Chap.20 Ecosystem Energetics
Chap.21 Decomposition and nutrient cycling
Chap.22 Biogeochemical cycles
Smith & Smith (2015) Elements of ecology. 9th. Ed. Pearson.
Part Six Ecosystem Ecology
生態體系 生態學
鄭先祐 (Ayo) 教授
生態科學與技術學系
國立臺南大學 環境與生態學院
Chap.21 分解與營養循環
Decomposition and
Nutrient Cycling
Smith & Smith (2015) Elements of ecology. 9th. Ed. Pearson.
鄭先祐 (Ayo) 教授
生態科學與技術學系
國立臺南大學 環境與生態學院
Colorful decomposers such as honey mushroom (Armillaria mellea) reside on
the forest floor throughout much of continental North America.
3
Chapter 21
Decomposition and Nutrient Cycling
http://myweb.nutn.edu.tw/~hycheng/
 Energy flow through an ecosystem is based on
the movement of carbon into and out of
organisms.
 Primary productivity depends on the movement
of carbon through the food chain and:
 Uptake of essential nutrients from the
atmosphere and rocks and minerals.
 Internal cycling (nutrient cycling) is the
transformation of organic nutrients into mineral
form and back into organisms.
 Decomposition and nutrient mineralization
4
21.1 Most Essential Nutrients Are
Recycled within the Ecosystem
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 Plants require all essential nutrients in inorganic
(or mineral) form.
 Nutrients are taken up in the soil solution
through the roots and the mineral is transformed
from an inorganic to organic form.
 As plant tissues age and die (senesce), nutrients
are returned to the soil surface in the form of
dead organic matter.
 Retranslocation or reabsorption of some
nutrients occurs.
5
Fig. 21.1 A generalized model of nutrient cycling in a terrestrial ecosystem.
6
21.1 Most Essential Nutrients Are
Recycled within the Ecosystem
http://myweb.nutn.edu.tw/~hycheng/
 Retranslocation or reabsorption of some
nutrients occurs
 In temperate regions, as days become shorter in
the autumn, chlorophyll production (responsible
for green leaf color) begins to decline
 Plant roots can reabsorb minerals (especially
nitrogen) from the leaves that will be lost from
the plant
 Yellow and organic pigments begin to show
and anthocyanins (花青素) are produced
7
8
21.2 Decomposition Is a Complex
Process Involving a Variety of Organisms
http://myweb.nutn.edu.tw/~hycheng/
Decomposition, the breakdown of chemical bonds of
organic molecules, is the key process in the recycling
of nutrients within the ecosystem.

Release of energy, carbon dioxide, and water
Decomposition includes many processes





Leaching (溶濾)
Fragmentation
Changes in physical and chemical structure
Ingestion (攝取)
Excretion (排泄) of waste products
9
21.2 Decomposition Is a Complex
Process Involving a Variety of Organisms
http://myweb.nutn.edu.tw/~hycheng/
Decomposers are organisms that feed on dead organic
matter or detritus (碎屑) (bacteria, fungi, and detritivores)

All heterotrophs function to some degree as decomposers
Decomposing organisms are categorized into groups
based on size and function.


Microflora: bactreria and fungi
Invertebrate detritivores
 Microfauna and microflora
 Mesofauna
 Macrofauna
 Megafauna
10
21.2 Decomposition Is a Complex
Process Involving a Variety of Organisms
http://myweb.nutn.edu.tw/~hycheng/
Bacteria are the dominant decomposers of dead animal
matter.


Aerobic
Anaerobic — fermentation of organic matter in
mud/sediments of aquatic habitats and in the ungulate rumen
Fungi are the major decomposers of plant matter.

