Fixation and defixation of ammonium in soils: a review

Fixed NH ₄ ⁺ (NH ₄ ⁺ f) and fixation and defixation of NH ₄ ⁺ in soils have been the subject of a number of investigations with conflicting results. The results vary because of differences in methodology, soil type, mineralogical composition, and agro-climatic conditions. Most investigators have determined NH ₄ ⁺ f using strong oxidizing agents (KOBr or KOH) to remove organic N and the remaining NH ₄ ⁺ f does not necessarily reflect the fraction that is truly available to plants. The content of native NH ₄ ⁺ f in different soils is related to parent material, texture, clay content, clay mineral composition, potassium status of the soil and K saturation of the interlayers of 2:1 clay minerals, and moisture conditions. Evaluation of the literature shows that the NH ₄ ⁺ f-N content amounts to 10-90 mg kg⁻¹ in coarse-textured soils (e.g., diluvial sand, red sandstone, granite), 60-270 mg kg⁻¹ in medium-textured soils (loess, marsh, alluvial sediment, basalt) and 90-460 mg kg⁻¹ in fine-textured soils (limestone, clay stone). Variable results on plant availability of NH ₄ ⁺ f are mainly due to the fact that some investigators distinguished between native and recently fixed NH ₄ ⁺ while others did not. Recently fixed NH ₄ ⁺ is available to plants to a greater degree than the native NH ₄ ⁺ f, and soil microflora play an important role in the defixation process. The temporal changes in the content of recently fixed NH ₄ ⁺ suggest that it is actively involved in N dynamics during a crop growth season. The amounts of NH ₄ ⁺ defixed during a growing season varied greatly within the groups of silty (20-200 kg NH ₄ ⁺ -N ha⁻¹ 30 cm⁻¹) as well as clayey (40-188 kg NH ₄ ⁺ -N ha⁻¹ 30 cm⁻¹) soils. The pool of recently fixed NH ₄ ⁺ may therefore be considered in fertilizer management programs for increasing N use efficiency and reducing N losses from soils.