Decomposers, Aquatic and
Nutrient Cycles
Heat
First Trophic
Level
Second Trophic
Level
Third Trophic
Level
Fourth Trophic
Level
Producers
(plants)
Primary
consumers
(herbivores)
Secondary
consumers
(carnivores)
Tertiary
consumers
(top carnivores)
Heat
Heat
Heat
Solar
energy
Heat
Heat
Detritvores
(decomposers and detritus feeders)
Heat
Three Major Types of Nutrient Cycles
• Hydrologic (or water) Cycle – water in the
form of ice, liquid water and water vapor
cycles through the biosphere.
• Atmospheric Cycle – a large portion of a
given element exists in a gaseous form in
the atmosphere.
• Sedimentary Cycle – An element does not
have a gaseous phase, or its gaseous
compounds do not make up a significant
portion of its supply.
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
Global Air Circulation & Regional Climates
• Uneven heating of the Earth’s Surface
– Air is more heated at the equator and less at
the poles.
Global Air Circulation & Regional Climates
• Seasonal changes in temperature and
precipitation
Insolation
A
B
C
Solar Energy
Rainy
Season
Seasonal shift in rainy/dry seasons
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
Nitrogen Cycle
• Nitrogen is used to make essential organic
compounds such as proteins (amino
acids), DNA, and RNA.
• Nitrogen is the atmosphere’s most
abundant element (global gaseous cycle).
• 78% of the volume is chemically un-reactive
nitrogen gas N2.
• Takes a lot of energy to break the triple
covalent bonds holding N N
• Microbes mostly responsible for N cycle
Have You Hugged Your Microbes Today?
Besides making beer, they are responsible for:
• Nitrogen fixation –conversion of gaseous nitrogen (by
Rhizobium, Azotobacter, and cyanobacteria) to ammonia
(N2 + 3H2  2NH3) which can be used by plants.
• Nitrification - Two-step process in which ammonia is
converted first to NO2- (by Nitrosomonas) and then to NO3(by Nitrobacter).
• Denitrification – conversion of nitrate ions (by
Pseudomonas or other anaerobic bacteria in waterlogged
soil or in the bottom sediments of a water body) into
nitrogen gas (N2) and nitrous oxide gas (N2O)
• Ammonification – the conversion (by decomposer
heterotrophic bacteria) of nitrogen-rich organic
compounds, wastes, cast-off particles, and dead bodies
into available ammonia (which can be used by plants).
Ecosystem 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
Energy and the Nitrogen Cycle
Proteins
Provides
Energy
Requires
Energy
Nitrate
Nitrogen Cycle
Phosphorous Cycle
• The phopsphorous cycle is slow, and on a human
time scale most phosphorous flows from the land
to the sea.
– Circulates through the earth’s crust, water, and living
organisms as phosphate (PO4)
– Bacteria are less important here than in the nitrogen
cycle
• Guano (bird poop), mined sediments, and ‘uphill’
movement of wastewater are the main ways
phosphorous is cycled in our lifetime
• Geologic process (mountain formations / uplifting
of ocean sediments) cycle phosphorus in
geologic time
Phosphorous Cycle
Guano
Food web
Soil
Ocean Water
Food web
Mining
Sediments
Phosphorous is Important
• Most soils contain very little
phosphorous; therefore, it is often the
limiting factor for plant growth on land
unless added as fertilizer.
• Phosphorous also limits primary producer
growth in freshwater aquatic ecosystems.
Phosphorous Cycle
Sulfur Cycle
• The sulfur cycle is a gaseous cycle.
– Sulfate (SO4) is the principal biological form
– Essential for some amino acids
– Usually not limiting, but the formation of iron sulfides
converts the insoluble form of phosphorous to a soluble
form
• 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
Volcanoes, Sea spray
Rapid Cycling
Excretion
Sulfur bacteria
Food Web
SO4
Aerobic Sulfideoxidizers
Organic Matter
Anaerobic Sulfurreducers
H2S
Heterotrophic
microorganisms
S
Sulfur bacteria
+Fe3
Very Slow
Flux Rate
OH
SH
FeS
FeS2
Black Anaerobic Mud
Soluble
Phosphorous
Sulfur Cycle
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
Heat
Energy
Primary Productivity
Solar Energy
CO2
Chemical
Energy (ATP)
Respiration
GPP
Photosynthesis
C6H12O6
NPP
O2
Biomass (g/m2/yr)
Available to
Consumers
Carbon Cycle
Atmospheric /
Aquatic CO2
Photosynthesis
Respiration
Combustion of
wood / fossil
fuels
Food Web
Weathering
Volcanic
Action
Sedimentation
Limestone Rocks
The Recyclers
• Detritus – parts of dead organisms and cast-off
fragments and wastes of living organisms
• Detritivores – organisms that feed on detritus
(detritus feeders and decomposers).
