By Fabián Fernández, University of Illinois
With a still fresh memory of the drought conditions during last year, recent rains have reduced concerns over water availability for the start of the 2013 growing season, but at the same time, concerns over nitrogen (N) loss have increased. Nitrogen loss is difficult to predict because it depends in many factors such as time of N application, type of N source, soil type and temperature, and the amount of precipitation received. While it is difficult to know how much N is lost without a direct analysis of soil N, I would like to provide some information that can help you determine what to do about N applications this growing season.
Most of the fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium. Nitrification, or the conversion of ammonium to nitrate (NO3-), is a bacteria-mediated transformation. The bacterium Nitrosomonas converts NH4+ into nitrite (NO2-) while the bacterium Nitrobacter converts NO2- to NO3-. The activity of these bacteria is minimal at temperatures below 50ºF. These bacteria also need aerobic conditions (unsaturated soil-water conditions) to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil is largely dependent on soil temperature and the time elapsed from application until the soil becomes saturated with water. Further, the nitrification process can be reduced with the use of nitrification inhibitors that reduces the activity of Nitrosomonas and allow N to stay in the ammonium form for a longer period of time.
When soils become saturated, the potential for N losses is directly related to the amount of N present in the nitrate (NO3-) form. While wet soil conditions this spring may be a reason for concern that some of the N applied last fall may be lost, the cold temperatures we had until recently likely substantially reduced nitrification. From now on, as temperatures increase, nitrification will also increase. When nitrate is present and soils warm up, N loss will start under saturated water conditions mostly through denitrification in fine-textured soils and through leaching in coarse-textured soils or intensively drained soils.
An important point to keep in mind is that the portion of the applied N that is in nitrate form is only subject to denitrification or leaching. However, the fact that N is in the nitrate form does not mean that N is lost; it simply means that it is susceptible to loss. Data from a study conducted a number of years ago (Table 1) provides a measure of the percent of ammonium that was transformed to nitrate by the end of May from three locations in Illinois depending on whether a nitrification inhibitor was used and when the application was done.
After determining how much of the N is in the nitrate form, it is possible to estimate how much N is potentially lost through denitrification based on soil temperature and the number of days the soil has been saturated. Current 4-inch depth soil temperatures can be accessed at http://www.isws.illinois.edu/warm/soiltemp.asp. In Illinois we have seen that for each day the soil is saturated with water, 1 to 2 % of the N in the nitrate form is lost via denitrification when temperatures are below 55°F. When temperatures are between 55 and 65°F the loss is 2 to 3%, and when temperatures are above 65 to 70°F losses are about 4 to 5%. Again, these losses are not for the total nitrogen applied, but rather for the portion that is in the nitrate form. Loss will vary depending on different factors, but these values are intended to provide an estimate.
The following calculation is a hypothetical situation given as an example using the data in Table 1 and current soil temperatures below 65°F:
Let’s assume that 180 lb N/acre were applied in early November with a nitrification inhibitor in a silty clay loam soil in DeKalb and soils were saturated for 5 days in late April.
First calculate N present as nitrate
N applied x % in nitrate form
180 lb N/acre x 0.55 = 99 lb N/acre
Second calculate N denitrified
N in nitrate form x % denitrified
99 x 0.15 (5 days x 3%/day)
15 lb N/acre lost
If leaching is a greater concern than denitrification, for each inch of precipitation, nitrate moves approximately 5 to 6 inches in silt loam and clay loam soils and approximately 12 inches in coarser textured-soils.