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United States Department of Agriculture

Agricultural Research Service

Title: Organic Matter in Soils: Biogeochemistry and Relation to Environmental Quality

Authors
item Kingery, William - MISSISSIPPI STATE
item Clapp, Charles

Submitted to: Environmental Soil Mineralology
Publication Type: Book / Chapter
Publication Acceptance Date: July 15, 1999
Publication Date: N/A

Interpretive Summary: It was observed centuries ago that the capacity of a soil to produce crops was related, in varying degrees, to the amount of organic matter it contained. In the same manner that a farmer chooses dark-colored soils as those considered to be highly fertile, soil organic matter (SOM) is the foundation for economically feasible and environmentally sound land use. This chapter, which is contained in the book, 'Environmental Soil Mineralogy', is a summary of recent research findings on the formation, chemical properties, distribution in soils, and potential impact on environmental quality of soil organic matter. It is intended as a reference text for beginning graduate students in soil science and related fields. The chapter begins with a brief description of how the organic matter in soils, which is largely made up of carbon, forms an integral component of global cycling of carbon in the atmosphere, through plants and dother organisms, and in oceans, rivers and other water bodies on or within the earth's surface. This discussion includes the recognition of the role SOM plays in the processes affecting the so-called greenhouse gases, which is a clear implication for environmental quality. A description is also given of the general importance of carbon in environmental processes. These include problems in the use and management of organic wastes, like animal manure, and carbon-containing compounds used in industry and agriculture. These materials interact with SOM in diverse ways and may retain contaminants against loss to the environment or in some cases, may enhance their potential risks. It is clear from what is known about SOM, that it is important to understand both its microscopic characteristics as well as its behavior in the landscape to optimize the use of this important resource.

Technical Abstract: Soil organic matter (SOM) is one of the most important of natural resources and the basis of many reactions important to environmental quality. The term SOM refers to the total organic C-containing substances in soil encompassing live organisms, their remains, and microbially and/or chemically synthesized products. Probably any compound synthesized by living organisms can be found in SOM. The largest portion of SOM is made u of humic substances (HS) (80 to 90 percent) and polysaccharides (15 to 20 percent). Research on HS shows that chemical characteristics of humic acids (HAs) and fulvic acids (FAs) are reflective of the environmental setting in which they reside. Current concepts point to a pseudo-macromolecular nature of HS, which are considered to be self-associations of smaller molecules and somewhat micelle-like. There are many potential bonding mechanisms between natural organic (SOM) and inorganic (clay minerals) colloids, making their interaction the rule. There is sound evidence of compositiona differences between the OM associated with the different sized soil separates. These properties have an important bearing on environmental quality since aggregate stabilization is related to HS and soil polysaccharides. The soil C cycle plays a fundamental role in environmental quality as the loss from soil of anthropogenic compounds to water resources is one of the keystone issues to environmental quality. Many natural organo-mineral complexes profoundly influence the behavior of soil-applied organic chemicals and nearly every aspect of metal chemistry connected to metal OM-interactions. Finally, the C biogeochemical cycles of natural ecosystems interact and an understanding of their impact on one another is essential to proper environmental management.

Last Modified: 8/30/2014
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