Nitrogen sequestration

When bacteria feed on soluble forms of nitrogen (ammonium and nitrite), they temporarily sequester that nitrogen in their bodies in a process called immobilisation. At a later time when those bacteria die, their nitrogen may be released as ammonium by the processes of mineralisation.
Protein material is easily broken down, but the rate of its decomposition is slowed by its attachment to the crystalline structure of clay and trapped between the clay layers. The layers are small enough that bacteria cannot enter. Some organisms can exude extracellular enzymes that can act on the sequestered proteins. However, those enzymes too may be trapped on the clay crystals.
Ammonium fixation occurs when ammonium replaces the potassium ions that normally exist between the layers of clay such as illite or montmorillonite. Only a small fraction of nitrogen is held this way.