TRANSFORMATION OF 15N-LABELLED AMMONIUM IN TWO SOILS DIFFERING IN NH+4-FIXING CAPACITY

Two soils differing in ammonium fixation capacity were incubated for 127 days with ¹⁵N-ammonium sulphate. In a gley soil with high NH⁺₄-fixing capacity caused by smectites with a charge up to 0.8 per formula unit, the major part of the added ammonium was first fixed by minerals and then released slowly during incubation. The proportion of labelled N in the nitrate fraction increased during the first weeks and then decreased permanently. In contrast, in a histosol with low NH⁺₄-fixing capacity, the exchangeable fraction contained most of the labelled NH⁺₄, this being highly available to microorganisms and therefore subject to nitrification. About 50% of the added ¹⁵NH₄ was lost from the histosol in 127 days, but only about 20 per cent was lost from the gley soil.