Extend hyphae into organic material to withdraw nutrients.
Bacteria and fungi secrete enzymes into plant and
animal tissue to break down organic molecules.
11
12
Fig. 21.2 (a) Fungi and
bacteria are major
decomposers of plant
and animal tissues.
(b) Mites and
springtails (躍尾蟲) are
among the most
abundant of small
detritivores.
(c) Earthworms and
millipedes (馬陸) are
large detritivores in
terrestrial ecosystems,
and
(d) Mollusks and crabs
play a similar role in
aquatic ecosystems.
21.2 Decomposition Is a Complex
Process Involving a Variety of Organisms
http://myweb.nutn.edu.tw/~hycheng/
 Invertebrate detritivores decompose leaves,
twigs, and other detritus and are classified by
body width.
 Microfauna and microflora (<100 m) include
protozoans and nematodes inhabiting the
water in soil pores.
 Mesofauna (100 m to 2 mm) include mites,
potworms, and springtails that live in soil air
spaces.
13
21.2 Decomposition Is a Complex
Process Involving a Variety of Organisms
http://myweb.nutn.edu.tw/~hycheng/
Invertebrate detritivores decompose leaves, twigs, and
other detritus and are classified by body width


Macrofauna (2 – 20 mm)
Megafauna (>20 mm)
Macro- and megafauna are represented by:


Terrestrial: snails, millipedes, and earthworms
Aquatic: annelid worms, crustaceans (e.g., amphipods and
isopods), mollusks, and crabs
Earthworms and snails dominate the megafauna
 Microbivores feed on bacteria and fungi
 This group includes protozoans (e.g., amoebas, springtails,
mites)
14
21.2 Decomposition Is a Complex
Process Involving a Variety of Organisms
http://myweb.nutn.edu.tw/~hycheng/
Ecologists study the process of
decomposition by designing experiments
that follow the decay of dead plant and
animal tissues through time.

Litter bags are used to examine the
decomposition of plant litter.
15
Fig. 21.3 Litterbag experiment. In this example, a known quantity of
senescent leaves is placed in mesh bags on the forest floor. Bags are
retrieved at various intervals, and the mass loss due to consumption by
decomposers is tracked through time.
16
21.3 Studying Decomposition Involves
Following the Fate of Dead Organic Matter
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 A fixed amount of litter material is placed in each
bag and the bags are examined after a set time
has elapsed
 The mass of litter remaining in the bags
decreases continuously as time progresses
 It is difficult to quantify how much of the
remaining mass is due to the contribution of
primary (original plant material) and
secondary (microbes doing the decomposing)
organic matter.
17
Fig. 21.4 Results of a litterbag experiment in central Virginia
designed to examine the decomposition of fallen leaves from red
maple, white oak, and sycamore (美國梧桐) trees.
18
21.3 Studying Decomposition Involves
Following the Fate of Dead Organic Matter
http://myweb.nutn.edu.tw/~hycheng/
M. Swift (University of Zimbabwe) estimated the
growth of fungi during decomposition by measuring
the change in chitin content (restricted to fungi)

By the end of the experiment, it was found that the
apparent decomposition rate (k) of 0.04/wk was doubled (to
0.09/wk)when calculated to exclude the fungal biomass.
A similar approach to litterbag
experiments is used in stream ecosystems.

To quantify the process of decomposition, plant
litter (that accumulates in areas of active
deposition) is placed in mesh bags (leaf packs)
that are anchored in place.
19
Fig. 21.5 (a) Inputs of plant litter from the surrounding
terrestrial environment can form areas of deposition, known
as leaf packs, in stream ecosystems.
20
Fig. 21.5 (b) Much like the use of litterbag experiments by
terrestrial ecologists, stream ecologists use mesh bags to
simulate natural leaf packs and examine the processes of
decomposition.
21
Quantifying Ecology 21.1 Estimating the
Rate of Decomposition
http://myweb.nutn.edu.tw/~hycheng/
Litterbag experiments are the primary means by
which ecologists study decomposition.
 Replicate litterbags are collected at regular intervals
during the process of decay.
 Researchers plot the proportion of mass loss
through time.
Original mass remaining = e–kt
t = time unit used
 k = decomposition coefficient (slope)
 The rate of organic decay is related to:
 Plant litter quality

22
Fig. 1 Data from two litterbag experiments that
examined the rate of decomposition for red maple and
Virginia pine leaf litter over a period of two years.
Each point represents the average mass remaining in
five replicate litterbags sampled during that period.
23
http://myweb.nutn.edu.tw/~hycheng/
24
Fig. 21.6
Decomposition rates
for leaf litter from five
tree species submerged
in stream.
Experiments used
submerged litter bags
(leaf packs) that were
sampled at five
intervals over a period
of 83 days.
25
21.4 Several Factors Influence the Rate
of Decomposition
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Plant litter quality determines its suitability
as habitat for decomposing organisms.
Types and quantities of carbon present
affect:


