Transcript Document

Nutrient Cycles
Matter Cycling in Ecosystems
• Nutrient – any atom, ion, or molecule an
organism needs to live, grow, or
reproduce
– Some (such as C, O, H, N, P, S, and Ca) are
needed in fairly large amounts
– Some (such as Na, Zn, Cu, and I) are only
needed in trace amounts.
Nutrient Cycles
• Compartment – represents a defined
space in nature
• Pool – amount of nutrients in a
compartment
• Flux rate – the quantity of nutrient
passing from one pool to another per
unit time.
Major Nutrient Cycle Pathways
Flux rate
Pool
Hypothetical Phosphorus
Nutrient Cycle
126
Plants
1.4
Herbivores
81
9
133
7
100
45
Water
9.5
Flux rate and pool size together define the nutrient
cycle within any particular ecosystem
19
Two Broad Types of Nutrient Cycles
• Local Cycle – operates within an ecosystem
• Nonvolatile elements
• No mechanism for long-distance transfer
• Global Cycle – involve exchanges (gaseous)
between the atmosphere and the ecosystem
• Volatile elements: Water, oxygen, carbon,
and nitrogen
• Easy exchange among ecosystems
Hydrologic Cycle
• Collects, purifies, and distributes the
Earth’s fixed supply of water – powered by
the sun.
• Distribution of Earth’s Water Supply:
– Salt water (oceans) = 97.4%
– Freshwater = 2.6%
• 80% in glaciers and ice caps
• 20% in groundwater
• 0.4% in lakes and rivers (0.01% of all water!)
– Anytime of year, the atmosphere holds only
0.0001% of water on the planet.
• Although large quantities are evaporated and
precipitated each year
• About 84% of water vapor comes from the ocean
Main Processes of the Hydrologic Cycle
1. Evaporation – conversion of water into water vapor
2. Transpiration – evaporation from leaves of water
extracted from soil by roots
3. Condensation – conversion of water vapor into
droplets of liquid water
4. Precipitation – rain, sleet, hail, and snow
5. Infiltration – movement of water into soil
6. Percolation – downward flow of water through soil
and permeable rock formations to groundwater
storage areas called aquifers
7. Runoff – downslope surface movement back to the
sea to resume cycle
Hydrologic Cycle
3
4
1
2
7
5
6
Nutrient Cycles in Forests
• Inputs – outputs = storage
• Nutrients accumulate in the leaves and
wood over time
Nutrient Storage in Trees is Temperature
and Vegetation Type Related
Organic matter (kg/ha)
Nitrogen (kg/ha)
Forest Region
#
Trees
Total
% Above
Ground
Trees
Total
% Above
Ground
Boreal coniferous
3
51,000
226,000
19
116
3,250
4
Boreal deciduous
1
97,000
491,000
20
331
3,780
6
Temperate coniferous
13
307,000
618,000
54
479
7,300
7
Temperate deciduous
14
152,000
389,000
40
442
5,619
8
Mediterranean
1
269,000
326,000
83
745
1,025
73
208,000
468,000
45
429
5,893
7
Average
In cold climates nutrients are tied up in the soil.
Nutrient Turnover Time is
Temperature Related
Mean turnover time (yr)
Forest Region
#
Organic
matter
N
K
Ca
Mg
P
Boreal coniferous
3
353
230.0
94.0
149.0
455.0
324.0
Boreal deciduous
1
26
27.1
10.0
13.8
14.2
15.2
Temperate coniferous
13
17
17.9
2.2
5.9
12.9
15.3
Temperate deciduous
14
4
5.5
1.3
3.0
3.4
5.8
Mediterranean
1
3
3.6
0.2
3.8
2.2
0.9
All Stands
32
12
34.1
13.0
21.8
61.4
46.0
Turnover time – the time an average atom will remain in
the soil before it is recycled into the trees or shrubs
Net Primary Production and
Nutrient Cycling
• In general, NPP is closely related to the speed of
nutrient cycling.
– Tracking the decay of a leaf and the cycling rate of
nutrients provides an indicator of biome
productivity.
Mean Residence Time (In Years)*
Biome
Organic
matter
Nitrogen
Boreal
forest
353
230
Temperate
forest
4
Chaparral
Tropical
rain forest
Phosphorous
Potassium
Calcium
324
94
149
455
360
5.5
5.8
1.3
3.0
3.4
540
3.8
4.2
3.6
1.4
5.0
2.8
270
0.4
2.0
1.6
0.7
1.5
1.1
900
* Mean residence time is the time for one cycle of decomposition.
Magnesium
NPP
(g C/m2/yr)
A Closed System
• The speed of nutrient cycling in the humid
tropics promotes high productivity, even
when soils are poor in nutrients.
– Nutrients are cycled so quickly there is little
opportunity for them to leak from the system!
• Because there is virtually no loss of
nutrients, many tropical forests have
virtually closed nutrient cycles.
