the elusive humic substances

Article for 23 April 2007:
Kelleher BP, Simpson AJ, 2006. Humic substances in soils: are they really chemically distinct? Environ. Sci. Technol. 40, 4605-4611.

For a long time it has been assumed that the humic substances are a distinct class of organic materials in soils and other natural systems, many workers believing them to be poorly ordered macromolecules formed by polycondensation reactions of a diverse range of biological molecules. Kelleher and Simpson's article suggests that this may not be the case, potentially changing our understanding of the largest pool of carbon in terrestrial environments. Their findings are based on advanced applications of NMR spectroscopy (¹H and ¹³C 2-D techniques, mostly; see Figure at right).

The main conclusion of the article seems to be that humic substances are mainly very complex mixtures of known compounds, rather than a distinct category in their own right. There are some disclaimers, of course; the authors allow for the possibility that distinctly "humic" macromolecules exist in amount below the fairly coarse detection limits of the NMR techniques, and also admit that older organic matter may contain chemically distinct substances.

One issue (at least) was unresolved, and confusing to us: if discrete biological compounds make up most of the humic mixture, than why do humic substances seem to be resistant to decomposition in soils?

Kelleher and Simpson's conclusion may be very important, though. As they point out, the dynamics of terrestrial carbon are critical to understanding global carbon cycles, a key to anthropogenic climate change scenarios. The chemical nature of the carbon compounds involved is a major determinant of carbon dynamics, so we do need to know what humic substances actually are.