Little is known about nitrogen (N) forms in ruminant feces, although this information is important to understand N dynamics in agro-ecosystems. We fed 15N labeled ryegrass hay to a sheep and collected 15N labeled feces. Nitrogen forms in the feces were characterized by chemical extractions, solid-state cross polarization 15N nuclear magnetic resonance spectroscopy (SS CP/MAS 15N NMR) and Curie-point pyrolysis-gas chromatography/mass spectrometry (Cp Py-GC/MS). A 4months incubation experiment was conducted to assess N release from the feces. Half of the fecal N could be ascribed to bacterial and endogenous debris and a third to undigested dietary N. About a tenth of the fecal N was mineralized during the incubation experiment. The 15N abundance of nitrate released during the incubation remained constant and close to the 15N abundance of the total feces N. The NMR analysis of the feces showed that most of the N was present in proteins, while some was present as heterocyclic N, amino acids and ammonium. The Cp Py-GC/MS analysis confirmed the presence of proteins, amino acids and heterocyclic N in the feces. Comparing these results to those obtained from the 15N labeled hay suggests that some N compounds present in the plant were not digested by the animal, and that the animal excreted de novo synthesized N compounds. The low content in ammonium and amino acids, the low rate of N release from these feces during the incubation and the relatively high fecal protein content, particularly the hard to mineralize undigested and microbially bound forms, can explain the low transfer of N from these feces to crops observed in a previous wor
