Summary and Conclusions
The organic chemistry of soil organic matter (SOM) in ADEs can be looked at at different scales, depending on the type of interest. At an ecological level, operationally defined SOM fractions with different turnover times and / or stability can be separated and characterized. SOM associated with different particle-size fractions vary in composition and turnover time, the sand fraction containing labile, particulate SOM, the silt fraction stable SOM, and the clay fraction microbially active SOM. Density fractionation separates particulate SOM in the light fraction from organo-mineral associated SOM in the medium fraction, and more or less pure minerals in the heavy fraction. To study the origin of SOM as well as its turnover mechanisms, analyzing the chemical composition of SOM might be more useful. For this aim, more or less specific biomarkers such as individual carbohydrates, lipids (especially n-alkanes), amino sugars, and benzenecarboxylic acids have been proven useful. Obviously the most powerful tool is a combination of SOM fractionation and biomarker analysis together with rather new techniques such as compound-specific stable isotope analyses where the origin of identical molecules can be traced. Future work may benefit from the application of microscopic and non-destructive microspectroscopic techniques to investigate SOM within its mineral and microbiological soil environment. For in-depth information on modern methodological approaches and concepts on SOM structure, the excellent reviews of Baldock and Nelson (2000), Hedges et al. (2000), and Koegel-Knabner (2000) are recommended.