Biogeochemical Cycles

 Biogeochemical cycles refer to the movement of chemical elements between living (biotic) and nonliving (abiotic) forms in the environment. Although many elements undergo this type of cycling to some extent, four elements— carbon, nitrogen, phosphorus, and sulfur—are most commonly discussed because of their importance (along with hydrogen and oxygen) for living or­ganisms. The extent and rate of the cycling of these elements has impor­tant consequences, such as influencing the amount of phosphate available to forests and the ability of the oceans to slow down global warming by ab­sorbing carbon dioxide.

Common Compounds

All the elements that undergo cycling are incorporated into compounds. Carbon may be found as inorganic CO₂ gas, carbonate ions (CO₃^2-) in rocks or the oceans, or in organic compounds, such as sugars and proteins, within living organisms. Nitrogen exists in the atmosphere as N₂ or ammonia (NH₃), in the soil as an ion such as nitrate (NO₃^-), and in living organisms in a variety of organic compounds, including proteins and nucleic acids. Wherever it occurs, phosphorus is largely bound to oxygen to make a phos­phate ion (PO₄^3-). Sulfur exists as sulfur dioxide gas (SO₂), sulfate ions (SO₄^2-) in rocks, and in living organisms incorporated into proteins.

The atmosphere, oceans, fresh water, rocks, soil, and living organisms can each be thought of as a “pool” for storing these compounds. The time spent in any one pool is quantified as the mean residence time (MRT). For instance, the MRT for phosphate in rock may be thousands of years, whereas the MRT for the phosphate in a stand of corn is less than one year.

Transport Mechanisms

Elements move from one pool to another through meteorological, geologic, biological, or anthropogenic mechanisms. Meteorological mechanisms re­volve around precipitation, such as rain carrying SO₂ into the soil. Geologic mechanisms include erosion, which can bring rock ions into solution, as well as sedimentation and volcanoes.

Biological mechanisms are those carried on by living organisms, such as photosynthetic conversion of CO₂ to sugar, or conversion of soil NH₃ to gaseous N₂ by soil bacteria. Marine birds can have a significant local impact on transport of phosphate and nitrogen from ocean to land. Many islands off the western coast of South America, for instance, are covered with a layer of white guano, dropped by generations of birds feasting on anchovies. Harvest of this rich fertilizer forms part of the economies of Peru, Chile, and Ecuador.

Soil erosion on a trail in the Adirondack mountains. Erosion is a geologic mechanism that helps to move chemical compounds between biotic and abiotic forms in the environment.

Anthropogenic mechanisms are those carried on by humans and are therefore a subset of biological mechanisms. Humans have a profound ef­fect on biogeochemical cycles through agriculture (for example, adding ni­trogen to the global nitrogen cycle through fertilizer applications), forestry, and especially the use of carbon-based fossil fuels. The release of vast amounts of carbon from stored pools is likely to raise the world’s tempera­ture by at least several degrees over the coming decades, with the potential for significant consequences on many forms of life. An important, yet unan­swered, question is whether the forests, soil, and especially the ocean can absorb this extra CO₂ and thereby reduce the extent of global warming.

References

Berner Elizabeth Kay, and Robert A. Berner. Global Environment: Water, Air, and Geo­chemical Cycles. Upper Saddle River, NJ: Prentice Hall, 1996.

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