15N-CPMAS nuclear magnetic resonance spectroscopy and biological stability of soil organic nitrogen in whole soil and particle-size fractions

Soil organic nitrogen was quantified by solid-state N-15 cross-polarization nuclear magnetic resonance spectroscopy (NMR) during a 14-month laboratory incubation of a sandy loam soil amended with N-15-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-15 and (2) correlation of spectral intensities with N-15 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-15, with correction of rate constants for field temperatures, suggested that the proteinanceous N-15 in the clay and fine silt fractions observed here had a mean residence time of 7 years in the field. (C) 2003 Elsevier Ltd. All rights reserved.