Energy gained by decomposers
Decomposition or consumption rate
26
21.4 Several Factors Influence the Rate
of Decomposition
http://myweb.nutn.edu.tw/~hycheng/
Carbon form available affects the
consumption (or decomposition) rates



Glucose and other simple sugars — highquality sources of carbon, small molecules,
high-energy bonds
Cellulose and hemicellulose — structurally
complex, more energy required to break bonds,
moderate quality
Lignin (and others) — very large and complex
molecules, slow to decompose, low quality

Basidiomycetes are the only group of decomposers to
decompose these molecules
27
21.4 Several Factors Influence the Rate
of Decomposition
http://myweb.nutn.edu.tw/~hycheng/
An experiment that studied rate of carbon
decay in straw found the following



Proteins, simple sugars, soluble
compounds (15 percent of carbon content) —
decomposed very quickly within the first few
days.
Cellulose and hemicellulose (60 percent of
carbon content) — decomposed more slowly and
were completely broken down in three weeks.
Lignins (20 percent) — the majority remained
intact by day 80.
28
Fig. 21.7 variation in
the rates of decay (mass
loss) of different classes
of carbon compounds in
an experiment
examining the
decomposition of straw
on the soil surface.
29
21.4 Several Factors Influence the Rate
of Decomposition
http://myweb.nutn.edu.tw/~hycheng/
The proportion of carbon contained in
lignin-based compounds is used as an
index of litter quality.
There is an inverse relationship
between decomposition rate for plant litter
and its lignin content at the start of
decomposition.

Terrestrial and aquatic ecosystems (Fig. 21.8)
30
Fig. 21.8 Relationship between initial lignin content of litter
material and rate of decomposition for a variety of plant
litters in (a) terrestrial and (b) aquatic environments. Each
point on the graphs represents an individual plant species.
31
Fig. 21.8 Relationship between initial lignin content of litter
material and rate of decomposition for a variety of plant
litters in (a) terrestrial and (b) aquatic environments. Each
point on the graphs represents an individual plant species.
32
21.4 Several Factors Influence the Rate
of Decomposition
http://myweb.nutn.edu.tw/~hycheng/
Carbon quality of plant litters can have a particularly
important influence on decomposition in coastal
marine environments.
 Decomposition in these plant litters is dependent on
the oxygen content of the water.
In aquatic habitats where oxygen levels are extremely
low (e.g., mud and sediments) the overall rate of
decomposition is slowed.

Only anaerobic bacteria are decomposing — there are no
aerobic bacteria or fungi.
33
Fig. 21.9 Results of a litterbag experiment designed to examine
the decomposition of Spartina alternifolia litter exposed to aerobic
(litterbags on the marsh surface) and anaerobic (buried 5-10 cm
below the marsh surface) conditions.
34
21.4 Several Factors Influence the Rate
of Decomposition
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The rate of organic decay is related to:
Plant litter quality
 Soil properties (texture and pH)
 Climate (temperature and precipitation)
 The rate of organic matter is directly affected by
temperature and moisture.
 Low temperatures and dry conditions reduce or
inhibit microbial activity.
 Warm and moist conditions are the optimum
environment for microbial action.

35
Fig. 21.10 Decomposition of red maple litter at three sites in
eastern North America: New Hampshire(circles), West
Virginia (triangles), and Virginia (squares).
36
21.4 Several Factors Influence the Rate
of Decomposition
http://myweb.nutn.edu.tw/~hycheng/
The direct influence of temperature on
decomposers results in a distinct diurnal
pattern of microbial activity and can be
measured by microbial respiration from the
soil.

