Release of Fixed Ammonium during Nitrification in Soils

Ammonium added to soils can be fixed by clay minerals and subsequently released. There is, however, uncertainty concerning how rapidly this release occurs. We evaluated the possibility that relatively rapid release of recently fixed NH + 4 may accompany nitrification of exchangeable NH + 4 in soils. Labeled urea was added to four soils at rates of 0, 100, or 200 mg N/kg, and the soils were incubated anaerobically for 20 d to allow ample time for urea hydrolysis and NH + 4 fixation to occur. Recovery of the labeled N as fixed NH + 4 at 20 d averaged 1% in a Nicollet (fine‐loamy, mixed, mesic Aquic Hapludoll) soil, 3% in a Bremer (fine, montmorillonitic, mesic Typic Argiaquoll) soil, 10% in a Webster (fine‐loamy, mixed, mesic Typic Haplaquoll) soil, and 23% in a Canisteo (fine‐loamy, mixed [calcareous], mesic Typic Haplaquoll) soil. Air was introduced after 20 d to promote nitrification of NH + 4 and release of fixed NH + 4 . In the two soils with greatest fixation, a mean of 45% of the labeled N present as fixed NH + 4 after 20 d was released in the next 15 d. These observations indicate relatively rapid release of fixed NH + 4 during nitrification. Failure to recognize the importance of release of fixed NH + 4 during nitrification in recently fertilized soils could result in underestimates of the amounts of N available to plants and microorganisms and in overestimates of mineralization‐immobilization turnover of N.