Environmental and Agronomic Fate of Fertilizer Nitrogen - American

Chapter 17

Environmental and Agronomic Fate of Fertilizer Nitrogen

Downloaded by EMORY UNIV on March 21, 2016 | http://pubs.acs.org Publication Date: December 3, 2003 | doi: 10.1021/bk-2004-0872.ch017

Robert G . Hoeft Department of Crop Science, University of Illinois, 1102 South Goodwin, Urbana, IL 61801

Maintenance of a quality environment and a competitive advantage in the world market place is a goal of all that recommend or use nitrogen fertilizer. To accomplish this goal, one must fully understand the biological and chemical reactions that nitrogen undergoes in a soil system. Mineralization, the process of conversion of organic nitrogen to plant available inorganic forms is affected by climatic and prior management practices. A s a result of the climatic influence, the rate of this reaction is unpredictable from year to year and thus it is difficult to predict the absolute optimum rate for any field in any year. Nitrification, the biological conversion of ammonium to nitrate is temperature dependent. Understanding this reaction allows producers to select the time of application that will minimize the potential for nitrogen to be in the nitrate form during the time period when denitrification and leaching are most likely to occur. Use of a nitrification inhibitor is another management tool that farmers can utilize to control the timing of the conversion of ammonium to nitrate. Plants recover from 30 to 40 percent of the fertilizer nitrogen in the year of application. A n equal amount is converted to organic form, immobilization, and is then available for release in subsequent years through the process of mineralization. The mineralization of newly immobilized organic compounds is about 7 times faster than native organic nitrogen compounds.

© 2004 American Chemical Society Hall and Robarge; Environmental Impact of Fertilizer on Soil and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Nitrogen management is quintessential for U.S. grain producers to maintain a competitive advantage in the world market place and at the same time a quality environment. Historically, grain prices have varied considerably, depending on supply and demand. Unfortunately, in the last several years, prices have trended down, and based on world grain supply, there is little hope that they will improve in the near future. Fertilizer prices also fluctuate based on supply and demand, with supply being dictated in part by cost and availability of raw material. Over the last 20 years, U.S. farmers paid on the average 14.6 cents per pound of nitrogen for fertilizer grade anhydrous ammonia. These prices varied from a low of 11.4 to a high of 20.1 cents per pound of nitrogen. Over the same time period, corn prices received by farmers varied from $1.54 to $3.30 with an average of $2.42 per bushel. Over the last five years, the average corn price has been $2.08 and fertilizer prices 17.1 cents per pound. Even though there has been a squeeze between fertilizer and grain prices, the use of nitrogen fertilizer is still very beneficial. Assuming that all producers used the optimum rate of nitrogen, the net value from increased production associated with nitrogen fertilizer use in Illinois would have been $680 million. A t the same time as economics are becoming tighter, pressure to improve nitrogen management because of environmental concerns are being stepped up by regulatory agencies. The Mississippi River/Gulf Hypoxia Watershed Nutrient Task Force reaffirmed the commitment to reduce Ν loss to the Mississippi River by 30%, with some suggesting that most of the gain will come from reduction of fertilizer use. While this is an amiable goal, the relationship between fertilizer sales and the size of the hypoxia zone is not strong (Figure 1). I HVpoa&SqLare miles

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Figure 1. Size of the hypoxia zone and Illinois nitrogen fertilizer sales. Hypoxia data provided by N.N. Rabalais, R.E. Turner, and W.J. Wiseman, Jr.

Hall and Robarge; Environmental Impact of Fertilizer on Soil and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

237 To meet the goals of having an economically and environmentally sound crop production system requires the use of Best Management Practices (BMP) for nitrogen management. These practices include setting the correct rate of application, taking credit for naturally produced nitrogen, and applying fertilizers at the correct time to avoid nitrogen loss. Use of the current scientific understanding of the fate of nitrogen in soil- the nitrogen cycle (Figure 2) provides guidance for developing B M P ' s .


Nitrogen Cycle Mineralization Mineralization, the microbial process that results in the conversion of organic nitrogen to inorganic nitrogen (plant available nitrogen), varies from year to year due to climatic variation. Under warm, moist conditions, the average rule of thumb is that there will be approximately 23 kg/ha of nitrogen released for each 1 percent organic matter. However, some of our recent work has shown that this can vary by at least 2 fold from year to year and as much as 4 fold depending on past management.

