Methods for studying soil organic matter: nature, dynamics, spatial accessibility, and interactions with minerals

Soil organic matter (SOM) is extremely complex because of the variety of its inputs (e.g., plant tissues, microorganisms, animals, exogenous organic matter) and their different stages of decomposition. This chapter presents methods designed for the characterization of its composition and turnover at contrasting scales. To separate different SOM pools (either chemically, thermally, or physically), fractionation methods can be used to separate meaningful subsets of SOM. The chemical composition of whole SOM or SOM fractions can be characterized with spectroscopic methods that do not require prior extraction of SOM. The study of specific components requires the utilization of wet chemical methods. Visualization methods allow for identification of the location of SOM within soil structure and deciphering of its interaction with soil minerals, even at nanometer scales. The dynamics of SOM can be determined by combining specific long-term experiments with 13C or 15N analyses at natural abundance and/or incubation studies after labeling. At sites where the use of 13C labeling is not possible, 14C measurements may allow for the determination of mean residence times in different soil horizons or fractions. An integrated understanding of the dynamics and functions of SOM in different pedological, land use, and climate contexts is best attained by combining fractionation methods with characterization of the composition and turnover rate of fractions, down to the molecular scale.