NUTRIENT CYCLES
READINGS:
FREEMAN, 2005 Chapter 54
Pages 1252-1259
NUTRIENT CYCLES:
ECOSYSTEM TO ECOSPHERE
• 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.
NUTRIENT CYCLES
• 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.
Nutrients: The Elements of Life
• 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
Na SODIUM Mn MANGANESE Fe IRON
Cl CHLORINE P PHOSPHORUS
Al ALUMINUM S SULFUR Mg MAGNESIUM Si SILICON K POTASSIUM
Ca CALCIUM N NITROGEN H HYDROGEN C CARBON O OXYGEN
A GENERALIZED MODEL OF NUTRIENT CYCLING IN AN
ECOSYSTEM
• 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.
Nutrient Compartments in a Terrestrial Ecosystem
• 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
Uptake of Inorganic Nutrients from the Soil
• With the exception of CO2 and O2 which enter though 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 Is a Source of Inorganic Nutrients
• The atmosphere acts as a reservoir for carbon dioxide (CO2), oxygen (O2) and water (H2O).
• 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 (N2);about 80% by volume.
Its entry into and exit from
Some Processes By Which Nutrients Are Recycled
• Cycling within an ecosystem involves a number of
processes.
• These are best considered by
focusing attention
on specific nutrients.
CARBON, HYDROGEN AND OXYGEN CYCLES IN
ECOSYSTEMS
• C, H & O basic elements of life; making up from about 98% of plant biomass.
• CO
2and O
2enter biota from the atmosphere.
• Producers convert CO
2and H
2O into carbohydrates (CH
2O compounds) and release O
2from water.
• Producers, consumers and decomposers
convert CH
2O compounds, using O
2, back
into CO and H O.
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.
PHOSPHOROUS CYCLE IN ECOSYSTEMS
• Phosphorus, as phosphate (PO4-3), 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 CYCLE IN ECOSYSTEMS
• Nitrogen (N2) makes up 78% of the atmosphere.
• Most living things, however, can not use atmospheric nitrogen to make amino- acids and other nitrogen containing compounds.
• They are dependent on nitrogen fixing bacteria to convert N2 into NH3(NH4+).
Sources of Nitrogen to the Soil
• Natural ecosystems receive their soil
nitrogen through
biological fixation and atmospheric deposition.
• Agricultural ecosystems receive additional
nitrogen through fertilizer addition.
Biological Sources of Soil Nitrogen
• 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.
Atmospheric Sources of Soil Nitrogen
• 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.
Agricultural Supplements to Soil Nitrogen
• 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.
Biological Nitrogen Fixation
• 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 N2 into ammonia (NH3) and ammonium (NH4+).
• Symbiotic bacteria (Rhizobium) in the nodules of legumes and certain other plants can also fix
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Nitrification
• Several species of bacteria can convert ammonium (NH
4+) into nitrites (NO
2-).
• Other bacterial
species convert
nitrites (NO
2-) to
nitrates (NO
3-).
Uptake of Nitrogen by Plants
• Plants can take in either
ammonium (NH4+) or nitrates (NO3-) 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.
Ammonification
• Decomposers convert organic nitrogen
(CHON) into ammonia (NH3) and ammonium (NH4+).
• A large number of
species of bacteria and fungi are capable of
converting organic molecules into
ammonia.
Denitrification
• A broad range of
bacterial species can convert nitrites, nitrates and nitrous oxides into molecular nitrogen (N2).
• They do this under
anaerobic conditions as a means of obtaining oxygen (O2).
• Thus, the recycling of N
NITROGEN CYCLE IN ECOSYSTEMS
• Molecular nitrogen in the air can be fixed into ammonia by a few species of prokaryotes.
• Other bacterial species convert NH4- into NO2- and others to N03-.
• Producers can take up NH4- and to N03- use it to make CHON.
• Decomposers use CHON and produce NH4-.
• Recycling is complete when still other species convert N03- and NO2- into N2.
NUTRIENT LOSS IN ECOSYSTEMS I
• 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
NUTRIENT LOSS IN ECOSYSTEMS II
NUTRIENT LOSS IN ECOSYSTEMS III
GLOBAL NUTRIENT CYCLES
• 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.
GLOBAL WATER CYCLE I
• 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 H2O.
• It has been estimated that all the earth’s water is split by plant cells and reconstituted by the biota about every
GLOBAL WATER CYCLE II
• 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.
GLOBAL WATER CYCLE III
• 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
GLOBAL WATER CYCLE IV
GLOBAL CARBON CYCLE I
• 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
2with 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.
GLOBAL CARBON CYCLE II
GLOBAL CARBON CYCLE III
GLOBAL NITROGEN CYCLE I
• 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.
