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Life and Biogeochemical Cycles
Life on earth is inextricably linked to climate through a variety of interacting
cycles and feedback loops. In recent years there has been a growing awareness
of the extent to which human activities, such as deforestation and fossil fuel
burning, have directly or indirectly modified the biogeochemical and physical
processes involved in determining the earth's climate. These changes in
atmospheric processes can disturb a variety of the ecosystem services that
humanity depends upon. In addition to helping to maintain relative climate
stability and a self-cleansing, oxidizing environment, these services include
protection from most of the sun's harmful ultraviolet rays, mediation of runoff
and evapotranspiration (which affects the quantity and quality of fresh water
supplies and helps control floods and droughts), and regulation of nutrient
cycling among others.
Before further discussion of these services, it is important to review briefly how
life and climate interact. The transport and transformation of substances in the
environment, through life, air, sea, land, and ice, are known collectively as
biogeochemical cycles. These global cycles include the circulation of certain
elements, or nutrients, upon which life and the earth's climate depend. One way
that climate influences life is by regulating the flow of substances through these
biogeochemical cycles, in part through atmospheric circulation. Water vapor is
one such substance. It is critical for the survival and health of human beings
and ecological systems and is part of the climatic state. When water vapor
condenses to form clouds, more of the sun's rays are reflected back into the
atmosphere, usually cooling the climate. Conversely, water vapor is also an
important greenhouse gas in the atmosphere, trapping heat in the infrared part
of the spectrum in the lower atmosphere. The water or hydrologic cycle
intersect with most of the other element cycles, including the cycles of carbon,
nitrogen, sulfur, and phosphorus, as well as the sedimentary cycle. The
processes involving each one of these elements may be strongly coupled with
that of other elements, and ultimately, with important regional and global scale
climatic or ecological processes.
Managing and finding solutions to many of the important environmental
problems facing humanity begins with understanding and integrating
biogeochemical cycles and the scales at which they operate. Examples of these
links include world climate and the potential threat of global climate change;
agricultural productivity and its strong reliance on climatic factors including
temperature and precipitation, and on the availability of nutrients; the cleansing
of toxics in soils and streams through precipitation and runoff; acid
precipitation and the perturbation of ecosystem processes; the depletion of
stratospheric ozone and its potential threat to human health and the food chain;
and the often destructive interaction with natural cycles of other manmade
compounds such as pesticides and synthetic hormones.
The Hydrologic and Sedimentary Cycles
While the total amount of water found on earth may seem huge, the amount of
precipitating freshwater available to people is a tiny fraction of this total.
Earth's renewable supply of water is continually distilled and distributed
through the hydrological cycle. It falls from the sky as precipitation, collects in
lakes, rivers and oceans or seeps into the ground, and eventually evaporates or
transpires, accumulating as water vapor in clouds, ready to begin the sun
powered cycle again. Water is transferred to the air from the leaves of plants
primarily from a process called transpiration. This, combined with evaporation
from bodies of water and the soil, is known as evapotranspiration. Evaporation
of ocean water is about six times as large globally as evapotranspiration on
land, although in the centers of continents evapotranspiration may be the main
local source of water vapor. Changes in the global climate may cause changes
in the hydrologic cycle. Increases in temperature and evaporation are expected
to cause increases in precipitation, which may further affect runoff and soil
moisture, and eventually influence vegetation patterns and world agriculture.
The sedimentary cycle is tied to the hydrological cycle through precipitation.
Water carries materials from the land to the oceans, where they can be
deposited as sediments. On a shorter time-scale, the sedimentary cycle includes
the processes of physical or chemical erosion, nutrient transport, and sediment
formation for which water flows are mostly responsible. On a geologically
longer time-scale, the processes of sedimentation, chemical transformation,
uplift, sea floor spreading, and continental drift operate. Both the hydrological
and sedimentary cycles are intertwined with the distribution of the amounts and
flows of six important elements - hydrogen, carbon, oxygen, nitrogen,
phosphorus, and sulfur. These elements, or macronutrients, combine in various
ways to make up more than 95% of all living things. Appropriate quantities of
them in proper balance and in the right places are required to sustain life.
