Stabilization of N-compounds in soil and organic-matter-rich sediments - what is the difference?

Most of the organic nitrogen in soils and sediments ultimately derives from living organisms where it is mainly present as peptides and amino acids. These biomolecules are considered to have a biologically labile chemical structure and are expected to be quickly mineralized during early stages of organic matter stabilization. In spite of this, nitrogen is still found in aged soils, recent and even fossilized sediments. To elucidate the nature of this recalcitrant nitrogen and the processes that are involved in its formation, solid-state N-15 nuclear magnetic resonance (NMR) spectroscopy was recently introduced into geosciences and applied to various environments differing in the origin of their organic matter precursors as well as in chemical and physical conditions of the environment. Results obtained with this approach indicate that survival of peptide-like structures is a ubiquitous phenomenon, although the mechanisms for their stabilization may differ in different ecological systems. However, a conspicuous change in organic nitrogen composition is observed in fossilized sediments and for organic matter formed by vegetation fires. Cyclization and rearrangement of peptide structures result in the formation of heteroaromatic N during fossilization, which was not detected for recent sediments and soils. From this, it may be concluded that such compounds are only formed in environments in which abiotic transformation of biogenic precursors dominates over biotic degradation. (C) 2004 Elsevier B.V. All rights reserved.