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Soil organic nitrogen was quantified by solid-state " N cross-polarization nuclear magnetic resonance spectroscopy (NMR) during a I4-month laboratory incubation of a sandy loam soil amended with '5N-clover. In whole soil and particle-size fractions, the clover-derived N was always 85-90% amide, 5 10% guanidinium N of arginine, and 5% amino. Quantitativeness of these results was suggested by (1) analysis of a standard containing a complex mixture of organic " N and (2) correlation of spectral intensities with 15N concentrations. Based on the unchanging proteinaceous NMR signature of clover-derived N throughout the incubation, differences in the mineralization/immobilization kinetics of clover-N among the different particle-size fractions appeared not to be linked to organic functional group. Kinetic analysis of the mineralization of " N, with correction of rate const;mts for field temperatures, suggested that the proteinanceous lsN in the clay and fine silt fractions observed here had a mean residence time of 7 years in the field. :(': 2003 Elsevier Ltd. All rights rescrvcd.