Overview of the symposium proceedings, Meaningful pools in determining soil carbon and nitrogen dynamics

Extraction of soil organic matter (SOM) fractions has been a long-standing 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 SOM stabilization. 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.