There are hundreds of nitrification inhibitors which are studied worldwide so far (McCarty,1999; Prasad et al.,1995). If you classify them by derivatives, there are about ten or twenty kinds. If you classify them by compounds, there are probably hundreds of kinds. Main types include: thiazole; 1, 2, 4-thiadiazole; 1-acylpyridines; Triazine classes; Halopyridine and so on. Although the effect of nitrification inhibitor is good, the price is expensive. It is limited in practical application. Therefore,most of the research focuses on general chemicals and natural resources. At present, there are more studies on 3, 4-dimethylpyrazole phosphate, 3, 5-dimethylpyrazole sulfate, acetylene, dicyandiamide and so on. In actual production and application, there are mainly the following nitrification inhibitors:

(1)Dicyandiamide (DCD)

DCD has stable physical and chemical properties. It is non-hygroscopic, non-volatile, soluble in water and it can be applied together with a variety of nitrogen fertilizers. DCD is widely used in agricultural production with strong inhibition effect. A large number of studies have shown that DCD can effectively inhibit the conversion of ammonium nitrogen into nitrate nitrogen in soil and make ammonium nitrogen retain in soil longer. Some studies have confirmed that DCD has a strong inhibitory effect on the activity of ammonia-oxidizing bacteria, and the nitrification inhibitory rate can be up to 21%-90%, but it has no effect on the activity of nitrite oxidizing bacteria and no direct effect on other soil microorganisms or heterotrophs. The main mechanism of DCD's inhibitory effect on nitroso bacteria may be that the C congruent C group of dicyandiamide reacts with the sulfhydryl or heavy metal groups of nitroso bacteria respiratory enzymes. The addition of 5% to 10% dicyandiamide in the nitrogen fertilizer can effectively inhibit the nitrification for six weeks, and dicyandiamide will eventually produce ammonium nitrogen and nitrate nitrogen in the soil, so it can also be used as a nitrogen fertilizer. Shenyang institute of applied ecology, Chinese academy of sciences used to add a certain amount of DCD to producyion to produce a new type of long-term ammonium bicarbonate for nitrogen fertilizer, and obtained a national patent. Long-term ammonium bicarbonate has the same advantage of common ammonium bicarbonate, it has high fertilizer efficiency, wide applicability and no pollution to environment. It also solved the disadvantages of common ammonium bicarbonate, such as instability and volatility. Long-term ammonium bicarbonate can improve the utilization rate of nitrogen fertilizer. The mechanism is that the dissociated free state NH4+ from ammonium bicarbonate interacts with DCD to form hydrogen bond, which reduces the apparent concentration of NH4+ in the soil, lowers the soil pH value, reduces the possibility of nitrate formation, and reduces the nitrogen loss.

(2)2-chloro-6 - (trichloromethyl) pyridine (CP)

2-chloro-6-trichloromethyl pyridine is also known as nitropyridine, and its commodity name is "n-serve", which is also known as CP in China. It is a white crystalline solid substance, almost insoluble in water, but soluble in alcohol. N-serve is a nitrification inhibitor product developed and produced by DOW chemical company in the United States based on the nitrification inhibition characteristics of 2-chloro-6-trichloromethyl pyridine. Nitropyridine is used in more than 10 million hm2 of farmland in the United States each year. Similar to DCD, nitropyridine shows activity of inhibiting nitrifying bacteria and had no effect on crops and other microorganisms in soil. However, studies have shown that nitropyridine has a certain inhibitory effect on rhizobia, which should be paid special attention to in the early stage of leguminous crop application. Nitropyridine is easily hydrolyzed into 6-chloropicolinic acid and HCL, resulting in its volatilization loss. In addition, nitropyridine is not suitable to be applied in the surface because it is easy to photolysis. Studies have also shown that when nitropyridine is applied to soil with high organic matter, the volatilization loss will be reduced due to the adsorption of organic matter. But even with many disadvantages, nitropyridine is still widely used in agricultural production, especially in Europe and America. Although DCD and nitropyridine have a wide range of application, they all have some shortcomings, which limit their application in a wider range. Therefore, the synthesis and development of new high-efficiency nitrification inhibitors are of great significance.

(3)3, 4-dimethylpyrazole phosphate (DMPP)

DMPP is gray powder (commercial products are light green granular), molecular weight is 194.2, the density is 1.51 g/cm3 (20 ℃), melting point is 165 ℃, unit weight is 440 kg/m3, solubility in water is 132 g/L (pH3.0, 25 ℃), and PH is 2.5-3.0. DMPP is a nitrification inhibitor developed by BASF company in Germany. Subsequently, a large number of field trials in Germany and Europe showed that DMPP was an effective, safe, non-toxic and cheap nitrification inhibitor. In 2001, the 14th world plant nutrition conference was held in Hanover, and 10 papers on DMPP in the conference attracted great attention from researchers from various countries. Studies have shown that DMPP has the following advantages: (1) small dosage, applying 0.5-1.5kg per hectare can play a very good nitrification inhibition effect; (2) it will not lose when reducing the loss of nitrate nitrogen significantly; (3) there is no harm to crop hormone effect, in the plant body residual minimal and it remains little in the plant; (4) it can be completely decomposed in the soil, and the period of validity can be up to 4-10 weeks; (5) it has no irritation on human skin and eyes. DMPP inhibits nitrification by Inhibiting nitrification of the activity of ammonia-oxidizing bacteria in the first step reaction. This is the same as DCD. It has no effect on the second step of nitrification. Most studies have shown that DMPP has no toxic effect on the environment. Therefore, it is an ideal nitrification inhibitor.