Diffusion of substrates to microorganisms is greatly slowed by drying; however, the relative humidity in the soil remains high. Rapid changes in soil water potential associated with rewetting cause microbes to undergo osmotic shock and induce cell lysis. A flush of activity by the remaining microbes, known as the Birch effect, results from mineralizing the labile cell constituents.
Different microbial communities are likely to be active over the range of water potentials commonly found in soils. A decline in microbial activity at low soil moisture levels can be explained as resulting from limited diffusion of soluble substrates to microbes or to reduced microbial mobility. Fungi are generally considered to be more tolerant of lower soil water potentials, i.e., drier soils, than are bacteria, presumably because soil bacteria are relatively immobile and rely more on diffusion processes for nutrition. Table 2.5 shows differences in the ability of different organisms to tolerate water stress. The nitrifiers, for example, typified by
Nitrosomonas, are less tolerant of stress than are the ammonifiers, typified by Clostridium and Penicillium. Ammonium may accumulate in a droughty soil because the nitrifiers do not have access to ammonium generated at water potentials at which the ammonifiers are still active.
ENVIRONMENTAL FACTORS, TEMPERATURE AND MOISTURE INTERACTIONS
Soil moisture and temperature are the critical climatic factors regulating soil biological activity. This control is affected by changes in the underlying rates of enzyme-catalyzed reactions. Where water is nonlimiting, biological activity may depend primarily on temperature, and standard Arrhenius theory can be used to predict temperature affects. But as soils dry, moisture is more controlling of biological processes than is temperature. These two environmental influences do not affect microbial activity in linear fashion but display complex, nonlinear, interrelated effects that likely reflect the individual responses of the various microorganisms and their associated enzyme systems.
The interaction of temperature, moisture, and organisms is exemplified by the current discussions about climate change. A hundred years ago, Swedish scientist Svante Arrhenius asked the important question "Is the mean temperature of the ground in any way influenced by the presence of the heat-absorbing gases in the atmosphere?" He went on to become the first person to investigate the effect that doubling atmospheric carbon dioxide would have on global climate. The globe is warming because of increased CO2 in the atmosphere from man's burning of past SOM depositions (fossil fuels) and from management changes such as cultivation of forested soils as in the Amazon. This in turn is causing the extensive organic deposits in frozen tundra to thaw. The melting allows the vegetation to change as the former tundra becomes boreal in nature. In turn, the trees will result in less light reflection and further increase warming and primary productivity. Will the generally warmer globe result in less overall organic matter as decomposition is increased? Theory such as the Arrhenius equation suggests that the more resistant organic matter compounds with high activation energy should be more decomposable at higher temperatures (Davidson and Janssens, 2006). But as always in soils there are interactions. What is the effect of physical protection by aggregates? Will there be more or fewer soil aggregates in a warmer climate with different vegetation-decomposer interactions, given that aggregates are formed by microorganisms and roots? Many of the environmental constraints affect decomposition reactions by altering organic matter (substrate) concentrations at the site at which all decomposition occurs, that of the enzyme reaction site. We must also consider decomposition rates at the enzyme affinity level; Michaelis-Menten models of enzyme kinetics are covered in Chap. 16 and energy yield in Chap. 9. Changes in microbial community structure (Chap. 8) also will have profound influences. The goal of this chapter is to provide an environmental boundary of the soil habitat and a description of its fundamental physical and chemical properties.
With this as a foundation, later chapters in this volume explore in detail information about organisms, their biochemistry, and their interactions.
references and suggested reading
Arrhenius, S. (1903). Nobel lecture.
Birkeland, P. W. (1999). "Soils and Geomorphology." 3rd ed. Oxford Univ. Press, Oxford.
Brady, N. C., and Weil, R. R. (2002). "The Nature and Properties of Soils." 13th ed. Prentice Hall, Upper Saddle River, NJ.
Davidson, E. A., and Janssens, I. A. (2006). Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 440, 165-173.
De Nobili, M., Contin, M., Mondini, C., and Brookes, P. C. (2001). Soil microbial biomass is triggered into activity by trace amounts of substrate. Soil Biol. Biochem. 33, 1163-1170.
Ettema, C. H., and Wardle, D. A. (2002). Spatial soil ecology. Trends Ecol. Evol. 17, 177-183.
Hillel, D. (1998). "Environmental Soil Physics." Academic Press, San Diego.
Kewei, Y., and Patrick, W. H., Jr. (2004). Redox window with minimum global warming potential contribution from rice soils. Soil Sci. Soc. Am. J. 68, 2086-2091.
McBride, M. B. (1994). "Environmental Chemistry of Soils." Oxford Univ. Press, Oxford, UK.
Paul, E. A., and Clark, F. E. (1996). "Soil Microbiology and Biochemistry." 2nd ed. Academic Press, San Diego.
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