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Where Does Your Nitrogen Go?

Robert Hoeft Robert Hoeft
Professor of Soil Fertility Extension
Department of Crop Sciences
(217) 333-4424
  Jeff Warren Jeff Warren
Senior Research Specialist in Agriculture
Department of Crop Sciences
(217) 333-4424

Illinois farmers consume approximately 0.9 million tons of nitrogen fertilizers each year. In addition, another 1.3 million tons become available from natural sources for crop production. The primary natural sources include organic matter mineralization (0.8 million tons), legumes (0.3 million tons), and livestock waste (0.2 million tons).

Scientists have long used the rule of thumb that soils mineralize (release inorganic N from organic matter) at the rate of 20 lb N/acre/year for each 1 percent of organic matter. Since this is a microbial process, it will vary depending in large part on soil moisture and temperature. For example, on the same set of plots, mineralization varied from 48 lb N/acre in 1995 (a cool, wet season) to 70 lb N/acre in 1996 (a warm, moist season). Recent work also has shown that the amount released will vary depending on previous nitrogen fertilization history of the field, with 70 lb N/acre being released on plots that had received no fertilizer N for the past 15 years as compared to 170 lb N/acre being released on plots that had received 180 lb N/acre over the same time period.

This vast input of nitrogen into agriculture goes through a number of reactions, transformations, and processes to: produce a huge supply of crop and livestock products that help to feed not only the residents of Illinois but also a great deal of the world population; and maintain the productivity of Illinois soils. Unfortunately, some of the nitrogen leaks back into the environment.

Harvested N: Slightly more than 1.2 million tons of N is contained in the harvested portion of Illinois crops and livestock each year. This harvested N accounts for about 65 to 70 percent of the total N taken up in the above-ground portion of the plant. Therefore, another 0.6 to 0.7 million tons of N taken up by plants is returned to the soil in crop residue each year. There is evidence that another 0.4 million tons of N is lost via canopy volatilization.

Fate of fertilizer N
Figure 1. Fate of fertilizer N.

Organic matter maintenance: Similar to mineralization, the rate of immobilization (conversion of inorganic N to organic N) is dependent on soil temperature and moisture and on the rate of N applied. At low rates of N application, from 30 to 35 percent of the N is converted to an organic form by the fall of the first growing season. As N rates increase, the rate of this conversion is decreased to about 20 percent (Figure 1).

Leaching and Denitrification: Total N recovered in the fall of the year in the plant-soil system ranged from 65 to 70 percent. The rest of the fertilizer N likely was lost via the processes of denitrification and/or leaching. Work in Champaign County has shown that a significant amount of the N loss in very wet years like 1998 could be via leaching, but in years of less excess water, the leaching loss is minor—less than 10 percent (Figure 2). As a rule of thumb, two-thirds of the N loss in Illinois soils occurs via the process of denitrification and the other one-third via leaching.

N loss from tile lines
Figure 2. N loss from tile lines.

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Department of Crop Sciences
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