ch04

Chapter 4 - THE BIOGEOCHEMICAL CYCLES

The Big Picture

Nutrients are elements or compounds that are essential for life. Nutrients, like all other elements in the Earth's crust, are finite and are recycled by natural processes. Nutrients may cycle between the lithosphere, hydrosphere, atmosphere, and biosphere. Thus, nutrient cycling is also referred to a biogeochemical cycling. Energy for biogeochemical cycling is derived in part from the solar radiation, which drives photosynthesis, and atmospheric and oceanic circulation. Energy for biogeochemical cycling is also derived from geologic processes such as subduction and uplift which are associated with plated tectonics and weathering, erosion, and rock formation which are part of the rock cycle. In order for organisms to survive, grow, and reproduce successfully, they must be supplied with nutrients in the appropriate concentrations and at the appropriate times. Human activities have greatly altered the Earth's landscape and consequently biogeochemical cycling. The global carbon cycling has been altered by deforestation, wood burning, and the combustion of fossil fuels. Nitrogen and phosphorous cycling has been altered by agriculture, industrial processing, and urbanization. Excessive inputs of these nutrients into waterways have resulted in eutrophication and a general decline in water quality in many waterways.

Frequently Asked Questions

What is a biogeochemical cycle?

A biogeochemical cycle is a pathway that a chemical follows as it cycles through the biota, soil, water, and atmosphere (Figure 4.2).
The cycle consists of reservoirs that are connected by pathways. In diagrams or models, the reservoirs are depicted as boxes (Figure 4.3b) and the pathways are depicted as arrows (Figure 4.3 a).
Models of biogeochemical cycles may be developed for local ecosystem cycles or for global cycles.

What elements are involved in biogeochemical cycles?

Twenty-four of the 103 known elements are involved in biogeochemical cycles (Table 4.1).
Because these 24 elements are used by organisms, they are considered nutrients.
Those nutrients needed in relatively large amounts by all organisms are called macronutrients (e.g., C,H,O,N,P, and S). These are usually needed as structural components or for common biochemicals.
Those nutrients need in relatively small amounts or by some organisms are called micronutrients. These are usually needed as coenzymes or for very specific functions.

How can nutrients be limiting factors?

If a required nutrient is present in concentrations so low that the metabolic needs of an organism, population, or species cannot be met, the nutrient can be considered a limiting factor.
Conversely, if a nutrient or other element is present in a toxic concentration, growth and survival may be limited.

What are some general concepts pertaining to biogeochemical cycling?

Biogeochemical cycling is essential for the continuation of life.
Elements with a gas phase (e.g., oxygen and nitrogen) usually cycle most rapidly.
Elements that readily become immobilized by geological process cycle slowly (e.g., phosphorous or carbon in fossil fuels).
Human activities and technologies have modified the Earth's landscape and altered biogeochemical cycling rates.

What natural processes are involved in nutrient cycling?

The tectonic cycle redistributes nutrients and other elements within the Earth's crust (lithosphere) as a consequence of shifting tectonic plates. The slowly shifting tectonic plates that comprise the Earth's crust cause physical and chemical environmental changes (Figure 4.5). Physical changes include the alteration of global atmospheric and oceanic circulation patterns, mountain uplift and rift formation. Chemically, crustal materials are changed when they are exposed to the extreme heat and pressure associated with burial or subduction along plate margins.
The hydrologic cycle is driven by the physical processes of evaporation and condensation. Water evaporates from the ocean, lake, and stream surfaces, soil, and vegetation (Figure 4.6). Atmospheric water vapor is then redistributed by local and global winds. Water vapor then condenses and returns to the oceans or land surfaces as precipitation. Flowing water erodes land forms and redistributes dissolved minerals and other elements. The oceans comprise the greatest global reservoir of water (97%); of course this is saline (35 ppt). The greatest reservoir of freshwater is frozen water in polar ice caps and glaciers (2%). The remaining 1% of global water is distributed within the groundwater surface waters, atmosphere, and biota.
The rock cycle is dependent upon the tectonic cycle and the hydrologic cycle. The rock cycle entails the transformation of parent rock material (formed by tectonic processes) into weathered rock material (weathered by water and other geologic, climatologic, and biologic processes) (Figure 4.7). Igneous rocks which are formed from molten magma and metamorphic rocks which have been subjected to great heat and pressure within the Earth's crust are uplifted, weathered, and their fragments are eroded. Eroded rock fragments and organic remains of plants and animals for soil and/or sedimentary rocks. Reburial of igneous, metamorphic, and sedimentary rocks by tectonic processes continues the rock cycle.

