NONHYDROLYZABLE ORGANIC NITROGEN IN SOIL SIZE SEPARATES FROM LONG-TERM AGRICULTURAL EXPERIMENTS

Total N (N-1), hydrolyzed N, NH3-N, and nonhydrolyzed N were determine d in soil particle-size separates from unfertilized or manured treatme nts in five long-term (15-108 Sr) experiments in Germany. The concentr ations of all N fractions (i) increased with decreases in particle siz e and (ii) were higher in samples from manured treatments. Irrespectiv e of particle size and soil management, nonhydrolyzed N accounted for 7 to 31% of N-1 (mean: 19%), On average, 53% of nonhydrolyzed N could be volatilized by pyrolysis. Field ionization mass spectra of the pyro lyzates of two hydrolysis residues showed that N heterocycles are majo r constituents of nonhydrolyzed N. In addition, 28 to 34% of total ion intensity was assigned to low-mass N compounds and aliphatic nitriles and amides. Shifts to higher volatilization temperatures with maxima at 450 to 520 degrees C in the thermograms of all N compounds indicate d that chemical stability, or strong bonds to soil minerals, are main reasons for the resistance of these molecules to acid hydrolysis. Curi e-point pyrolysis-gas chromatography/mass spectrometry using a N-selec tive detector and library searches enabled the identification of aliph atic, carbocyclic, and aromatic amines and nitriles, benzothiazole, su bstituted imidazoles, substituted pyrroles and pyrrolidine, substitute d pyrazoles, an isoquinoline derivative, substituted pyrazines and pip erazine, pyridine, and methylpyridine. In addition, low-mass N compoun ds such as hydrocyanic acid, N-2, nitrogen monoxide, isocyanomethane, and hydrazoic acid were assigned so that, in total, 37 compounds were identified in the pyrolyzates of nonhydrolyzed N. Within this fraction , we distinguished (i) proteinaceous materials, nonhydrolyzable probab ly due to binding or occlusion by pedogenic oxides, and (ii) highly al kyl-substituted N heterocycles, which are structural constituents of s table humic substances.