Figure 2. The Nitrogen Cycle


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This differential in nitrogen release created by differential nitrogen management in the past affects the amount and relative proportion of nitrogen taken up by plants from soil and current fertilizer application (Figure 3). The data provided below comes from a study in which the nitrogen rates listed were applied each year for 15 years prior to collection of the data in the figure. During that time, the excess nitrogen from fertilizers obviously resulted in a build-up o f easily mineralizable nitrogen that was released and taken up by the corn plants. In addition, the higher the nitrogen rate in the year of application, the greater the fertilizer nitrogen uptake by plants. In terms of fertilizer recovery, the greatest recovery occurred at the rate that was near the optimum for crop production, approximately 160 kg/ha of nitrogen.

Figure 3. Relative source of corn Ν uptake.

Nitrification Nitrification is the biological conversion o f ammonium to nitrate with an intermediate production of nitrite. As shown in figure 2, plants can utilize both ammonium and nitrate nitrogen. However, the majority of the nitrogen taken up by plants is in the nitrate form. The rate of nitrification is temperature dependent (Figure 4) and is affected by the addition of a nitrification inhibitor (Figure 5). A n understanding of the nitrification process is important in terms of management of fertilizer application to avoid the potential for nitrogen loss.


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Days after application Figure 5. Disappearance of ammonium nitrogen from soils over time at different temperatures when nitrapyrin is included with the fertilizer.



240 Denitrification and leaching Once it reaches the nitrate form, nitrogen is susceptible to loss by the processes of denitrification or leaching. Denitrification is most likely to occur on medium to heavy textured soils, whereas leaching is most likely to occur on lighter textured (sandy) soils. Neither of these loss mechanisms will occur unless the soils are excessively wet. In the case of denitrification, the soils must be saturated for a period of at least 3 days under warm moist conditions. Torbert et al., 1992 reported nitrogen loss values ranging from 2 to 10 percent of the fertilizer applied for each day the soils are saturated. While leaching is more of a problem on sandy soils, it does occur in heavier soils and is accentuated by the use of tile drain systems. The amount of nitrogen lost via tile line leaching is influenced by rate of nitrogen application and by the amount of water deposited on the land. In an excessively wet year (1999), nitrogen loss was equivalent to over 30 percent of the amount of fertilizer nitrogen applied, but in a dry year (2000) on the same fields, this loss was reduced to an equivalence of less than 5 percent of the fertilizer nitrogen applied (Figure 6).

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Figure 6. Percent of applied fertilizer lost from tile lines at 11 experimental locations over 3 years

Immobilization Immobilization is the biological process of converting inorganic nitrogen to organic nitrogen. From 30 to 50 percent of the fertilizer nitrogen is immobilized (converted to organic nitrogen) during the growing season in which it was applied (Figure 7). This resulting organic nitrogen material will not be available for loss via either denitrification or leaching until it has been nitrified.



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Figure 7. Fate offertilizer nitrogen during thefirstgrowing season after application Stevens et al, 1997

Best Management Practices Proper nitrogen rate Since there are several biological reactions that influence the availability of nitrogen for crop use, it is difficult to establish a nitrogen rate that will be accurate for every field in every year. The economical optimum nitrogen rates varied from 80 to 260 kg/ha over the 19 years of a study at the Northwestern Illinois Research and Demonstration Center (Figure 8). This difference in response across years was in large part due to climatic differences. The years of low nitrogen need were generally characterized as being years of good mineralization or years of low yield. In contrast, the years in which the riitrogen rate required was high were generally characterized as being years of low mineralization or high nitrogen loss due to denitrification. Results similar to this occur regularly in most fields across the Corn Belt. Therefore, producers are forced to use a rate that will over the long run give them an economic optimum return. Current University of Illinois recommendation for the field used in the experiment for Figure 8 would have been approximately 170 kg/ha, a level that would have resulted in less than optimum yield in but a few of the years of the study. Long term use of nitrogen rates in excess of those recommended will result in a marked increase in the loss of nitrogen from tile lines (Figure 9). Irrespective of the historical rate of nitrogen used, losses will be greatest in the year in which the nitrogen has been applied—the corn year.


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Environmental and Agronomic Fate of Fertilizer Nitrogen - American

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