Biogenic Nitrogen in Soils as Revealed by Solid-State Carbon-13 and Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy
- Heike Knicker *
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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