What is the role of ecosystems in biogeochemical cycling?

An ecosystem consists of a community of organisms and their nonliving environment.
One aspect of ecosystems is the processing of "nonliving" materials (i.e., nutrients) by the biota of an ecosystem. Most of the energy used to process nutrients in ecosystems is derived from solar radiation.
Nutrients may be cycled internally with the same ecosystem, such as the recycling of nutrients from decomposed leaves by the trees that produced the leaves.
Ecosystems are open systems, thus exchange materials or nutrients with other ecosystems.
Nutrients may be translocated from one ecosystem to another via such means as flowing water, air movement, and migrating animals.
Ecosystems that export a relatively large portion of their nutrients are called leaky ecosystems (e.g., a tidal salt marsh that exports nutrients in detritus to adjacent estuaries with each ebbing tide).

How does the carbon cycle function?

Carbon is the basic structural element of all organic molecules, thus essential to all life.
Even though life is carbon-based, carbon comprises a relatively small component of the Earth's crust (0.032% by weight).
Carbon occurs in gaseous forms (CO₂, CO, CH₄), dissolved in water (HCO₃, H2CO₃), in sediments (detritus and humus) and rock (limestone and fossil fuels), and incorporated into living tissue (organic molecules). Carbon also occurs in inorganic forms (e.g., graphite and diamonds).
Carbon is initially incorporated into the biota via photosynthesis (CO₂ is converted to glucose) (Figure 4.13).
Some carbon (as glucose) is converted into new plant tissue. Some plant tissue is consumed by animals and converted into animal tissue (Figure 4.14).
Carbon is aerobically respired by plants, animals, and microbes and returns to the atmosphere or water as CO₂. Carbon is also returned to the atmosphere via anaerobic respiration and combustion.
Annually, approximately 15% of the total atmospheric carbon is fixed by photosynthetic plants and respired by plants, animals, and microbes.
In certain anaerobic conditions such as exists in some wetlands or where decomposition is very slow such as cold deep sea sediments or polar regions, carbon accumulates, is buried, and forms fossil fuel deposits.
Humans have altered the carbon cycle through burning fossil fuels and vegetation which releases stored carbon, and through deforestation, which reduces the amount of atmospheric carbon taken up through photosynthesis.
Although understanding the global carbon cycle is problematic, atmospheric monitoring studies have indicated an increase in atmospheric carbon (a greenhouse gas) over the past 35 years.

What is the carbon-silicate cycle?

Carbon dioxide readily dissolved in water to form carbonic acid (H₂CO₃).
As water containing carbonic acid migrates through groundwater and surface water systems, the acid dissolves silicate rock and releases calcium and bicarbonate ions.
Calcium and bicarbonate ions are used by planktonic organisms and mollusks to construct shells.
These shells, along with bones and other forms of carbon in sediments, accumulate on the sea floor and form limestone. If this limestone is subducted by tectonic processes, the carbon may be released as CO₂by volcanic activity (Figure 4.15).

How does the nitrogen cycle function?

Nitrogen is a constituent of proteins, thus essential for life.
Free nitrogen (N₂) makes up approximately 80% of the atmosphere.
Free nitrogen cannot be used directly by plants and animals so must be converted to a usable form. This conversion is accomplished by various species of bacteria. (Lightning also oxidizes N₂ to usable NO₃-.)
Bacteria living symbiotically with plants (e.g., legumes) and algae, or free living in the soil or water convert N₂ to ammonia (NH₃) (which can be used by plants); this process is called nitrogen fixation (Figure 4.16).
Nitrogen-fixing bacteria also live in the stomachs of some animals such as the ruminants and help provide as much as half the nitrogen requirements for the animal.
When plant and animal tissue or animal wastes decompose, decomposer bacteria convert organic molecules back to usable ammonia (NH₃) or nitrate (NO₃-).
Under certain anaerobic conditions (such as occurs in wetlands), denitrifying bacteria convert nitrate (NO₃-) or nitrite (NO₂-) to free nitrogen; this process is called denitrification.
Industrial processes can be used to artificially fix nitrogen for use in commercial fertilizers.

How does the phosphorous cycle function?