The daily temperature pattern is closely
paralleled by the release of CO2
37
Fig. 21.11 Diurnal changes in air temperature and decomposition in a
temperate deciduous forest. Decomposition rate is measured indirectly
as the release of CO2 from decomposing litter on the forest floor.
38
Field Studies: Edward A.G. (Ted) Schuur
http://myweb.nutn.edu.tw/~hycheng/
 The warm, wet environments of the tropical rain
forest support the highest rates of net primary
productivity (NPP) and decomposition of any
terrestrial ecosystem on Earth.
 However, Ted Schuur
(University of Florida)
found a different pattern
in the montane forests of
Maui (Hawaiian Islands)
 Schuur found a pattern
of decreasing NPP with
increasing annual
precipitation — why?
39
Fig. 1 Typical forest of the Makawao and Koolau Forest Reserves.
40
Field Studies: Edward A.G. (Ted) Schuur
http://myweb.nutn.edu.tw/~hycheng/
 Schuur measured the chemical composition of
leaves from trees along the mountainside.
 Two characteristics varied systematically as rainfall
increased and might limit nutrient availability for
NPP.
 Leaf nitrogen concentration decreased
 The concentration of lignin increased
41
Field Studies: Edward A.G. (Ted) Schuur
http://myweb.nutn.edu.tw/~hycheng/
 Schuur examined how both plant and physical
characteristics influenced the process of nutrient
cycling.
 Litterbag experiments were used to examine carbon
and nitrogen cycling during decomposition.
 A measurable decline was noted in
decomposition rates and nitrogen cycling across
the gradient of mean annual rainfall.
42
Fig. 2 Rate of decomposition (k: blue circles) and nitrogen loss
(green circles) as a function of rainfall for the six study sites.
43
Field Studies: Edward A.G. (Ted) Schuur
http://myweb.nutn.edu.tw/~hycheng/
Differences in the rate of nitrogen release affected the
availability of soil nitrogen  NPP

Oxygen availability decreased with increasing annual
precipitation.
In the montane forests of Maui, the litter
decomposition rates and nutrient release slow with
increasing rainfall due to decreased soil oxygen
availability and low-quality litter.
44
Fig. 3 Relationship between net primary productivity
and mean annual precipitation.
45
Field Studies: Edward A.G. (Ted) Schuur
http://myweb.nutn.edu.tw/~hycheng/
Schuur's work has important implications
for the debate over how climate change
will influence terrestrial ecosystems
What is the real effect of increased
precipitation on NPP and decomposition?
46
21.5 Nutrients in Organic Matter Are
Mineralized During Decomposition
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The nutrient quality of dead organic material varies
greatly — the higher the nutrient content, the higher the
nutrient value for the decomposer
The net mineralization rate is the difference between
the rates of mineralization and immobilization.


Mineralization (礦化) is the transformation of nutrients
contained in organic compounds into inorganic forms
Immobilization(固定) is the uptake and assimilation of
minerals by microbial decomposers
47
Fig. 21.12 Diagram illustrating the exchanges between litterbag (delineated by the
dashed line) and soil in a standard litterbag experiment used to quantify the process
of decomposition.
48
21.5 Nutrients in Organic Matter Are
Mineralized During Decomposition
http://myweb.nutn.edu.tw/~hycheng/
Changes in the nitrogen content conform to three
stages
A. Amount of nitrogen in leaf litter declines as watersoluble compounds are leached from the litter.
B. Nitrogen content increases as microbial decomposers
immobilize nitrogen from outside the litter.
A.
Due to the nitrogen content of the decomposers (bacteria and
fungi)
C. Finally as carbon quality declines, the mineralization
rate exceeds the immobilization rate and nitrogen is
released to the soil.
49
Fig. 21.13 Idealized graph showing the change in nitrogen
content of plant litter during decomposition.
50
Fig. 21.14 Results from a litterbag experiment designed to
examine the changing composition of decomposing winter rye
(裸麥) in an agricultural field.
(a) Mass loss continued throughout the 100 days of the
experiment.
51
Fig. 21.14 Results from a litterbag experiment designed to
examine the changing composition of decomposing winter rye
(裸麥) in an agricultural field.
(b) The proportion of the remaining mass in plant and
microbial (fungal) biomass (living and dead).
52
Fig. 21.14 Results from a litterbag experiment designed to examine
the changing composition of decomposing winter rye (裸麥) in an
agricultural field.
(c) Because the ratio of carbon to nitrogen (C:N) of the microbial
biomass is much lower than that of the remaining plant litter, there
is a general pattern of decline in the C:N during decomposition.
53
21.5 Nutrients in Organic Matter Are
Mineralized During Decomposition
http://myweb.nutn.edu.tw/~hycheng/
 If the nitrogen content of the litter material is
high, then mineralization may exceed the rate of
immobilization from the onset of decomposition
(Fig, 21.15).
54
Fig. 21.15 Change in the nitrogen content of leaf litter from two tree species
inhabiting the forests of central Virginia: American hornbeam and Virginia pine.
Note the difference between the two species in the initial nitrogen content of the
leaf litter and the subsequent rates of immobilization.
55
21.5 Nutrients in Organic Matter Are
Mineralized During Decomposition
http://myweb.nutn.edu.tw/~hycheng/
 The pattern of dynamics during composition is a
function of the nutrient content of the litter and
the demand for the nutrient by the microbial
population (Fig. 21.16)
56
http://myweb.nutn.edu.tw/~hycheng/
Fig. 21.16 Patterns of immobilization and mineralization for
sulfur (S), calcium (Ca) , and manganese (Mn) in decomposing
needles of Scots pine. Results are from a litterbag experiment
during a period of five years.
57
21.6 Decomposition Proceeds as Plant Litter Is
Converted into Soil Organic Matter
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 As the decomposition process continues, the litter
degrades into a dark brown/black homogeneous
organic matter called humus.
 As humus becomes embedded in the soil matrix,
it is referred to as soil organic matter.
 B. Berg (Swedish University of Agricultural Sciences)
examined the decomposition of leaf litter in a
pine forest over five years.
 As decomposition proceeds, mass loss continues
 Mineral nitrogen is immobilized due to the high
C:N.
58
21.6 Decomposition Proceeds as Plant Litter Is
Converted into Soil Organic Matter
http://myweb.nutn.edu.tw/~hycheng/