– Detritus feeders – extract nutrients from partially
decomposed organic matter in leaf litter, plant
detritus, and animal dung (crabs, carpenter ants,
termites, earthworms).
– Decomposers (certain types of bacteria and fungi)
are very important in recycling nutrients in an
ecosystem
Detritus Feeders and Decomposers
Without detritus feeders and decomposers, the lack of
nutrients would quickly stop primary production!
Turnover and Residence Times
• Turnover rate – the fraction of the total amount of
a nutrient in a compartment that is released (or
that enters) in a given period
• Turnover time – the time needed to replace a
quantity of a substance equal to its amount in the
compartment
• Residence time – the time a nutrient stays in a
compartment (similar to turnover time)
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)
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
The Tropics: 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
– Waters in local streams and rivers can have as
few nutrients as rain water
• 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
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.
Watershed Biogeochemistry
• Watershed – catchment or drainage basin of a river
• Streams and rivers are main conduits of nutrient
loss
• Vegetation type can influence nutrient loss:
Mean calcium concentrations (% dry wt) in three plant species.
Species
Bark
Wood
Twigs
Leaves
Chestnut Oak
1.25 ± 0.17
0.09 ± 0.01
0.68 ± 0.06
0.58 ± 0.07
Flowering Dogwood
2.36 ± 0.26
0.11 ± 0.01
0.80 ± 0.06
1.85 ± 0.11
Rhododendron
0.30 ± 0.10
0.07 ± 0.31
0.99 ± 0.24
1.20 ± 0.29
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%
Riparian Buffer Zone
• Areas of trees, shrubs and other vegetation,
that are adjacent to a body of water, that are
managed for several purposes:
– to maintain the integrity of stream channels and
shorelines;
– to reduce the impact of upland sources of
pollution by trapping, filtering, and converting
sediments, nutrients and other chemicals;
– to supply food, cover and thermal protection to
fish and other wildlife.
• The main purpose of a riparian buffer is to
help control non-point source pollution.
Three Zone Riparian Buffer
Other Methods to Control Erosion
• Silt Fence / hay bales
– Allows water to pool so that sediment is
dropped.
What is Soil?
•
Complex mixture of eroded rock, mineral
nutrients, decaying organic matter, water,
air, and billions of living organisms
(mostly decomposers)
•
Soil is created by
1) Weathering of rock
2) Deposit of sediments by erosion
3) Decomposition of organic matter in dead
animals
Soil Horizons (Profiles)
O horizon - Consists mostly of
freshly fallen and partially
decomposed leaves, twigs, animal
wastes, fungi, and other organic
materials.
A horizon - A porous mixture of
partially decomposed organic
matter (humus) and some
inorganic mineral particles.
Humus is a sticky, brown residue
of partially decomposed organic
material.
•B Horizon (sub-soil) and C
horizon (parent material) - Contain
most of a soil’s inorganic matter.
Mostly broken-down rock
consisting of varying mixtures of
sand, silt, clay, and gravel.
Soil Horizons (Profiles)
Immature soil
O horizon
Leaf litter
A horizon
Topsoil
Regolith
B horizon
Subsoil
Bedrock
C horizon
Young soil
Parent
material
Mature soil
Life in Soil
• The two top layers of most well-developed
soils teem with bacteria, fungi,
earthworms, and small insects that
interact in complex food webs and
nutrient cycles.
Soil Texture
• Clay – very fine particles
• Silt – fine particles
• Sand – medium-size particles
• Gravel – Coarse to very coarse particles
Loam – roughly equal mixtures of clay, sand, silt,
and humus
Soil Texture
Topsoil – Renewable Resource?
• Is regenerated by renewable resources,
but it takes 200 - 1,000 years to produce
about an inch of topsoil in tropical and
temperate climates
– Rate depends on climate and soil type
• If erosion exceeds regeneration, then the
resource is not renewable
Soil erosion – movement of soil components,
especially surface litter and top soil, from one
place to another.
- Typically caused by flowing water and
wind
Any activity that destroys plant cover makes soil
vulnerable to erosion (e.g., farming, logging,
construction, over-grazing by livestock, off-road
vehicles, and deliberate burning of vegetation).
Moving Water Causes Most Soil Erosion
• Sheet Erosion – fairly uniform sheets of soils
are removed as surface water flows over a
slope or across a field in a wide flow.
• Rill Erosion – occurs when surface water
forms fast-flowing rivulets that cuts small
channels in the soil.
• Gully Erosion – occurs when rivulets of fastflowing water join each other and with each
succeeding rain cut the channels wider and
deeper until they become ditches or gullies.
Harmful Effects of Soil Erosion
1) Loss of soil fertility and its ability to hold
water
2) Runoff of sediment that pollutes water,
kills fish and shellfish, and clogs
irrigation ditches, boat channels,
reservoirs, and lakes.