Phosphorus does not have a gas phase so the cycle is somewhat simpler than the carbon or nitrogen cycles (Figure 4.17).
Phosphorus is a component of the DNA and ATP molecules.
Phosphorus cycles in its oxidized form, phosphate (PO₄), which is taken up directly by plants, algae, and some bacteria.
One source of phosphate is the waste and remains of animals and plants. Bird and bat guano contains high concentrations of phosphate. Ecosystems containing the feeding grounds, rookeries, and roosts of the animals are often very productive (e.g., ocean upwellings).
Phosphate is also found in high concentrations in sedimentary rocks containing the fossilized wastes or remains of marine plants and animals. This phosphate can be mined and used in commercial fertilizers.
Phosphate is often the most limiting nutrient in freshwater ecosystems. Artificial inputs of phosphate into waterways from agricultural, residential, and urban sources can result in eutrophication.

Ecology In Your Backyard

In many waterways of the United States, eutrophication is a water quality problem. Is eutrophication a problem in the waters in your region? If so, what evidence of eutrophication is apparent? What are the possible sources of excessive nutrients that are causing the problem?

Some state and local agencies have citizen water quality monitoring programs in which you can participate. Simple water quality test kits used to measure nutrient concentrations and other water quality parameters are usually provided to participants. Although simple field kits are not highly precise, general trends in water quality can be determined.

Please respond to these questions or send your thoughtful examples and comments to:

BackYard@wiley.com

The best responses will be posted on the Wiley Environet Website, so check the page regularly for updates to see if your email is posted!

Hardcopy Links In The Library

Gilliland, M.W. 1988. A study of the nitrogen-fixing biotechnologies for corn in Mexico. Environment 30: 3.
Kasting, J. F., O. B. Toon, and J. B. Pollack. 1988. How climate evolved on the terrestrial planets. Scientific American 258 (2): 90-97.
Pomeroy, L.R. 1974. Benchmark Papers in Ecology: Cycles of Essential Elements. Dowden, Hutchinson and Ross, Inc. Stroudsburg, PA.
Post, W.M., T. Peng, W. R. Emanual, A.W. King, V. H. Dale, and D. L. DeAngelis. 1990. The global carbon cycle. American Scientist 78:310- 326.
Schlesinger, W. H. 1992. Biogeochemistry: An Analysis of Global Change. San Diego: Academic Press.

Ecolinks On the Web

http://pubs.usgs.gov/publications/text/dynamic.html - This USGS website pertains to plate tectonics.

http://www.science.ubc.ca/~geol202/s/rock_cycle/rockcycle.html - The rock cycle is depicted and discussed.

http://www.giss.nasa.gov/Research/Intro/gornitz.02/ - This NASA website discusses the hydrologic cycle and sea-level rise.

http://hammock.ifas.ufl.edu/txt/fairs/1608 - Plant nutrients (N and P) as limiting factors are discussed.

http://www.eurekalert.org/E-lert/current/public_releases/deposit/impact.html - The impact of human activities on global nitrogen cycling is discussed.

http://www.giss.nasa.gov/Research/Intro/fung.01/ - This NASA website addresses the issue of increased atmospheric CO2 concentrations that have resulted from human activities.

http://www.giss.nasa.gov/Research/Intro/matthews.01/ - This NASA website discusses CO₂ and fossil fuel combustion.

http://wwwrvares.er.usgs.gov/nawqa/CIRC-1136.html - This is a USGS- National Water Quality Assessment Program web site pertaining to nutrient problems in the waters of the United States (see the Table of Contents for details).

http://www.gmpo.gov/gulfweb/hypoxia/boesch.html - A discussion of eutrophication in Chesapeake Bay.

http://www.grida.no/soeno95/ - See the subsection pertaining to eutrophication at this Norwegian environmental web site.

Note: If any of these links are not working, please see if alternative links are available at the Ecolink Update Site.

Ecotest Online

1. Which of the following is not a macronutrient?

a. phosphorous

b. nitrogen

c. silicon

d. sulfur

2. If a nutrient or element is present in a very low concentration or excessively high or toxic concentrations such that the survival and growth of an organism, population, or species is affected, the nutrient becomes a ______.

a. response variable

b. control parameter

c. flux factor

d. limiting factor

3. The largest reservoirs of freshwater are ______.

a. the oceans

b. lakes and streams

c. polar ice caps and glaciers

d. groundwater aquifers

4. Ecosystems that readily exchange a relatively large portion of their nutrients with other ecosystems are called ______ ecosystems.

a. leaky

b. fluctuating