Although great stocks of all of these nutrients exist in the earth's crust in
different (but not always accessible) forms, at any one time the natural supply
of these vital elements is limited. Therefore, they must be recycled for life to
regenerate continuously. We describe three of these cycles critical to important
ecosystem services in the following sections.
The Nitrogen Cycle
Nitrogen exist in a variety of forms in natural systems and its compounds are
involved in numerous biological and abiotic processes. Nitrogen, in its gaseous
form of N2, makes up almost 80 percent of the atmosphere. This constitutes the
major storage pool in the complex cycle of nitrogen through ecosystems. Some
of this gas is converted in the soils and waters to ammonia (NH3), ammonium
(NH4+), or many other nitrogen compounds. The process is known as nitrogen
fixation, and, in the absence of industrial fertilizers, is the primary source of
nitrogen to all living things. Biological nitrogen fixation is mediated by special
nitrogen-fixing bacteria and algae. On the land, these bacteria often live on
nodules on the roots of legumes where they use energy from plants to do their
work. In freshwater and, possibly, in marine systems, cyanobacteria fix
nitrogen. Once nitrogen has been fixed in the soil or aquatic system, it can
follow two different pathways. It can be oxidized for energy in a process called
nitrification or assimilated by an organism into its biomass in a process called
ammonia assimilation.
Plants incorporate the appropriate forms of fixed nitrogen into their tissues
through their root systems. The plants then use it to manufacture amino acids
and convert it into proteins. Nitrogen, fixed as proteins in the bodies of living
organisms, eventually returns via the nitrogen cycle to its original form of
nitrogen gas in the air. The process of denitrification starts when plants
containing the fixed nitrogen are either eaten or die. Fixed nitrogen products in
dead plants, animal bodies, and animal excreta encounter denitrifying bacteria
that undo the work done by the nitrogen-fixing bacteria. Generally, N2 is the
end-product of denitrification, but nitrous oxide (N2O) is also produced in
much smaller quantities (up to ten percent).
The disruption of the nitrogen cycle by human activity plays an important role
in a wide-range of environmental problems ranging from the production of
tropospheric (lower atmosphere) smog to the perturbation of stratospheric
ozone and the contamination of groundwater. Nitrous oxide, for example, is a
greenhouse gas like carbon dioxide and water vapor that can trap heat near the
earth's surface. It also destroys stratospheric ozone. Eventually nitrous oxide in
the stratosphere is broken down by ultraviolet light into nitrogen dioxide (NO2)
and nitric oxide (NO), which can catalytically reduce ozone. Nitrogen oxides
are chemically transformed back to either N2 or to nitrate or nitrite compounds,
which may later get used by plants after they are washed by the rain back to the
earth's surface. Nitrate rain is acidic and can cause ecological problems as well
as serve as a fertilizer to vegetation.
The Sulfur Cycle
Another example of a major biogeochemical cycle of significance to climate
and life is the sulfur cycle. Living things require certain safe, low levels of this
nutrient. The sulfur cycle can be thought of as beginning with the gas sulfur
dioxide (SO2) or the particles of sulfate (SO4=) compounds in the air. These
compounds either fall out or are rained out of the atmosphere. Plants take up
some forms of these compounds and incorporate them into their tissues. Then,
as with nitrogen, these organic sulfur compounds are returned to the land or
water after the plants die or are consumed by animals. Bacteria are important
here as well since they can transform the organic sulfur to hydrogen sulfide gas
(H2S). In the oceans, certain phytoplankton can produce a chemical that
transforms to SO2 that resides in the atmosphere. These gases can re-enter the
atmosphere, water, and soil, and continue the cycle.
