The chemistry of soil organic nitrogen: a review

Abstract

1. From the data presented herein it is possible to deduce the following distribution of total N in humic substances and soils: proteinaceous materials (proteins, peptides, and amino acids) – ca. 40%; amino sugars – 5–6%; heterocyclic N compounds (including purines and pyrimidines) – ca. 35%; NH₃–19%; approximately 1/4 of the NH₃ is fixed NH₄ ⁺. Thus, proteinaceous materials and heterocyclics appear to be major soil N components.

2. Natural ¹⁵N abundance levels in soils and humic materials are so low that direct analysis by ¹⁵N NMR is very difficult or impossible. To overcome this difficulty, the soil or humic material is incubated with ¹⁵N-enriched fertilizer. Even incubation in the laboratory for up to 630 days does not produce the same types of ¹⁵N compounds that are formed in soils and humic materials over hundreds or thousands of years. For example, very few ¹⁵N-labelled heterocyclics are detected by ¹⁵N NMR. Does this mean that heterocyclics are not present? Or are the heterocyclics that are present not labelled under these experimental conditions and therefore not detected by the ¹⁵N NMR spectrometer ? Another possibility is that a large number of N heterocyclics occur in soils, but each type occurs in very low concentrations. Until the sensitivity is improved, ¹⁵N NMR will not provide results that can be compared with data obtained from the same soil and humic material samples by chemical methods and mass spectroscopy.

3. What is most important with respect to agricultural is that all major N forms in soils are available to organisms and are sources of NH₃ or NH₄ ⁺ for plant roots and microbes. Naturally, some of the NH₃ will enter the N cycle.

4. From chemical and pyrolysis-mass spectrometric analyses it appears that N heterocylics are significant components of the SOM, rather than degradation products of other molecules due to pyrolysis. The arguments in favor of N heterocyclics as genuine SOM components are the following:

a) Some N-heterocyclics originate from biological precursors of SOM, such as proteinaceous materials, carbohydrates, chlorophyll, nucleic acids, and alkaloids, which enter the soil system as plant residues or remains of animals.

b) In aquatic humic substances and dissolved organic matter (DOM) at considerably lower pyrolysis temperatures (200 to 300°C), free and substituted N-heterocyclics such as pyrroles, pyrrolidines, pyridines, pyranes, and pyrazoles, have been identified by analytical pyrolysis (Schulten et al 1997b).

c) Their presence in humic substances and soils was also detected without pyrolysis by gel chromatography – GC/MS after reductive acetylation (Schnitzer and Spiteller 1986), by X-ray photoelectron spectroscopy (Patience et al. 1992), and also by spectroscopic, chromatographic, chemical, and isotopic methods (Ikan et al. 1992).

5. While we can see light at the end of the tunnel as far as soil-N is concerned, further research is needed to identify additional N-containing compounds such as N- heterocyclics, to determine whether these are present in the soil or humic materials in the form in which they were identified or whether they originate from more complex structures. If the latter is correct, then we need to isolate these complex N-molecules and attempt to identify them.