What is the purpose of adding Nitrification Inhibitor to BOD samples

BOD is defined as the amount of oxygen required by microorganisms to digest organic carbon. However, there are bacteria that use oxygen to convert NH4+ to NO2- and NO2- to NO3- by a process called nitrification. Adding Nitrification Inhibitor (N-Allylthiourea) to the samples, prevents the nitrification process so that only oxygen demand due to carbon digestion is measured.