Biogeochemical Cycles: Along with the circulation of the atmosphere and oceans, there are energy and mass circulating through and interacting with the various life subsystems of Earth. The corporate name for these translocations is biogeochemical cycles.
The term is apt in that it connotes the interplay of life and its chemical environment on, over, and under Earth’s landscapes. In toto, the magnitudes of these flows are unimaginably vast and complicated but their components have been studied to the extent that the basics are well known. The cycles provide needed supplies of mass energy because these commodities are finite on our planet. In short, the biogeochemical cycles bathe the planet in ways that provide for sustained life. Although it can be shown that humans have had major, unintentional impacts on some of these cycles, it is evident that there is much resilience in the way they function.
The energy cycle is usually the first to be noted because it is the fuel by which all the other cycles act. A huge amount of solar energy is incident on the Earth system. Instantaneously, the amount is about 174 petawatts (1015 watts), which is billions of times the rate of electric energy generated by human devices. As solar energy passes through the atmosphere there is all manner of pathways as described elsewhere.
The key energy for life is supplied by photosynthesis. Although incredibly complex with many nuances yet to be understood, photosynthesis is responsible for energizing the large bulk of biomass on the planet and, ultimately, is the source of energy on which the human organism depends. Photosynthesis occurs in plants and uses carbon dioxide and water commonly available in the environment. Not all solar energy received is used. Perhaps 0.6 percent of solar energy incident on Earth’s surface interacts with the above ingredients in plant cells through the auspices of the green pigment chlorophyll. The radiant solar energy is converted into a chemical form as a series of carbohydrates. This storage allows plants a steady supply of energy for respiration, the processes that keep the plant alive. Of incredible importance to our planet is the oxygen released as a byproduct; the bulk of our atmospheric oxygen supply was produced by photosynthesis.
Plants manufacturing carbohydrates from solar energy are known as primary producers. Once fixed into chemical form, solar energy is available for further use by other organisms. Some of the primary producers are consumed by animals that use carbohydrates to sustain their life processes and these animals are known as primary consumers. Of course, some animals eat other animals thereby gaining the energy the primary consumers gained from plants; in this case, the animals are known as secondary consumers. Plants and animals are intertwined by energy and mass pathways known as food chains. This is, perhaps, oversimplification in that most ecosystems are complex webs of recycled energy and matter. The relationships do not represent perfect usage of energy. At every step, about 90 percent of the energy is lost. Ironically, any top predator like an eagle or a shark is several steps away from the capture of solar energy by plants and is dependent on the existence of huge amounts of biomass to survive. Humans have survived because of their ability to either eat plants directly or eat animals that have eaten plants.
Geographers are interested in global net primary productivity (NPP). It is the measure of photosynthesis minus respiration and is commonly given as kilograms of carbon per square meter per year. NPP is geographically varied by climate and landscape environments. Tropical rainforests have the greatest net primary productivities, on the order of 2.2 kg/m2 /yr, whereas tundra manages only 0.14 kg/m2 /yr. Although some upwelling areas of the oceans are quite productive, there are large stretches that are unproductive. For example, the stable waters in the subtropical highs might have NPPs of .002 kg/m2 /yr. Moreover, these numbers are only for the top waters into which solar energy can penetrate.
The hydrologic cycle is the most massive of the cycles on the planet with about 400,000 cubic kilometers leaving the oceans each year. Water is a part of all Earth’s life so its circulation around the planet is of major importance to the sustenance of life.
Oxygen is plentiful in the atmosphere and is well known for its use by the animal life on our planet. Like other components of our atmosphere, oxygen has evolved over time. In Earth’s earliest atmospheres, the two- and three-atom oxygen so common now could not exist in quantity because of the propensity of unmitigated solar energy to break apart these molecules.
Two things happened to increase the oxygen supply.
- (1) The formation of the ozone (O3) layer that screens most of the ultraviolet energy from the lower atmosphere, and
- (2) The buildup of breathable oxygen (O2) as a chemical byproduct of photosynthesis that was allowed once the ozone layer was established. Additionally, oxygen enters the atmosphere as part of water vapor during evapotranspiration and via the weathering of rocks (oxygen is the most prominent component of rock at the planetary surface). Oxygen leaves the atmosphere through the precipitation of water and inhalation by animals. At present, Earth’s atmospheric oxygen supply is stable and there are no worries that it will become scarce in the foreseeable future.
The nitrogen cycle is interesting in the way it provides useful nitrogen to plants and animals. Nitrogen forms the bulk of the atmospheric gases and is, yet, not usually useful to life until it is fixed into one of a series of compounds called nitrates. Plants can directly take up these compounds to supply life processes. Only a tiny fraction of gaseous nitrogen is fixed. On land, most nitrates are produced by bacteria in the root nodules of certain plants and in the soil itself. Exotically, cosmic rays and lightning both produce small amounts of nitrates. Also, minor amounts are fixed by marine life. Fixed nitrogen can return to the atmosphere by denitrification caused by bacteria. Humans manufacture and concentrate nitrates as fertilizer for agricultural purposes. Runoff from fields and other fertilized surfaces has had unintentional, deleterious effects on life in streams and small water bodies because increased nitrate concentrations decrease dissolved oxygen.
The carbon cycle is significant to live and has had major interference from human activities. Carbon, hydrogen, and oxygen atoms in combination are building blocks for myriad organic molecules. Carbon enters plants via photosynthesis and animals via the consumption of plants. Carbon-based carbohydrates include chemical energy derived from solar radiation. As animals consume plants this carbon is passed through the web of life. Animals exhale this carbon dioxide. Organism death and decay result in the release of carbon into the atmosphere. Inorganic sources such as the outgassing of volcanoes and the weathering of certain rocks also provide carbon back to the atmosphere. Every burning process, natural or human-related, combines oxygen and carbon that becomes carbon dioxide.
Of great concern is the agricultural, industrial, and transportation processes now releasing carbon dioxide into the atmosphere in amounts that surpass natural sources. Since the start of the Industrial Revolution in the middle 1800s, the carbon dioxide content of the atmosphere has risen by over a third. Although intake by photosynthesis and solution into oceans buffer the planetary atmosphere from containing all of this carbon dioxide, it is unknown how much capacity these “sinks” possess. Such an increase in carbon dioxide is not toxic nor will it decrease breathable oxygen in the atmosphere but carbon dioxide is one of the primary greenhouse gases such that humans may be causing unintended impacts on Earth’s energy budget.
In a short article about the cycles, it is impossible to convey a full sense of how interconnected they are. A change in one cycle can markedly affect the functioning of another. There are a number of other biogeochemical cycles that have not been mentioned here (for example, calcium, zinc, sulfur, and phosphorus). Although vital at various times and places, they do not have the overall volumes and energy contents of the major cycles illustrated above; life can be threatened if these other cycles are somehow interrupted.