chapter 5 element cycling © 2013 elsevier, inc. all rights reserved. from fundamentals of ecosystem...

Chapter 5Chapter 5

Element CyclingElement Cycling

© 2013 Elsevier, Inc. All rights reserved.From Fundamentals of Ecosystem Science, Weathers, Strayer, and Likens (eds).

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Figure 5.1 Inorganic and organic materials follow many pathways as they move through ecosystems, but in general they flow between abiotic and biotic pools as they are taken up by organisms and incorporated into living biomass (production) and broken down (decomposition). They also move within inorganic pools (e.g., through soil weathering) and within organic pools (e.g., in trophic transfer). Materials may be added or lost from these pools as elements are transferred across ecosystem boundaries.


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Figure 5.2 Element cycles are linked as materials flow through ecosystems. Water transports elements such as hydrogen, sulfur, and nitrogen from the atmosphere to soil, where new elements such as calcium, potassium, sodium, and aluminum are encountered in the soil solution on the soil exchange complex. Interactions among these elements may result in a different assemblage of elements in soil solution—some of which may be taken up by plants or lost via leaching—thereby altering the chemical composition of water and changing what is retained in biotic and abiotic pools in the ecosystem.


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Figure 5.3 Nutrient spiraling, in which downstream transport draws out nutrient cycles into spirals.


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Figure 5.4 Uptake length of two forms of dissolved inorganic nitrogen in streams of different sizes, synthesized from 54 published studies by Tank et al. (2008). Symbol colors and shapes differentiate streams by their discharge, the volume of water moving downstream per unit time. Streams with greater discharge tend to be larger. This analysis suggests several conclusions: (1) that ammonium is taken up much more rapidly (shorter uptake lengths) than nitrate, especially in small streams (those with lower discharge); (2) that uptake length increases with discharge (which is a function of current velocity and the cross-sectional area of the stream, so either or both of these factors can be important); (3) that there is a lot of variation in uptake length among streams; and (4) that there is a paucity of research on nutrient dynamics in larger (high discharge) streams (which are logistically difficult to study).


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Figure 5.5 Periodic table of elements. An element’s reactivity—determined in part by its chemical structure and hence position within the periodic table—is important in element cycling because it affects reactions within cycles and variety of forms an element will take as it moves among different pools in an ecosystem. (From http://periodic.lanl.gov/index.shtml.)


http://periodic.lanl.gov/index.shtml

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Figure 5.6 Sulfur moves, sticks, and changes as it cycles through a northern hardwood ecosystem. Sulfur may enter a forest through atmospheric deposition of, for example, gaseous sulfur dioxide (SO2), dissolved sulfuric

acid (H2SO4), or dissolved or particulate ammonium sulfate ((NH4)2SO4). In the canopy, ions such as potassium

(K+) and magnesium (Mg2+) may be more abundant than in the atmosphere, so these may be picked up and become associated with sulfate (SO4

2−) in solution as water moves through the canopy. Sulfate moves readily

from atmosphere to forest, soil, and stream water with little sticking and usually few chemical changes. Sulfur sticks when taken up by plants and adsorbed in soils, and this may or may not involve a change in form. Sulfur can change between abiotic and biotic forms when it is taken up by plants or later when organic material is mineralized from organic to inorganic forms during decomposition (SOM=soil organic matter). The loss of sulfur from a forested ecosystem is most often as sulfate in solution, although in anoxic soils sulfate reduction can lead to the production of hydrogen sulfide gas (appendix).


chapter 5 element cycling © 2013 elsevier, inc. all rights reserved. from fundamentals of ecosystem...

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