In its reduced oxidation state, the nutrient sulfur plays an important part in the
structure and function of proteins. In its fully oxidized state, sulfur exists as
sulfate and is the major cause of acidity in both natural and polluted rainwater.
This link to acidity makes sulfur important to geochemical, atmospheric, and
biological processes such as the natural weathering of rocks, acid precipitation,
and rates of denitrification. Sulfur is also one of the main elemental cycles most
heavily perturbed by human activity. Estimates suggest that emissions of sulfur
to the atmosphere from human activity are at least equal or probably larger in
magnitude than those from natural processes. Like nitrogen, sulfur can exist in
many forms: as gases or sulfuric acid particles. Sulfuric acid particles
contribute to the polluting smog that engulfs some industrial centers and cities
where many sulfur containing fuels are burned. Such particles floating in air
(known as sulfate aerosols) can cause respiratory diseases or cool the climate
by reflecting some extra sunlight to space.
The lifetime of most sulfur compounds in the air is relatively short (e.g. days).
Superimposed on these fast cycles of sulfur are the extremely slow
sedimentary-cycle processes or erosion, sedimentation, and uplift of rocks
containing sulfur. In addition, sulfur compounds from volcanoes are
intermittently injected into the atmosphere, and a continual stream of these
compounds is produced from industrial activities. These compounds mix with
water vapor and form sulfuric acid smog. In addition to contributing to acid
rain, the sulfuric acid droplets of smog form a haze layer that reflects solar
radiation and can cause a cooling of the earth's surface. While many questions
remain concerning specifics, the sulfur cycle in general, and acid rain and smog
issues in particular are becoming major physical, biological, and social
problems.
The Carbon Cycle
Carbon, the key element of all life on earth, has a
complicated biogeochemical cycle of great
importance to global climate change. The carbon
cycle includes four main reservoirs of stored
carbon: as CO2 in the atmosphere; as organic
compounds in living or recently dead organisms; as
dissolved carbon dioxide in the oceans and other
bodies of water; and as calcium carbonate in
limestone and in buried organic matter (e.g. natural
gas, peat, coal, and petroleum). Ultimately, the
cycling of carbon through each of these reservoirs
is tightly tied to living organisms.
Plants continuously extract carbon from the
atmosphere and use it to form carbohydrates
and sugars to build up their tissues through the
process of photosynthesis. Animals consume
plants and use these organic compounds in
their metabolism. When plants and animals
die, CO2 is formed again as the organic
compounds combine with oxygen during
decay. Not all of the compounds are oxidized,
however, and a small fraction is transported and redeposited as sediment and
trapped where it can form deposits of coal and petroleum. Carbon dioxide from
the atmosphere also dissolves in oceans and other bodies of water. Aquatic
plants use it for photosynthesis and many aquatic animals use it to make shells
of calcium carbonate (CaCO3). The shells of dead organisms (e.g.
phytoplankton or coral reefs) accumulate on the sea floor and can form
limestone that is part of the sedimentary cycle. The relevant time-scales for
these different processes vary over many orders of magnitude, from millions of
years for the rock cycle and plate tectonics to days and even seconds for
processes like photosynthesis and air-sea exchange.
CO2 is a trace gas in the earth's atmosphere that has a substantial effect on
earth's heat balance by absorbing infrared radiation. This gas, like water vapor
(H2O), CH4, and N2O, has a strong greenhouse effect. Life can alter the global
concentration of CO2 over very short time periods. During the growing season,
CO2 decreases in the atmosphere of the temperate latitudes due to the
increasing sunlight and temperatures which help plants to increase their rate of
carbon uptake and growth. During the winter dormant period, more CO2 enters
the atmosphere than is removed by plants, and the concentration rises because
plant respiration and the decay of dying plants and animals occurs faster than
photosynthesis. The land mass in the northern hemisphere is greater than in the
southern hemisphere, thus the global concentration of CO2 tracks the
seasonality of terrestrial vegetation in the northern hemisphere.