Ssj50111 967..974

Extraction of soil organic matter (SOM) fractions has been a longstanding approach to elucidating the pivotal roles of SOM in soil processes. Several types of extraction procedures are commonly used, and all provide partial information on SOM function. This report and accompanying papers summarize the information regarding SOM functions in real-world issues that has been gained through physical or chemical fractionations. Each procedure has its strengths and weaknesses; each is capable to some degree of distinguishing labile SOM fractions from nonlabile fractions for studying soil processes, such as the cycling of a specific soil nutrient or anthropogenic compound, and each is based on an agent for SOM stabilization. Physical fractionations capture the effects on SOM dynamics of the spatial arrangement of primary and secondary organomineral particles in soil, but they do not consider chemical agents for SOMstabilization. They appear better suited for C cycling than N cycling. Chemical fractionations cannot consider the spatial arrangement, but their purely organic fractions are suitable for advanced chemical characterization and can be used to elucidate molecular-level interactions between SOM and nutrients or other organic compounds. During all fractionations, the potential exists for sample alteration or mixing of material among fractions. We call for better coordination of research efforts by (i) developing integrated fractionation procedures that include physical, chemical, and/or biological components, and (ii) categorizing fractionations by their most suitable applications, defined by the nutrient, compound, or soil process in question, land use or crop type, crop management strategies, soil type, and possibly other factors. Selecting the most suitable fractionation procedure for a given research application would enable more precise approximation of the functional SOM pool. SOIL ORGANIC MATTER plays pivotal roles in several processes of the soil ecosystem, including nutrient cycling, soil structure formation, C sequestration, water retention, detoxification of anthropogenic chemicals, and energy supply to soil microorganisms. These processes lie at the heart of leading agricultural and environmental issues. Our prediction and management of these processes has been limited, though, in part because we do not understand the underlying mechanisms of SOM activity. Despite considerable attention, SOM remains a mysterious soil component. Mechanistic investigations of SOM are often based on the premise that SOM is composed of a continuum of materials, whose cycling rates vary from weeks to millennia (Jenkinson, 1981). This broad continuum can be conceptually divided into discrete kinetic pools, and researchers strive to characterize those pools that contribute to their research issues. Most agricultural and environmental issues develop over timescales ranging frommonths to decades, which calls for the isolation and study of relatively young SOM pools. A daunting research challenge, though, has been to extract from soil meaningful SOM fractions: those which correspond to the conceptual pools and whose cycling rates are both measurable and relevant to the issue in question. An ideal fractionation of SOM provides for the exhaustive extraction from all soils of unaltered material that is free of contaminants (Stevenson, 1994). All these conditions may never be achieved simultaneously. A useful, if not ideal, extraction procedure isolates meaningful fractions that are significant proportions of total SOMand are responsive in quantity and chemical nature to land management within the appropriate timescale. Such fractions can be used to elucidate (i) the effects of land use on SOM pools that are cycling at relevant rates, and (ii) the altered contributions of these SOM pools to soil processes and the research issue. The extraction of SOM fractions that differ in cycling rate can be based on any of several modes for SOM stabilization in soil. Thesemodes are of physical, chemical, or biological nature, including recalcitrant SOM chemical structures, microbially mediated condensations, binding to mineral surfaces, binding to polyvalent cations, protection within aggregates, inaccessibility in small micropores, and inhibited microbial activity (Sollins et al., 1996). During more than two centuries of research, SOM fractions have been extracted by many different procedures. Most studies used only one type of extraction procedure. Chemical extractions were dominant until recent decades, when physical extractions have become increasingly popular and more elaborate analyses for microbial characteristics have been developed. Comprehensive reviews are available for extractions involving particulate organic matter and the light fraction (Gregorich and Janzen, 1996; Wander, 2004), particle size fractions (Christensen, 1992), aggregate classes (Six et al., 2004), and chemical extractants (Hayes, 1985; Stevenson, 1994). The modes of SOM stabilization have proven to vary in significance by the situation, depending on soil type, land use, and other factors. No SOM extraction procedure has proven to be the most suitable in all cases. Researchers have tended to cluster into groups centered on preferred fractionations. Unfortunately, collaboration and dialog among the different groups have been lacking. Chemical extractions are used for advanced chemical characterization of SOM and in studies of aquatic systems and decomposition of organic materials, but they have not been linked to physical extractions that D.C. Olk, USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-4420; E.G. Gregorich, Agric. & Agri-Food Canada, Central Experimental Farm, Ottawa, ON K1A 0C6, Canada. Received 6 Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:967–974 (2006). Symposium: Meaningful Pools in Determining Soil C and N Dynamics doi:10.2136/sssaj2005.0111 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: SOM, soil organic matter. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 967 Published online April 19, 2006