, the extra carbon source can be compost topdressing. With Carbon to Nitrogen ratios between 10:1-20:1, you can build soil humus levels. With ratios less than 10:1 you lose soil carbon. With ratios over 20:1 you rob N from the plants to decompose the organic material. The microbes will eat at the food source (compost, protein meals, manures, etc) first, and the plants get the leftovers.
  5. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,335

    I think the point of the study was to show that using crop yield as an indicator of N requirements was leading to over-fertilization. Turf managers do not use yield as a measuring stick to determine fertilizer rates.

    It is also important to consider the differences between crop residue and grass clippings/compost with respect to C:N. Fresh grass clippings can range anywhere from 10-25, high quality compost 10-20, corn residue (per the crop examined in the study) generally runs around 65-75. 25-30 is generally considered the N limiting threshold.

    I do agree the study can be used to show what could potentially happen in a system that has been continuously over-fertilized.
  6. muddstopper

    muddstopper LawnSite Silver Member
    Messages: 2,345

    I agree, but one should probably take into consideration the homeowner that takes care of their own lawn, as well as many Lawn maintenance guys that like to see green, fast growing turf. Yield in the terms of dark green turf is still over applications of N to the soil with the same resulting loss in soil humus.
  7. americanlawn

    americanlawn LawnSite Fanatic
    from midwest
    Messages: 5,958

    We use humic acid in our liquid fert for turf & tree feddings, but our slow-release granular products do not offer humus. Our dry ferts range from 30% to 50% slow-release balanced fert plus micros in all of our turf apps.

    I grew up on a NW Iowa farm -- 540 acres of corn & soybeans. 35 bushels of beans/90 bushels of corn was a great yield back then. So why are we now seeing much higher yield regarding row crops? Also, why are corn & soybeans reaching new heights in quality regarding food processing or ethynol?

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