As plant litter is consumed, a significant portion
of carbon is lost to the atmosphere through
microbial respiration.
The nitrogen concentration of the residual
organic matter increases with mass loss. (Fig.
21.17 (a))
The decrease in carbon (lost to respiration) and
increase in nitrogen content in residual organic
matter result in a decline in C:N as
decomposition proceeds. (Fig.21.17 (b))
59
Fig. 21.17 patterns of (a) mass loss and nitrogen dynamics
during a five-year experiment examining the decomposition of
Scots pine leaf litter in central Sweden..
60
Fig. 21.17 patterns of (b) changes in nitrogen concentration of
residual organic matter during a five-year experiment examining
the decomposition of Scots pine leaf litter in central Sweden..
61
Fig. 21.17 patterns of (c) ratio of carbon to nitrogen during a
five-year experiment examining the decomposition of Scots pine
leaf litter in central Sweden.
A low C:N does not indicate an increase in available nitrogen for
microbial decomposers.
62
Fig. 21.17 patterns of (d) concentration of lignin in residual
organic matter during a five-year experiment examining the
decomposition of Scots pine leaf litter in central Sweden..
Residual organic matter consists of complex lignin-based
compounds and nitrogen is bound up in these recalcitrant (hard to
break down) compounds
63
21.6 Decomposition Proceeds as Plant Litter Is
Converted into Soil Organic Matter
http://myweb.nutn.edu.tw/~hycheng/
 Soil organic matter typically has a residence time of
20 to 50 years.
 It can range from one to two years in a
cultivated field to thousands of years in
environments with slow rates of decomposition
(cold or dry)
 Humus decomposes very slowly, but as it is
abundant, it represents a significant portion of
carbon and nutrients released from soils.
64
21.7 Plant Processes Enhance the Decomposition
of Soil Organic Matter in the Rhizosphere
http://myweb.nutn.edu.tw/~hycheng/
The rhizosphere is the region of the soil where plant
roots function, an active zone of root growth and death
with intense microbial and fungal activity.

Makes up virtually all of the soil in fine-rooted grasslands.
Decomposition in the rhizosphere is more rapid than
in the bulk soil.

