Nonhydrolyzable Organic Nitrogen in Soil Size Separates from Long‐term Agricultural Experiments

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Abstract
Total N (Nt), hydrolyzed N, NH3‐N, and nonhydrolyzed N were determined in soil particle‐size separates from unfertilized or manured treatments in five long‐term (15–108 yr) experiments in Germany. The concentrations of all N fractions (i) increased with decreases in particle size and (ii) were higher in samples from manured treatments. Irrespective of particle size and soil management, nonhydrolyzed N accounted for 7 to 31% of Ni (mean: 19%). On average, 53% of nonhydrolyzed N could be volatilized by pyrolysis. Field ionization mass spectra of the pyrolyzates of two hydrolysis residues showed that N heterocycles are major 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°C in the thermograms of all N compounds indicated that chemical stability, or strong bonds to soil minerals, are main reasons for the resistance of these molecules to acid hydrolysis. Curie‐point pyrolysis‐gas chromatography/mass spectrometry using a N‐selective detector and library searches enabled the identification of aliphatic, carbocyclic, and aromatic amines and nitriles, benzothiazole, substituted imidazoles, substituted pyrroles and pyrrolidine, substituted pyrazoles, an isoquinoline derivative, substituted pyrazines and piperazine, pyridine, and methylpyridine. In addition, low‐mass N compounds such as hydrocyanic acid, N2, 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 probably due to binding or occlusion by pedogenic oxides, and (ii) highly alkyl‐substituted N heterocycles, which are structural constituents of stable humic substances.

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