READINGS:
FREEMAN
Chapter 54

• Nutrient cycling occurs at the local level through the action of the biota.
• Nutrient cycling occurs at the global level through geological processes, such as, atmospheric circulation, erosion and weathering.

• The atoms of earth and life are the same; they just find themselves in different places at different times.
• Most of the calcium in your bones came from cows, who got it from corn, which took it from rocks that were once formed in the sea.
• The path atoms take from the living (biotic) to the non-living (abiotic) world and back again is called a biogeochemical cycle .

• Of the 50 to 70 atoms
(elements) that are found in living things, only 15 or so account for the major portion of living biomass.
• Only around half of these 15 have been studied extensively as they travel through ecosystems or circulate on a global scale.
O OXYGEN
C CARBON
H HYDROGEN
N NITROGEN
Ca CALCIUM
K POTASSIUM
Si SILICON
Mg MAGNESIUM
S SULFUR
Al ALUMINUM
P PHOSPHORUS
Cl CHLORINE
Fe IRON
Mn MANGANESE
Na SODIUM

• The cycling of nutrients in an ecosystem are interlinked by an a number of processes that move atoms from and through organisms and to and from the atmosphere, soil and/or rocks, and water.
• Nutrients can flow between these compartments along a variety of pathways.

• The organic compartment consists of the living organisms and their detritus.
• The available-nutrient compartment consists of nutrients held to surface of soil particles or in solution.
• The third compartment consists of nutrients held in soils or rocks that are unavailable to living organisms.
• The fourth compartment is the air which can be found in the atmosphere or in the ground.

• With the exception of CO
2 and O which enter though
2 leaves, the main path of all other nutrients is from the soil through the roots of producers.
• Even consumers which find
Ca, P, S and other elements in the water they drink, obtain the majority of these nutrients either directly or indirectly from producers.

• The atmosphere acts as a reservoir for carbon dioxide
(CO
2
), oxygen (O water (H
2
O).
2
) and
• These inorganic compounds can be exchanged directly with the biota through the processes of photosynthesis and respiration.
• The most abundant gas in the atmosphere is nitrogen
(N
2
);about 80% by volume.
Its entry into and exit from the biota is through bacteria.

• Cycling within an ecosystem involves a number of processes.
• These are best considered by focusing attention on specific nutrients.

• C, H & O basic elements of life; making up from about 98% of plant biomass.
• CO
2 and O
2 enter biota from the atmosphere.
• Producers convert CO
2 carbohydrates (C H
2 release O
2 from water.
and H
2
O into
O compounds) and
• Producers, consumers and decomposers convert C H into CO
2
2
O compounds, using O
2 and H
2
O .
, back

CARBON, HYDROGEN AND OXYGEN
CYCLES IN ECOSYSTEMS
• Carbon and oxygen cycle come out of the air as carbon dioxide during photosynthesis and are returned during respiration.
• Oxygen is produced from water during photosynthesis and combines with the hydrogen to form water during respiration.

• Phosphorus, as phosphate (PO
4
-3 ), is an essential element of life.
• It does not cycle through atmosphere, thus enters producers through the soil and is cycled locally through producers, consumers and decomposers.
• Generally, small local losses by leaching are balanced by gains from the weathering of rocks.
• Over very long time periods
(geological time) phosphorus follows a sedimentary cycle.

• Nitrogen (N
2
) makes up
78% of the atmosphere.
• Most living things, however, can not use atmospheric nitrogen to make aminoacids and other nitrogen containing compounds.
• They are dependent on nitrogen fixing bacteria to convert N
2 into NH
3
(NH
4
+ ).

• Natural ecosystems receive their soil nitrogen through biological fixation and atmospheric deposition.
• Agricultural ecosystems receive additional nitrogen through fertilizer addition.

• Only a few species of bacteria and cyanobacteria are capable of nitrogen fixation.
• Some are fee-living and others form mutualistic associations with plants.
• A few are lichens.

• Lightning was the major source of soil nitrogen until recent times when the burning of fossil fuels became a major source of atmospheric deposition.
• Nitrogen oxides come from a variety of combustion sources that use fossil fuels. In urban areas, at least half of these pollutants come cars and other vehicles.

• Various forms of commercial fertilizer are added to agricultural fields to supplement the nitrogen lost through plant harvest.
• Crop rotation with legumes such as soybeans or alfalfa is also practiced to supplement soil nitrogen.

