Microbial nitrification and subsequent denitrification releases nitrogen as gas to the atmosphere. Natural processes that generate nitrous oxide may be classified as nitrification and denitrification. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The conventional removal is nitrification, followed by denitrification. And the denitrification is basically the reverse process of nitrification. Techniques can include reforestation, pest control, nitrification and denitrification, irrigation, coppicing and hedge laying. It is a two-step process, consisting of nitrification followed by denitrification. The nitrification process transforms ammonia into nitrate. Microbial communities in contaminated soil have shown reductions in respiration and nitrification. Others convert ammonium to nitrite and nitrate ions, a process known as nitrification. In nitrification, ammonium is converted to nitrite. The transformation of ammonia to nitrite is usually the rate limiting step of nitrification. By extension, animal waste contributes to environmental pollution through nitrification and acidification of soil. To prevent fish deaths, nitrification via aeration prior to discharge is often desirable. Nitric oxide and nitrous oxide are also produced during nitrification.