Biogenic Nitrogen in Soils as Revealed by Solid-State Carbon-13 and Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy
Knicker, Heike
Solid-state nuclear magnetic resonance (NMR) spectroscopy represents a valuable nondestructive alternative to common chemolytic and thermolytic analytical approaches for characterizing the formation of humified organic N from biogenic precursors in soils. In this review, recent studies using solid-state ¹⁵N NMR spectroscopy for the examination of the fate of biogenic N in soils are summarized. From their results it can be assumed that most of the N occurs as peptide-like structures. Heterocyclic aromatic-N was not identified to a large extent in naturally humified material but was observed in spectra obtained from a humic acid of a soil incubated with ¹⁵N-labeled trinitrotoluene. The dominance of amide-N in humified organic N is supported by the application of dipolar dephasing (DD) solid-state 13C NMR spectroscopy. This technique can be used to estimate the relative content of N-substituted aliphatic carbons and thus, to calculate the relative contribution of peptides to the total C and N content of a sample. Applying this technique to degraded plant and algal material and to a humic fraction obtained from a natural soil indicates that peptides comprise more than 80% of the total organic N in the examined samples. The solid-state ¹⁵N NMR spectrum of a clay fraction of the Chinese Loess Plateau (age: 10 000 yr) reveals that some peptide-like material can survive prolonged pedogenesis. Several explanations for the protection of these commonly thought labile compounds are available and discussed.
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