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

Abstract

Summary Two soils differing in ammonium fixation capacity were incubated for 127 days with 15 N‐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 15 NH 4 was lost from the histosol in 127 days, but only about 20 per cent was lost from the gley soil.