Wednesday, November 15, 2006 - 8:00 AM

Molecular Approaches to Soil Organic Matter Composition and Origin: a Review.

Ingrid Kögel-Knabner, Lehrstuhl für Bodenkunde, Department für Ökologie, Freising-Weihenstephan, Germany

This review focuses on research breakthroughs since the mid-1980s and delineates frontiers in molecular soil organic matter chemistry for the upcoming decade. However, significant contributions early in the last century are highlighted. Beginning in the 1980s, soil organic matter paradigms shifted from operational definitions based solely on chemical extraction procedures, which give humic and fulvic acids to physical fractionations, as related to the role that organic matter plays in soil structure and soil function. These fractionation procedures aim to partition SOM into components that differ in their "turnover time", chemistry (composition, structure and types of functional groups), and origin (plant-derived versus microbially-derived). Most of the early work in soil humic substances research was dedicated to unraveling the precise structure of soil organic matter in extractable humic fractions. It was already then shown that many components that could be isolated can be traced back to molecules well known from plant or microbial origin. A major problem were harsh degradation or pyrolysis procedures that often were considered to produce artefacts and thus the isolated molecules did not resemble any more the original humic components. Modern methods now widely used for the molecular-level analysis of soil organic matter include (thermo)chemolysis in combination with GC-MS and spectroscopic (FTIR, 13C and 15N NMR, XANES for C and N) techniques. This also led to the detection of charcoal as a major organic component in many soils. A major advantage is that we now can ascribe a turnover time or a radiocarbon age to some of the molecular components of soil organic matter. Nonetheless a large part of soil organic matter remains uncharacterized at the molecular level and thus more fundamental research on soil organic matter still needs to be done to fully elucidate its multiple roles in the development and properties of soils.