Soil nitrogen (N), whether it is from fertilizer or soil organic matter, is only available to the plant when it is in the nitrate or ammonium form. In the ammonium form it is considered stable or protected since the negative charged soil particles hold the positive charged ammonium. The negative charged nitrate cannot be held by the soil and is vulnerable to various loss mechanisms. Eventually the ammonium will be converted to nitrate by soil bacteria in a process called nitrification. The first bacterium Nitrosomonas converts ammonium-N to nitrite-N. The second bacterium Nitrobacter converts nitrite-N to nitrate-N. And as you know, nitrate-N is the form we are most concerned about being lost (whether by leaching or denitrification).

The purpose of a nitrification inhibitor is to delay the nitrification process by suppressing the bacteria Nitrosomonas in the area where ammonium is to be present. Thus inhibitors extend the time that soil N stays in the ammonium form. There are three common nitrification inhibitors that are commercially available: 2-chloro-6-(trichloromethyl)-pyridine (nitrapyrin), dicyandiamide (DCD), and ammonium thiosulfate (ATS). Nitrapyrin is the active ingredient found in the Dow product N-Serve and Instinct. The biochemical activity of nitrapyrin and its ability to suppress growth of Nitrosomonas has been known since the 70s and it was initially registered in 1974. It is quite effective even at relatively low rates. Dicyandiamide (DCD) is the active ingredient in nitrification inhibitors such as Agrotain Plus, SuperU and Guardian.  Dicyandiamide is required at a significantly larger concentration to prevent nitrification. Correct concentration level is critical for all nitrification inhibitors to be effective.

Others products claim to be nitrification inhibitors. Some of them have been proven not to even inhibit nitrification. Research is lacking on some products and thus cannot be recommended at this time. North Dakota State University has a good discussion on nitrification inhibitors that may be found at the following URL:

Generally we do not recommend nitrification inhibitors for anhydrous ammonia applied shortly before planting or as a side-dress treatment, since the risk of nitrogen loss is low. No-till situations are more likely to show positive yield results than conventional till systems for spring applied anhydrous. 

Same would be true for products added to 28 percent solutions. Also keep in mind that the nitrification inhibitor would only affect the ammonium portion of solution and would not prevent N loss from the nitrate portion.

Nitrification inhibitors are less likely to show an economic benefit when high N rates are used in the field.  Nitrogen losses at high application rates are not likely to affect yield as much if lower rates are applied.

In summary, there are commercial products available that will inhibit nitrification for a short time period when applied at the right concentration. When N is applied near planting or at side-dress, the benefit from a nitrification inhibitor is minimal since the potential for N loss is low at this time.