<P> Mechanism based inhibitors can be defined as compounds that interrupt the normal reaction catalyzed by an enzyme . This method occurs by the inactivation of the enzyme via covalent modification of the product, which ultimately inhibits nitrification . Through the process, AMO is deactivated and one or more proteins is covalently bonded to the final product . This is found to be most prominent in a broad range of sulfur or acetylenic compounds . </P> <P> Sulfur - containing compounds, including ammonium thiosulfate (a popular inhibitor) are found to operate by producing volatile compounds with strong inhibitory effects such as carbon disulfide and thiourea . </P> <P> In particular, thiophosphoryl triamide has been a notable addition where it has the dual purpose of inhibiting both the production of urease and nitrification . In a study of inhibitory effects of oxidation by the bacteria Nitrosomonas europaea, the use of thioethers resulted in the oxidation of these compounds to sulfoxides, where the S atom is the primary site of oxidation by AMO . This is most strongly correlated to the field of competitive inhibition . </P> <P> N - heterocyclic compounds are also highly effective nitrification inhibitors and are often classified by their ring structure . The mode of action for these compounds is not well understood: while nitrapyrin, a widely used inhibitor and substrate of AMO, is a weak mechanism - based inhibitor of said enzyme, the effects of said mechanism are unable to correlate directly with the compound's ability to inhibit nitrification . It is suggested that nitrapyrin acts against the monooxygenase enzyme within the bacteria, preventing growth and CH / NH oxidation . Compounds containing two or three adjacent ring N atoms (pyridazine, pyrazole, indazole) tend to have a significantly higher inhibition effect than compounds containing non-adjacent N atoms or singular ring N atoms (pyridine, pyrrole). This suggests that the presence of ring N atoms is directly correlated with the inhibition effect of this class of compounds . </P>

Which of the following is not an intermediate step in the cycling of nitrogen