Submitted to: Encyclopedia of Soil Science
Publication Type: Book / Chapter
Publication Acceptance Date: August 15, 2001
Publication Date: June 30, 2002
Citation: Cambardella, C.A. 2002. Aggregation and organic matter. New York City: Encyclopedia of Soil Science: Marcel Dekker, Inc. p. 41-44. Technical Abstract: Soil organic matter (SOM) consists of plant, animal and microbial residues, and organic decomposition products that are associated with the inorganic soil matrix. Soil organic mater turnover is coupled to the agradation/degradation of soil aggregates through the activity of soil microorganisms. This chapter discusses how soil structure can mediate SOM turnover and formation and also discusses the effects of SOM on soil aggregation processes. Soil primary particles interact with SOM on several different scales. At the smallest scale, clay particles are organized into units called clay domains which are joined to quartz particles through organic matter mediated bonds. These organo-mineral complexes can be further structured into larger discrete units call microaggregates held together by organic matter-polyvalent cation bridges. Microaggregates are organized into larger units called macroaggregates which are held together by a network of fine roots and fungal hyphae. Differences in the relative stability of macro- and microaggregates are a function of their relative size, as well as the location and availability of the organic binding agents within the aggregates. The bonds that hold together macroaggregates are more easily disrupted than those that bind microaggregates because the organic matter binding microaggregates together is more humified and physically less available to microorganisms. As a result, microaggregates are more stabilized against decomposition relative to macroaggregates. Numerous studies have supported the assertion that SOM decomposition rates are higher in macroaggregates than in microaggregates for a variety of soil and land-use management protocols.