– The opposite would be an open system, in
which nutrients are washed out rapidly
Rapid Cycling in the Tropics
• Reasons for rapid cycling in the tropics:
–
–
–
–
Warm climate
No winter to retard decomposition
An army of decomposers
Abundant mycorrhizal fungi on shallow roots
• Fungi that grow symbiotically with plant
roots
• Facilitate water and nutrient uptake
• Waters in local streams and rivers can
have as few nutrients as rain water!
Tropical Rain Forest Paradox
• Most tropical rain forests are poor in
nutrients – especially oxisol.
• When the forests are cleared for farmland,
the land can only support three or four
harvests.
• Well, how can they support the amount of
primary production we find in a tropical
rain forest?
Standing Biomass
• Standing Biomass - all the plant matter in
a given area.
• Nutrients are either found in the soil or in
the standing biomass.
• In a temperate forest system, recycling is
slow.
– Consequently, at any given time, a large
proportion of nutrients are in the soil.
– So when the land is cleared, it is fertile and can
support many years of agriculture
Tropical Soils
• In the humid tropics, as little as 10% of the
total nutrients are in an oxisol (soil) at any
given time.
– Hence, when the logging trucks take the trees,
they are carrying the majority of the nutrients!
• An increase in soil acidity often follows
timber removal to the point that available
phosphorous is transformed to an
insoluble form.
Normal Nutrient Loss
• Rain runoff is the major vector of nutrient
loss from most ecosystems
Precipitation (mg/L) Streamwater (mg/L)
Calcium
0.21
1.58
Magnesium
0.06
0.39
Potassium
0.09
0.23
Sodium
0.12
0.92
Aluminum
---a
0.24
Ammonium
0.22
0.05
Sulfate
3.10
6.40
Nitrate
1.31
1.14
Chloride
0.42
0.64
Bicarbonate
--- a
1.90
Dissolved silica
--- a
4.61b
a Not determined, but very low; b Watershed 4 only
Deforestation Can Increase Loss of
Nutrients From Areas Due to Runoff
Stream Nitrite Concentration
Note Scale Change
Other stream nutrient increase two years after the deforestation:
Calcium 417%, Magnesium 408%, Potassium 1,558%, Sodium 177%
Carbon Cycle
• Carbon is the basic building block of organic
compounds necessary for life.
• The carbon cycle is a global gaseous cycle
– Carbon dioxide makes up 0.036% of the troposphere
and is also dissolved in water
• Key component of nature’s thermostat
– Too much taken out of the atmosphere, temp’s
decrease
– Too much added to atmosphere, temp’s increase
CO2 Uptake and Release
• Terrestrial producers remove CO2 from the
atmosphere and aquatic producers remove
CO2 from water via photosynthesis.
• The cells in oxygen-consuming producers,
consumers, and decomposers break down
the organic compounds and release CO2
back to the atmosphere or water.
• The link between photosynthesis and
respiration is a major part of the global
carbon cycle
Heat
Energy
Primary Productivity
Solar Energy
CO2
Chemical
Energy (ATP)
Respiration
GPP
Photosynthesis
C6H12O6
NPP
O2
Biomass (g/m2/yr)
Available to
Consumers
Other Links of the Carbon Cycle
• Fossil Fuels – large stores of carbon which are
not released as CO2 unless extracted and burned.
– In only a few hundred years, we have extracted and
burned fossil fuels that took millions of years to
form.
• Limestone (CaCO3) – largest storage for the
earth’s carbon is in sedimentary rocks such as
limestone.
– Carbon reenters the cycle as some of the rock
releases dissolved CO2 back to the atmosphere.
– Geologic processes can bring sediments to the
surface and expose carbonate rock to the
atmosphere.
The Ocean and CO2
• The oceans are the second largest storage
reservoir in the carbon cycle.
– Some is dissolved as CO2 and some reacts
with seawater to form carbonate (CO32-) and
bicarbonate (HCO3-).
– As water warms, more CO2 is squeezed out of
the water into the atmosphere (like a hot coke).
• Ions can react with Ca++ to form CaCO3 to
build the shells and exoskeletons of marine
organisms.
– When these organisms die, tiny particles of
their shells and bone settle to the oceans
bottom and may be buried for eons.
• May eventually be converted to limestone rock
Carbon Cycle
Atmospheric /
Aquatic CO2
Photosynthesis
Respiration
Combustion of
wood / fossil
fuels
Food Web
Weathering
Volcanic
Action
Sedimentation
Limestone Rocks
The Carbon Cycle (Terrestrial)
Atmosphere
(mainly carbon dioxide)
volcanic action
Terrestrial
rocks
weathering
photosynthesis
aerobic
respiration
Land food webs
combustion of
wood (for clearing
land; or for fuel
sedimentation
producers, consumers,
decomposers, detritivores
Soil water
(dissolved carbon)
leaching
runoff
death, burial, compaction
over geologic time
Peat,
fossil fuels
Fig. 4.29, p. 93
The Carbon Cycle (Aquatic)
diffusion between
atmosphere and ocean
combustion of fossil
fuels
Carbon dioxide
dissolved in
ocean water
photosynthesis
aerobic
respiration
Marine food webs
producers, consumers,
decomposers, detritivores
incorporation
death,
into sediments sedimentation
uplifting over
geologic
time
Marine sediments, including
formations with fossil fuels
sedimentation
Fig. 4.29, p. 92-93
Nitrogen Cycle
• Nitrogen is used to make essential organic
compounds such as amino acids,
proteins, DNA, and RNA.
