Nitrogen in humic substances

The relationship of humin to kerogen, and the role of these substances as precursors to coal and petroleum are discussed by Hatcher et al. in Chapter 11. Schnitzer discusses the nature of nitrogen in humic substances in the last chapter (Chapter 12) of the geochemistry section. 

The purpose of this chapter is to highlight our current state of knowledge on the distribution of different types of nitrogen compounds in humic substances and to project future research needs, especially with regard to the identification of the unknown nitrogen in humic substances and the relationship between carbon and nitrogen. 

TABLE 7. Distribution of Hot Acid Hydrolyzable Nitrogen in Humic Substances from Different Climates (Mean Values)... 

From the data presented here one can conclude that in some humic fractions up to 18.6% of the total nitrogen, or close to 40% of the unidentified nitrogen, may occur as nucleic acid bases. Thus, the nitrogen in purines and pyrimidines appears to constitute a greater proportion of the unidentified nitrogen in humic substances than has been assumed in the past. 

As has been shown earlier, at least 50% of the nitrogen in humic substances still remains to be identified, and this after more than 100 years of research... 

From the information presented in this chapter it becomes apparent that there exists a voluminous literature on nitrogen in humic substances. But this literature deals mainly with types and concentrations of amino acids and amino sugars and the amounts of ammonia that are found in acid hydroly-zates of humic substances. About one-third to one-half of the total nitrogen still remains to be identified (Sowden et al., 1977). There are indications that this unidentified nitrogen can be transformed by microorganisms (Ivarson and Schnitzer, 1979) and chemically (Schnitzer and Hindle, 1980) to NH3. However, little is known about the chemical structure of the major components of this very substantial nitrogen fraction. Over the years a number of proposals have been made as to the identities of these constituents. These proposals were based on results of laboratory studies, philosophical considerations, and wishful thinking. Unfortunately, none of the proposed compounds has so far been detected in humic substances. 

Anderson, H. A., Bick, W., Hepburn, A., and Steward, M. (1989). Nitrogen in humic substances. In Humic Substances II In Search of Structure, ed. Hayes, M. H. B., MacCarthy, P., Malcolm, R. L., and Swift, R. S., Wiley-lnterscience, Chichester, West Sussex, England, 223-253. 

Schnitzer, M. 1985. Nature of nitrogen in humic substances. In G. R. Aiken, D. M. McKnight, R. L. Wershaw, and P. MacCarthy, eds. Humic substances in soil, sediment, and water. Wiley, New York, pp. 303-325. 

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