Formation of Heteroaromatic Nitrogen after Prolonged Humification of Vascular Plant Remains as Revealed by Nuclear Magnetic Resonance Spectroscopy

Abstract

In the search for the mechanisms involved in the immobilization of organic nitrogen in humified remains of vascular plants, the efforts of the present investigation were directed toward the examination of the transformation of nitrogenous compounds during the peat and coal stage by means of solid-state nuclear magnetic resonance (NMR) spectroscopy. While accumulation of heteroaromatic-N is not detected in most of the studied peat layers, a clear shoulder in the chemical shift region of pyrrole- or indole-N is observed in the solid-state 15N NMR spectrum of material from the deepest (and thus oldest) peat layer underlying the sapropel from Mangrove Lake, Bermuda (10000 years). This points to the assumption that transformation of nitrogen occurs between an advanced stage of peatification and an early stage of coalification. The observed sudden alteration in nitrogen functionality indicates that continuous accumulation of newly synthesized or selectively preserved biogenic structures is not responsible for the presence of heteroaromatic-N in these fossilized deposits. It seems rather likely that abiotic conditions, occurring during advanced sediment maturation, have an effect on the observed N transformation. With increasing coalification, pyrrole-type-N becomes the dominant form in the macromolecular coal network. Pyridine-type-N was only detected in a coal of anthracite rank.

Evolution of Concepts of Environmental Natural Nonliving Organic Matter

“…Knicker (2000) has referred to her NMR evidence that shows that > 80% of the organic N in soils is in peptide -like structures. Knicker et al (2002) did detect a clear shoulder in the chemical shift region for pyrrole -or indole -heteroaromatic N ( − 145 to − 220 ppm) in the 15 N -NMR spectrum of the deepest layer of a peat that was at least 10,000 years old. However, this peat could be considered to be at the beginning of the coalifi cation stage, and there is abundant evidence for heterocyclic N in coal.…”

Alkyl amides in two organic-rich anoxic sediments: A possible new abiotic route for N sequestration

“…The identity of this amide functionality belongs to proteins and/or peptides but other amides such as amino sugars are also possible. In contrast, N-compounds in thermally mature sediments are generally considered to be of the pyridinic and aromatic form (Knicker et al, 2002;Boudou et al, 2008;Schimmelmann and Lis, 2010).…”

Does Solid-state 15N NMR Spectroscopy Detect all Soil Organic Nitrogen?

“…On the other hand, incubations of plant material with 15 N-labeled urea (Benzing-Purdie et al 1992;Skene et al 1997) and humic fractions with 15 N-labeled nitrite (Thorn and Mikita 2000) formed considerable quantities of non-amide N. Incubations of soil with 15 N-labeled trinitrotoluene (TNT) also resulted in the formation of non-amide TNT-breakdown products containing various forms of organic N (Knicker et al 1999;Bruns-Nagel et al 2000;. Two types of organic matter in which the signal for heterocyclic N does constitute most of the 15 N NMR signal detected are chars (Derenne et al 1993;Knicker et al 1996b;Keleman et al 2002), including the char fraction of soil (Knicker and Skjemstad 2000), and fossil organic matter such as coal and kerogen Knicker et al 1996a;Keleman et al 2002;Knicker et al 2002).…”

“…This problem is expected to be more severe for 15 N CP NMR spectra, as the lower gyromagnetic ratio of the 15 N nucleus results in a weakening of the critical dipolar coupling responsible for polarization transfer. Furthermore, the short contact times of 1 ms or less that are usually employed to characterize organic N (see, for example, Knicker, 2000a, b;Knicker and Hatcher 2001;Knicker et al 2002) can bias the acquisition of signal intensity toward directly protonated nitrogen types such as amide, at the expense of that derived from non-protonated N. Keleman et al (2002) showed using model compounds that long contact times were required to acquire signal from non-protonated N efficiently. Furthermore, Keleman et al (2002) found that the detection of non-protonated N was sensitive to the magnetic field strength of the spectrometer.…”