Roots alter the chemistry of the rhizosphere by secreting
carbohydrates into the soil.
65
21.7 Plant Processes Enhance the Decomposition
of Soil Organic Matter in the Rhizosphere
http://myweb.nutn.edu.tw/~hycheng/
 The growth of bacteria in the rhizosphere is
supported by the high-quality root exudates
(carbon) and is limited most strongly by nutrient
availability (e.g., nitrogen).
 Bacteria must acquire their nutrients by
breaking down soil organic matter.
66
21.7 Plant Processes Enhance the Decomposition
of Soil Organic Matter in the Rhizosphere
http://myweb.nutn.edu.tw/~hycheng/
 The interplay between microbial decomposers and
microbivores determines the rate of nutrient cycling in the
rhizosphere and strongly enhances the availability of mineral
nutrients to plants.
 The soil microbial loop



Plants supplement carbon to microbial decomposers in the rhizosphere.
Microbes are preyed on by microbivores that release minerals and
nutrients back to the soil.
Enhancement of mineral cycling and an increase in nutrient availability
to plants.
67
Fig. 21.18 Illustration of the soil microbial
loop in which energy-rich carbon exudates
from the plant roots within the rhizosphere
enhance the growth of microbial
populations and the breakdown of soil
organic matter.
68
Nutrients immobilized in microbial biomass are then liberated to
the soil through predation by microbivores, providing increased
mineral nutrients to support plant growth.
69
21.7 Plant Processes Enhance the Decomposition
of Soil Organic Matter in the Rhizosphere
http://myweb.nutn.edu.tw/~hycheng/
Populations of protozoa and nematodes fluctuate.

As populations decline, their readily decomposable tissues
enter the detrital food chain.
As much as 70 percent of soil respiration can be due
to protozoa.
As much as 15 percent of soil respiration can be due
to nematodes.

Production rates of microbivores can be 10 to 12 times
their standing biomass.
70
21.7 Plant Processes Enhance the Decomposition
of Soil Organic Matter in the Rhizosphere
http://myweb.nutn.edu.tw/~hycheng/
 The rhizosphere mediates virtually all aspects of
nutrient cycling.
 According to some estimates, the rhizosphere
processes utilize 50 percent of the energy
fixed by photosynthesis and contribute ~50
percent of the total CO2 from terrestrial
ecosystems.
71
21.8 Decomposition Occurs in Aquatic
Environments
http://myweb.nutn.edu.tw/~hycheng/
 Decomposition in aquatic systems is similar to
the patterns already discussed.
 Coastal environments
 Submerged plant litters decompose more
rapidly than those on the surface because they
are more accessible to detritivores and
provide a more stable physical environment to
microbial decomposers.
72
Fig. 21.19 Decomposition of leaves of arrow arum (Peltandra virginica) in a
tidal freshwater marsh. In litterbags under three conditions: irregularly
flooded high marsh exposed to alternate periods of wetting and drying, creek
bed flooded two time daily (tidal), and permanently submerged.
73
21.8 Decomposition Occurs in Aquatic
Environments
http://myweb.nutn.edu.tw/~hycheng/
 Flowing water ecosystems
 Aquatic invertebrates
 Shredders (切碎者) fragment organic particles
in the process of eating bacteria and fungi on
the surface of the litter.
 Filtering and gathering collectors filter fine
particles and fecal material of the shredders.
 Grazers and scrapers (刮削者) feed on
material growing or collecting on rocks.
 Algae take up nutrients and dissolved organic
matter from the water.
74
21.8 Decomposition Occurs in Aquatic
Environments
http://myweb.nutn.edu.tw/~hycheng/
 Decomposition in aquatic systems is similar to
the patterns already discussed
 Open water (ponds, lakes, ocean)
 Particulate organic matter (POM) is
ingested, digested, and mineralized as it
makes its way to the bottom
 Bottom-dwelling detritivores may further
decompose the organic matter
75
21.8 Decomposition Occurs in Aquatic
Environments
http://myweb.nutn.edu.tw/~hycheng/
 Water column
 Dissolved organic matter (DOM) are the
free-floating macroalgae, phytoplankton, and
zooplankton that dissolve with 15 to 30
minutes after their death.
 Ciliate and zooplankton excrete nutrients as
exudates and fecal pellets
76
21.9 Key Ecosystem Processes Influence
the Rate of Nutrient Cycling
http://myweb.nutn.edu.tw/~hycheng/
 The internal cycling of nutrients through the
ecosystem depends on the processes of primary
production and decomposition.
 How do these processes interact to limit the rate of
internal nutrient cycling?
77
21.9 Key Ecosystem Processes Influence
the Rate of Nutrient Cycling
http://myweb.nutn.edu.tw/~hycheng/
The maximum rate of photosynthesis is strongly
correlated with nitrogen concentrations in the leaves
(e.g. chlorophyll)

