Immobilisation of 15NH4+ in three soils by chemical and biological processes

An investigation was made of the effect of chemical and biological processes on ammonium immobilisation. Three soils, autoclaved and unautoclaved, were used. Labelled (15NH4)2SO4 was added to soils that were then incubated at 25°C for 0, 1, 5, 10 or 20 days. At the end of incubation, clay-fixed 15NH4+, organic 15N and KCl-extractable mineral 15N was determined. In autoclaved soil, fixation by clay accounted for up to 31% of applied 15NH4+ over a 20 day incubation. In unautoclaved soils 15NH4+N fixed by clay decreased when nitrification reduced the quantity of KCl-extractable 15NH4+N. Incorporation of 15NH4+ into organic matter in unautoclaved soil accounted for 6–13% of applied N, whereas in autoclaved soil, only 2–7% was recovered in the organic N pool. The data show that both chemical and biological processes play important roles in immobilisation of 15NH4+ in soils.