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6 - Characterization of soil organic matter
- Edited by Werner L. Kutsch, Max-Planck-Institut für Biogeochemie, Jena, Michael Bahn, Leopold-Franzens-Universität Innsbruck, Austria, Andreas Heinemeyer
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- Book:
- Soil Carbon Dynamics
- Published online:
- 11 May 2010
- Print publication:
- 07 January 2010, pp 91-126
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Summary
INTRODUCTION
Soil organic matter (SOM) generally refers to the non-living organic material within the soil matrix that was once part of, or produced by, a living organism. It is usually determined on soil that has passed through a 2-mm sieve, and therefore is free of coarse animal residues, surface litter and large roots. Soil organic matter can be of plant, animal or microbial origin, and consists of a continuum of materials in various stages of alteration due to both biotic and abiotic processes (Baldock and Skjemstad, 2000). Methods used in the past to estimate directly SOM content involved the destruction of the organic matter by treatment with hydrogen peroxide (H2O2) or by ignition of the soil at high temperature (Nelson and Sommers, 1996). Both of these techniques, however, are subject to significant error: oxidation of SOM by H2O2 is incomplete, and some inorganic soil constituents decompose upon heating.
While different elements such as C, N, P, S etc. are bound into organic compounds, we will concentrate on soil organic carbon (SOC) for the purposes of this chapter because it is the dominant element, and because of its role in the global carbon cycle. Organic carbon to SOM conversion factors for surface soils typically range from 1.72 to 2.0 g SOM g−1 C (Nelson and Sommers, 1996). Direct measurement of total soil carbon involves the conversion of all forms of carbon to carbon dioxide (CO2) by wet or dry combustion and subsequent quantification of the evolved CO2.
9 - Measuring soil microbial parameters relevant for soil carbon fluxes
- Edited by Werner L. Kutsch, Max-Planck-Institut für Biogeochemie, Jena, Michael Bahn, Leopold-Franzens-Universität Innsbruck, Austria, Andreas Heinemeyer
-
- Book:
- Soil Carbon Dynamics
- Published online:
- 11 May 2010
- Print publication:
- 07 January 2010, pp 169-186
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- Chapter
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Summary
INTRODUCTION
The role of microbiota in soil carbon dynamics is a fitting example of the ‘borderland character’ mentioned in the introduction of this book. For years, the flux community largely followed the unspoken paradigm that ‘everything is everywhere’, which means that there is a universal potential of micro-organisms to decompose all kinds of organic matter, and that soil carbon dynamics, therefore, only depend on soil organic matter quality, soil properties and climate. A key distinction that controls whether the ‘everything is everywhere’ perspective is workable, however, is considering whether environmental conditions are in steady-state or non-steady-state conditions. Under steady-state conditions, microbial influences will be least apparent as they will have acclimatized to the existing physical, chemical or climatic constraints. Under non-steady-state conditions, however, extant microbial populations may reflect past, rather than present, conditions and their behaviour may be paramount and potentially counter-intuitive.
There is evidence that micro-organisms respond sensitively to changing environmental conditions by: (1) adjusting their intra- as well as extracellular enzymatic repertoire and, consequently, their physiological performance; (2) changes in the species composition and (3) growth or reduction of the microbial biomass.
There are several approaches to analyze the response of micro-organisms to changing environmental conditions.
Microbial eco-physiology (Anderson, 1994) focuses on the microbial biomass and its performance. More recent approaches (Lynch et al., 2004; Zak et al., 2006) include community oriented approaches that allow linking metabolic pathways to species composition or, at least, functional groups.
Microbial biochemistry analyses to determine the production and activity of microbial components, in particular enzymes. Since microbiota release a high portion of their enzymes, extracellular enzymes are included here (Sinsabaugh, 1994).
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