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A biogeochemical cycle refers to the cycling and transport of a chemical element or compound, usually in multiple forms and physical states, through the biotic (living) and abiotic (nonliving) components of the earth system. Some of the most commonly examined biogeochemical cycles include carbon, nitrogen, oxygen, water, and phosphorous, which are highly interdependent and connected to both the physical environment and human activity. Biogeochemical processes include cycling to and from living organisms in the biosphere, rock minerals in the lithosphere, hydrological processes in the hydrosphere, and air circulation in the atmosphere, making the spatial and temporal variability of biogeochemical cycles quite complex. Biogeochemistry attempts to understand the physical processes that control and make up these cycles, as well as the natural and anthropogenically-induced variation in these cycles, including potentially harmful alterations. Humans depend upon biogeochemical cycles for, among other things, food production, water supplies, and oxygen, so the dynamics and disturbances of these processes are a major concern for environmental scientists and policymakers. Similarly, concerns related to global warming, air pollution, and biodiversity also require an understanding of biogeochemical cycles, as well as their interaction with humans and human activity across the world.
Biogeochemistry is an interdisciplinary science because it requires knowledge of living and nonliving processes that occur at various temporal and spatial scales in all components of the earth system, including the world’s oceans, forests, and urban areas. A biogeochemical cycle may include the occurrence of pools or sinks (where an element or compound is stored for longer periods) and sources (where an element or compound is freed from a sink, often in a short time and in relatively large quantities). Both human and nonhuman processes and activities (such as fire) may alter the spatial and temporal cycling of elements such as carbon and oxygen from sources and sinks, which can make it difficult to clearly distinguish “natural” and “human” perturbations of biogeochemical cycles. Furthermore, biogeochemical cycles occur and can be altered at a range scale from molecular to global, making it challenging to study entire biogeochemical cycles at one time. As a result, biogeochemistry examines past, current, and future time scales though the use of paleo-ecology, physical science, and statistical modeling.
Carbon and Nitrogen
Biogeochemistry attempts to determine the interrelated and multidirectional connections and feedback loops that make up the physical environment. Biogeochemical cycles interact with other chemicals and compounds, human and nonhuman processes, and various components of the earth’s spheres. The carbon and nitrogen cycles are provided as examples of the complex interactions that constitute biogeochemistry.
Carbon is one of the most studied elements in biogeochemistry because it is the primary element of living tissue, is essential for plant photosynthesis, and is an important greenhouse gas (as carbon dioxide and methane) in the earth’s atmosphere. Carbon cycles through plants, animals, oceans, vegetation, the atmosphere, and lithosphere, and is driven largely by photosynthesis and respiration in plants, animals, and other living organisms. This cycle has been dramatically altered through human activity, such as the burning of fossil fuels, cement production, urban development, and grazing, all of which can release carbon dioxide into the atmosphere. Higher concentrations of carbon dioxide since the industrial revolution have been shown to contribute to global warming by increasing the atmosphere’s greenhouse effect, which may raise global temperatures, cause a rise in sea level due to the melting of sea ice, alter precipitation patterns around the world, and change storm frequency and intensity.
At the same time, however, human activities that increase atmospheric CO2 have also been shown to alter the rate at which plants take up carbon through photosynthesis. Specifically, some studies have shown that higher carbon dioxide levels in the atmosphere may increase the rate at which some plants photosynthesize, offsetting some carbon dioxide emissions (in what has been called CO2 fertilization). These spatial and temporal changes, howerver, must be understood within the complete biogeochemical cycling of carbon, however, must be because the potential for short-lived increases in CO2 uptake during increased photosynthesis is unlikely to offset decades of human increases of atmospheric CO2.
Similarly, human activity has been shown to alter the nitrogen cycle through the application of fertilizers, production of power, combustion engines, and increases in human and animal waste. In an attempt to increase plant growth and photosynthesis, humans have applied fertilizers containing nitrogen and phosphorous to agricultural lands worldwide. Though productivity rates may be temporarily improved, the excess nitrogen released into the biosphere has damaged certain aquatic habitats through eutrophication, where the excessive growth of particular organisms can deplete the water of oxygen. Acid rain has also been attributed to increases in certain forms of nitrogen, which can disturb water systems and aquatic life, leach important nutrients from soils, and damage plants and buildings. The biogeochemistry of both carbon and nitrogen reveal the connections between various processes and components of the earth system, both human and nonhuman.
Biogeochemical cycles are of incredible significance to society because the science related to the biogeochemical cycling of chemicais becomes the basis for various policies, programs, and actions by individuals, states, and corporations. For example, climate policy is based upon the understanding of changes in carbon sources and sequestration, while interactions of the land and atmosphere with hydrological systems become the basis for water quality measures. Because biogeochemistry examines multiple elements and systems, it is essential to understand the spatial and temporal variability of biogeochemical cycles and the complex connections between human activity and physical responses. Isolating any one particular part of a biogeochemical cycle may not accurately characterize the complete interaction between the physical environment and human activity. Biogeochemistry works to uncover and more fully understand the connections between various elements, parts of the earth system, and biotic and abiotic processes.
- Todd S. Glickman, ed., Glossary of Meteorology, 2nd (The American Meteorological Society, 1999);
- R. Kump, J.F. Kasting, and R.G. Crane, The Earth System, 2nd ed. (Pearson, 2004);
- Stephen P. Long, Elizabeth A. Ainsworth, Andrew D. B. Leakey, Josef Nosberger, Donald R. Ort, “Food for Thought: Lower-Than-Expected Crop Yield Stimulation with Rising CO2 Concentrations,” Science (312: 5782, 2006);
- William Schlesinger, Biogeochemistry: An Analysis of Global Change (Academic Press, 1997).