BSC 314 Lecture Notes - Lecture 66: Biotic Component, Water Cycle, Hydrosphere
28 Jun 2018
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Materials Used by Organisms Are Recycled
The Earth is essentially a closed chemical system through which the elements
necessary for life are reused and move from abiotic reservoirs to the biota and back in
global biogeochemical cycles. Some elements are held as gases in the atmosphere,
others are components of the lithosphere (rocks and soil of the Earth's crust), many
move through the hydrosphere (marine and freshwaters) before or after their sojourn in
the biosphere (the living components).
The cycles through the lithosphere are said to be sedimentarycycles (from the
sedimentary rocks in which the elements reside) and are of such long duration that the
elements are essentially removed from further cycling until tectonic (mountain building)
or volcanic eruptions expose the rock layers to new weathering. Elements have shorter
residence times in the air in the atmospheric cycles and generally the least of all in the
biota. A surprisingly small amount of the world's matter is held in living organisms at any
time; the reservoirs for the elements of concern to life are almost entirely abiotic ones.
Plants are more than merely users of the chemicals of the Earth; through their metabolic
processes they exert a considerable influence on the cycling of the major chemicals.
Plants have been indispensable through deep (geologic) time in maintaining the
steady state condition of most of the biogeochemical cycles. All of the ecologically ‐
significant chemical elements have both an abiotic and a biotic component. Carbon,
hydrogen, and oxygen enter plants from the air and from the decomposition of organic
matter, but the other 14 essential nutrients are taken from the soil, as are the
miscellaneous other elements used in small amounts by a variety of organisms.
Nutrients released from weathered rocks enter the soil solution and move
by diffusion and mass flow to the sites of biological activity. Rock weathering is a
long term process that adds small quantities of minerals slowly, over time to the ‐
ecosystem. Plants and other organisms, therefore, obtain most of the minerals they
need by recycling existing organic matter.
The Water Hydrologic Cycle
All life depends on water and in its absence life ceases. The kind of vegetation present
at a site depends upon the amount of free water available and a principal factor in
terrestrial net primary production is the amount of precipitation a site receives. The
movement of water on the face of the Earth affects the rate of weathering, the amount
of materials carried to the seas and lost to the bottom sediments, the erosion of soils, as
well as the global heat balance and rainfall patterns from pole to pole.
Water is the carrier of the elements, and all of the biogeochemical cycles include
portions of time in the hydrosphere. Water itself cycles and would do so in the absence
of organisms (unlike the other major elements, which require organisms in their cycles).
Oceans, which cover over three quarters of the Earth's surface, are the reservoir for ‐
water. Evaporation from their surfaces amounts to 425,000 km 3 per year. About 90
percent of the water returns to the oceans as precipitation, and the remaining 10
percent falls as precipitation on the land. Transpiration (the loss of water from plants)
and evaporation from the soil, together called evapotranspiration, add 71,000 km 3 to
the atmosphere yearly. The amount of water in the atmospheric reservoir is small,
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Document Summary
The earth is essentially a closed chemical system through which the elements necessary for life are reused and move from abiotic reservoirs to the biota and back in global biogeochemical cycles. Elements have shorter residence times in the air in the atmospheric cycles and generally the least of all in the biota. A surprisingly small amount of the world"s matter is held in living organisms at any time; the reservoirs for the elements of concern to life are almost entirely abiotic ones. Plants are more than merely users of the chemicals of the earth; through their metabolic processes they exert a considerable influence on the cycling of the major chemicals. Plants have been indispensable through deep (geologic) time in maintaining the steady state condition of most of the biogeochemical cycles. All of the ecologically significant chemical elements have both an abiotic and a biotic component.
