Quantitative Determination of Recently Fixed Nonexchangeable Ammonium in Soils

Abstract

The importance of nonexchangeable NH4 as a potential source of plant‐available N is widely recognized. However, quantitative determination of nonexchangeable NH4 in soils poses a challenging analytical problem. The use of the KOBr pretreatment in the methods currently used to determine nonexchangeable NH4 in soils was found to cause a substantial loss of this fraction. For example, a second KOBr pretreatment prior to digestion of air‐dried samples reduced recovery of fixed NH4 by 6.4 to 32.8%. Therefore, a more reliable method was developed that avoids KOBr pretreatment. The proposed method quantitatively recovers (98–99%) nonexchangeable NH4 including recently fixed (RF) as well as native clay fixed NH4 while avoiding significant recovery (<1%) of N in soil organic matter or in microbial cells. Values obtained by the proposed method were similar to methods employing KOBr when the amounts of fixed NH4 were determined in 17 air‐dried soil samples that ranged widely in clay and organic matter content. However, when these samples were treated to enrich RF NH4, values obtained by methods employing KOBr were an average of 35% less than those obtained by the proposed method. These differences were attributed to loss of RF NH4 during KOBr oxidation. In the absence of KOBr pretreatment, determination of NH4 by steam distillation resulted in values inflated by the partial recovery of organic N. The proposed method determines the amount of NH4–N in 5 M HF:1 M HCl soil digests by a NH3 membrane diffusion technique that is highly accurate, minimizes recovery of organic N, and avoids the health hazards posed by the use of KOBr.