Download Lecture 11: Nutrient Cycles

NUTRIENT CYCLES
READINGS:
FREEMAN
Chapter 54
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
on a global scale.
O OXYGEN
K POTASSIUM
P PHOSPHORUS
C CARBON
Si SILICON
Cl CHLORINE
H HYDROGEN
Mg MAGNESIUM
Fe IRON
N NITROGEN
S SULFUR
Mn MANGANESE
Ca CALCIUM
Al ALUMINUM
Na SODIUM
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 found in the atmosphere or in the ground.
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
the biota is through bacteria.
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.
• CO2 and O2 enter biota from the atmosphere.
• Producers convert CO2 and H2O into
carbohydrates (CH2O compounds) and
release O2 from water.
• Producers, consumers and decomposers
convert CH2O compounds, using O2, back
into CO2 and H2O.
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 aminoacids 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
nitrogen.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Nitrification
• Several species of
bacteria can convert
ammonium (NH4+)
into nitrites (NO2-).
• Other bacterial
species convert
nitrites (NO2-) to
nitrates (NO3-).
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
is complete.
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
in Figure 54.15.
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
2,000,000 years.
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
ground water.
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.
GLOBAL CARBON CYCLE II
• The rate at which the biota exchanges CO2
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.
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.
GLOBAL NITROGEN CYCLE II
• 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.
GLOBAL NITROGEN CYCLE III
NUTRIENT CYCLES
READINGS:
FREEMAN
Chapter 54