Denitrification is a microbiological process through which nitrogen, usually in the form of NO3-, is returned from the soil to the atmosphere. As a result of the recent rainfall, particularly in northwest Minnesota, some discussion has been generated concerning possible denitrification in the soils of our area as they approach saturation. Most growers in our area don’t get too excited about soil denitrification until we’ve had plenty of rainfall over a period of a week or more. Well, at the time of the writing of this article, the forecast (which changes almost daily in this part of Minnesota) is for rain throughout the rest of this week or longer. A short discussion of the conditions necessary for soil denitrification to occur and the general pathways of soil nitrogen might be of interest to some.

The loss of soil nitrogen due to denitrification has been estimated to be as low as 5% in some soils in our area to possibly as high as 20% per year. Northwest Minnesota has been experiencing several years of lower rainfall, thus, tendencies toward the lower end of this estimate would be a best guess.

The general requirements for denitrification are (1) anaerobic conditions or restricted O2 availability (saturated soil conditions), nitrogen oxides (much NO3-), and warm conditions (good microbial activity).

The pathway of nitrogen oxide reduction during denitrification is generally thought to be:

NO3- → NO2- + Oconsumed → NO → N2O → N2

In saturated soils (low O2) the microbial organisms will convert the NO3- (nitrate) to NO2- (nitrite) + oxygen and thus, consume the oxygen. This reductive sequence has been hypothesized for some time, and a significant amount of evidence has accumulated supporting the role of each nitrogen oxide in this pathway. Almost all denitrifying bacteria are aerobic organisms capable of anaerobic growth only in the presence of nitrogen oxides (see pathway above).

The capacity to denitrify has been reported to be present in many genera of bacteria. However, only a few are commonly isolated from soils, mainly species from the genera Alcaligenes and Pseudomonas.

It should be noted here that as soils increase in temperature during the summer months the denitrification process increases. Reports of minimum temperatures for the occurrence of denitrification in soil are about 50º F. Some reports suggest that as temperature decreases, the minimum soil water content for denitrification to occur increases. At 50º F little, if any, denitrification is observed even in effectively flooded soils. The literature reports that maximum temperatures for denitrification in soil seem to be as high as167º F, and the optimum temperatures have been observed to be around 149º F. These temperatures are much higher than one would likely encounter in a field situation, especially given the soil water content necessary for significant denitrification to occur. The point is that as soil temperatures increase above 50º F in fields that are saturated for long periods of time, and given there is enough nitrogen oxide (NO3-) in the soil, denitrification will likely occur.