The chemical fixation ofl5N‐ammonia by partly decomposed Scots pine needle litter was studied. The material had been extracted with water using ultra‐sound treatment. This treatment, together with the addition of HgCl2had killed microorganisms and prevented microbial activity. Fixation of15N‐ammonia decreased with increasing accumulated mass loss and was inversely proportional to concentrations of sulfuric‐acid lignin as well as to concentrations of total nitrogen.