• Normally in short supply and often limits
the rate of primary production.
– Why most commercial fertilizers contain
biologically useful compounds such as
ammonium nitrate (NH4NO3).
• However, nitrogen is the atmosphere’s
most abundant element
– 78% of the volume is chemically un-reactive
nitrogen gas N2.
Nitrogen Fixation
• The nitrogen cycle is a global gaseous
cycle.
• Atmospheric nitrogen must be ‘fixed’ or
combined with H or O to provide
compounds that plants can use.
– Lightning stimulates production of nitrogen
oxides: N2 + O2  2NO
– Certain bacteria in the soil and aquatic
systems can convert nitrogen gas into
compounds that can enter food webs.
Nitrogen Cycle
• Nitrogen fixation – specialized bacteria
convert gaseous nitrogen to ammonia (N2
+ 3H2  2NH3) which can be used by
plants.
– Done mostly by cyanobacteria in water and
soil and Rhizobium bacteria living in small
nodules of some plants
Nitrification and Assimilation
• Nitrification - Two-step process in which
ammonia is converted first to NO2- (toxic
to plants) and then to NO3- (easily taken up
by plants).
• Assimilation – A process by which plants
roots absorb inorganic ammonia,
ammonium ions, and nitrate ions to make
nitrogen containing organic molecules
(DNA, RNA, Proteins).
– Animals then get their nitrogen by eating
plants.
Ammonification
• Ammonification – the conversion (by
decomposer bacteria) of nitrogen-rich
organic compounds, wastes, cast-off
particles, and dead bodies into:
– Simpler nitrogen-containing inorganic
compounds such as ammonia (NH3)
– Water-soluble salts containing ammonium ions
(NH4+)
Denitrification
• Other specialized bacteria (mostly anaerobic
bacteria in waterlogged soil or in the bottom
sediments of a water body) convert NH3 and
NH4+ back to nitrite (NO2- ) and nitrate (NO3-)
ions and then into nitrogen gas (N2) and
nitrous oxide gas (N2O) to complete the
cycle.
Nitrogen Cycle
Gaseous N2
Nitrogen Fixation
Ammonification
Ammonia: NH3, NH4+
1. Nitrification
Nitrite: NO2-
Food Web
2. Nitrification
Nitrate: NO3Loss by
Leaching
Denitrification
Nitrogenous
Waste
-III
-II
NITROGEN OXIDATION STATE
Biochemical
Transition of
Nitrogen
Ammonium
NH+4
-I
Hydroxylamine
NH2OH
0
+I
Nitrogen Gas
N2
Nitroxyl
NOH
Nitrous Oxide N20
+II
Nitric Oxide NO
+III
Nitrite
NO-2
NO-2
+IV
+V
Nitrate
NO-3
Aerobic
Anaerobic
Nitrification
Denitrification
Nitrogen Cycle
Phosphorous Cycle
• The phosphorous cycle is a sedimentary
cycle.
– Circulates through the earth’s crust and living
organisms
– Bacteria are less important here than in the
nitrogen cycle
– At the temperature’s and pressures normally
found on the earth, phosphorous and its
compounds are not gases.
• The phopsphorous is slow, and on a
human time scale much phosphorous
flows from the land to the sea.
Phosphorous
• Phosphorous is usually found as
phosphate salts containing phosphate
ions (PO43-) in terrestrial rock formations
and ocean bottom sediments.
• Most soils contain very little phosphorous,
so it is often the limiting factor for plant
growth on land unless added as fertilizer.
• Phosphorous also limits primary producer
growth in aquatic ecosystems.
Phosphorous Cycle
Sulfur Cycle
• The sulfur cycle is a gaseous cycle.
– Although much of the earth’s sulfur is stored
underground in rocks and minerals.
– Sulfur is important in some amino acids
• Sulfur enters the atmosphere from several
natural sources.
– Hydrogen sulfide (H2S) is released by volcanic
activity and by the breakdown of organic
matter in swamps, bogs, and tidal flats (you
can smell this at low tide in the salt marsh).
– Sulfur dioxide (SO42-) enters from volcanoes.
– Particles of sulfate (SO42-) salts, such as
ammonium sulfate, enter as seas spray.
Sulfur Cycle
All values are
1012 g S/yr