Plantand Soil 162: 31-37, 1994. (~) 1994 Kluwer AcademicPublishers.Printed in the Netherlands. Labelling of animal manure nitrogen with 15N R SCrensen l, E.S. Jensen 1 and N.E. Nielsen 2 1 Plant Biology Section, Environmental Science and Technology Department, Rise National Laboratory, DK-4000 Roskilde, Denmark and 2 Section of Soil, Water and Plant Nutrition, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark Received23 June 1993. Accepted in revisedform 18 January 1994 Key words: homogeneity of labelling, mineralization, mobile nylon bag, ryegrass hay, sheep manure, water-soluble N Abstract A sheep was fed on 15N-labelled ryegrass hay during a period of 9 days in order to obtain 15N-labelled manure. After 9 days of feeding, the total N in faeces contained 3.70 atom % 15N excess, which was equivalent to 82% of the 15N enrichment of the hay N. The easily-decomposable fraction of the faecal N was less labelled (2.89 atom % 15N excess) than the slowly-decomposable fraction. The 15N enrichment of mineralized faecal N did not change significantly during 32 weeks of incubation in sand. About 25% of the faecal N was water-soluble. This N had a higher 15N enrichment than the total faecal N, indicating that a part of the water-soluble N was indigestible feed N. The faeces contained only small amounts of NH4 +-N, which had a 15N enrichment similar to the 15N enrichment of N mineralized during incubation in sand. It is suggested that the labelled faecal N obtained after a few days of feeding on labelled feed could be divided in two N pools: A decomposable N fraction (about 60%) with a 15N enrichment similar to the enrichment of N mineralized in sand (2.89 4- 0.09 atom % 15N excess), and a very slowly-decomposable N fraction (about 40%) with a 15N enrichment similar to that of the feed (4.52 atom % 15N excess). Introduction Due to the many processes involved in the turnover and losses of manure N in soils (Castellanos and Pratt, 198t; Thompson et al., 1987; van Faassen and van Dijk, 1987), the plant-availability of nitro- gen in animal manures is difficult to estimate. The fate of animal manure nitrogen applied to soil may be studied by using 15N-labelled manure. Differ- ent methods have been used for labelling of animal manures with 15N. Herbst et al. (1987) and Vilsmey- er and Gutzer (1990) labelled the NH+-N pool of a cattle slurry by adding a small amount of high- ly enriched NH+-N. Labelling the organic N fraction of manure homogeneously with 15N is more difficult. Rauhe et al. (1973) and Peschke (1982) fed cows on 15N-labelled plant material for a single day. Rauhe and Bornhak (1970) used the excrements of a cow, that had been given a feed containing 15N labelled urea. Kirchmann (1985) produced 15N labelled poul- try manure by feeding a cock on 15N enriched barley grain. The aim of the present study was to evaluate the homogeneity of ~SN enrichment of sheep faecal N. The sheep was fed on 15N-enriched ryegrass hay for 9 days. Theoretical considerations According to its origin, faecal N from ruminants may be partitioned into the following fractions: a) indigestible feed N, of which the pool size is depen- dent upon the nature of the feed; b) microbial N from the rumen, contributing .about 17% of faecal N, as estimated from Danf~er (1990); c) nitrogen in digestive secretions, mucus and dead cells from tissues in the digestive tract;