Quantitative Determination of Recently Fixed Nonexchangeable Ammonium in Soils
2000
Paramacivam, Aru. (Aruṇakiri) | Breitenbeck, G. A.
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.
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