Labelling of animal manure nitrogen with ¹⁵N

A sheep was fed on N-15-labelled ryegrass hay during a period of 9 days in order to obtain N-15-labelled manure. After 9 days of feeding, the total N in faeces contained 3.70 atom % N-15 excess, which was equivalent to 82% of the N-15 enrichment of the hay N. The easily-decomposable fraction of the faecal N was less labelled (2.89 atom % N-15 excess) than the slowly-decomposable fraction. The N-15 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 N-15 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 N-15 enrichment similar to the N-15 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 N-15 enrichment similar to the enrichment of N mineralized in sand (2.89 +/- 0.09 atom % N-15 excess), and a very slowly-decomposable N fraction (about 40%) with a N-15 enrichment similar to that of the feed (4.52 atom % N-15 excess).