• Nitrogen fixation is the largest source of soil nitrogen in natural ecosystems.
• Free-living soil bacteria and cyanobacteria (blue-green
“algae”) are capable of converting N
2 into ammonia
(NH
3
) and ammonium (NH
4
+ ).
• Symbiotic bacteria (Rhizobium ) in the nodules of legumes and certain other plants can also fix nitrogen.
QuickTime™ and a
TIFF (Uncompressed) decompressor are needed to see this picture.

• Several species of bacteria can convert ammonium (NH
4
+ ) into nitrites (NO
2
).
• Other bacterial species convert nitrites (NO
2
) to nitrates (NO
3
).

• Plants can take in either ammonium (NH
4
+ ) or nitrates (NO
3
) and make amino acids or nucleic acids.
• These molecules are the building blocks of proteins and DNA, RNA,
ATP, NADP, respectively.
• These building blocks of life are passed on to other trophic levels through consumption and decomposition.

• Decomposers convert organic nitrogen
(CHON) into ammonia
(NH
3
(NH
4
) and ammonium
+ ).
• A large number of species of bacteria and fungi are capable of converting organic molecules into ammonia.

• A broad range of bacterial species can convert nitrites, nitrates and nitrous oxides into molecular nitrogen (N
2
).
• They do this under anaerobic conditions as a means of obtaining oxygen (O
2
).
• Thus, the recycling of N is complete.

• Molecular nitrogen in the air can be fixed into ammonia by a few species of prokaryotes.
• Other bacterial species convert
NH
4
into NO
2
and others to N0
3
.
• Producers can take up NH
4
to N0
3
use it to make CHON.
and
• Decomposers use CHON and produce NH
4
.
• Recycling is complete when still other species convert N0
3
-
NO
2
into N
2
.
and

• The role of vegetation in nutrient cycles is clearly seen in clear cut experiments at
Hubbard Brook.
• When all vegetation was cut from a 38-acre watershed, the output of water and loss of nutrients increased; 60 fold for nitrates, and at least 10 fold for other nutrients.
• Freeman describes the experiments on page 1254 and in Figure 54.15.

NUTRIENT LOSS IN ECOSYSTEMS II

NUTRIENT LOSS IN ECOSYSTEMS III

• The loss of nutrients from one ecosystem means a gain for another. (Remember the law of conservation of matter.)
• When ecosystems become linked in this manor, attention shifts to a global scale. One is now considering the
ECOSPHERE or the whole of planet earth.

• Water is the solvent in which all the chemistry of life takes place and the source of its hydrogen.
• The earth’s oceans, ice caps, glaciers, lakes, rivers, soils and atmosphere contains about 1.5 billion cubic kilometers of H
2
O.
• It has been estimated that all the earth’s water is split by plant cells and reconstituted by the biota about every
2,000,000 years .

• Oceans contain a little less than 98% of the earth’s water.
• Around 1.8% is ice; found in the two polar ice caps and mountain glaciers.
• Only 0.5% is found in the water table and ground water.
• The atmosphere contains only 0.001% of the earth’s water, but is the major driver of weather.

• The rate at which water cycles is shown in Figure
54.16 (Freeman, 2005).
• Evaporation exceeds precipitation over the oceans; thus there is a net movement of water to the land.
• Nearly 60% of the precipitation that falls on land is either evaporated or transpired by plants; the remainder is runoff and ground water.

• All but a small portion of the earth’s carbon (C) is tied up in sedimentary rocks; but the portion that circulates is what sustains life.
• The active pool of carbon is estimated to be around
40,000 gigatons.
• 93.2 % found in the ocean;
3.7% in soils; 1.7% in atmosphere; 1.4% in vegetation.

• The rate at which the biota exchanges CO
2 with atmosphere has been estimated to be every 300 years.
• The rate at which carbon cycles through various components of the ecosphere is summarized in Figure 54.17 in Freeman
(2005).
• Since the industrial revolution, a new source of stored sedimentary carbon has been added to the atmosphere from the burning of fossil fuels causing a concern with respect to climate change.

• 99.4% of exchangeable N is found in the atmosphere; 0.5% is dissolved in the ocean;
0.04% in detritus ; 0.006% as inorganic N sources; 0.0004% in living biota.
• Figure 54.19 in Freeman
(2005) gives major pathways and rates of exchange.

• Humans are adding large amounts of N to ecosystems. Some estimates of are given in
Figure 54.20 in Freeman (2005).
• Among the fossil fuel sources, power plants and automobiles are important sources of atmospheric nitrogen deposition in the US.
• Investigations of native plant and natural ecosystem responses to nitrogen deposition and global warming will be a focus of study.

READINGS:
FREEMAN
Chapter 54