Nitrogen availability directly affects rates of ecosystem
primary productivity via the influence of nitrogen on
photosynthesis and carbon uptake
The quantity and quality of organic matter as a food
source for decomposers directly influence the rates of
decomposition and nitrogen mineralization.
A feedback system exists in the internal cycling of
nutrients within an ecosystem.
78
Fig. 21.20
Feedback that
occurs between
nutrient
availability, net
primary
productivity and
nutrient release in
decomposition for
initial conditions of
low and high
nutrient
availability.
79
21.9 Key Ecosystem Processes Influence
the Rate of Nutrient Cycling
http://myweb.nutn.edu.tw/~hycheng/
J. Pastor (University of Minnesota) examined the
aboveground production and nutrient cycling in a
series of forest stands along a gradient of soil texture



Tree species producing higher-quality litter (lower C:N ratio)
dominated sites with finer coil texture.
Higher-quality litter resulted in a higher rate of nutrient
mineralization.
Higher rates of nutrient availability resulted in a higher
rate of primary productivity and nutrient return in litter
fall.
80
Fig. 21.21 Relationship between (a) liter quality (C:N) and nitrogen
mineralization rate (N availability) and (b) nitrogen mineralization rate and
nitrogen returned in annual litterfall for a variety of forest ecosystems on
Blackhawk island, Wisconsin.
81
Ecological Issues: Nitrogen Fertilizers
http://myweb.nutn.edu.tw/~hycheng/
 There is a tight link between net primary productivity (NPP)
and decomposition.
 NPP determines the quantity and quality of organic matter
available to decomposers
 In agriculture, this balance is disrupted because plants (and the
nutrients that they contain) are harvested and the organic matter
does not return to the soil, so nutrient supplements (fertilizers)
are added.
 Historical development of chemical fertilizers.
 Natural fertilizers: manures, ground animal bones.
 Chemical fertilizers: natural and synthetic sources.
82
Ecological Issues: Nitrogen Fertilizers
http://myweb.nutn.edu.tw/~hycheng/
 Three elements are necessary in large quantities
for plant growth
 Potassium (K)
 Phosphorus (P)
 Nitrogen (N)
 Originally, these came from mineral deposits
 K — potash (草鹼)
 P — phosphate rocks (磷岩)
 N — saltpeter (硝石)
83
Ecological Issues: Nitrogen Fertilizers
http://myweb.nutn.edu.tw/~hycheng/
As the demand for food increased with human
population, there was a growing concern about the
depletion of nitrogen for chemical fertilizers.
 In the early 1900s, F. Haber developed the synthetic
ammonia process (N2 + 3 H2  2 NH3) that made
ammonia manufacture economically feasible.
 C. Bosch translated this to a large-scale process using
a catalyst and high-pressure methods.
84
Ecological Issues: Nitrogen Fertilizers
http://myweb.nutn.edu.tw/~hycheng/
 The Haber–Bosch process has changed the way
nitrogen fertilizers are produced and used.
 The bounty of food produced comes with
environmental cost.
 Nitrates pollute drinking water.
 Nitrogen runoff from agricultural fields disrupts
the normal constraints on primary productivity.
85
Ecological Issues: Nitrogen Fertilizers
http://myweb.nutn.edu.tw/~hycheng/
 Excess nitrogen deposited in aquatic ecosystems
leads to eutrophication, the explosive growth of
algae.
 High inputs of organic matter result in a
corresponding increase in decomposition and
respiration  huge reduction in O2 content of
water.
 Many native organisms cannot survive these
subsequent low-oxygen conditions.
86
Ecological Issues: Nitrogen Fertilizers
http://myweb.nutn.edu.tw/~hycheng/
 The Pew Oceans Commission reported to Congress
in 2003 that nitrogen fertilizer is the main source
of pollution in the ocean
 Humans must reduce the negative environmental
consequences of the process that enables us to
feed the world's population.
87
21.10 Nutrient Cycling Differs between
Terrestrial and Open-Water Aquatic Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
 In virtually all ecosystems, there is a vertical
separation between the zones of production
(photosynthesis) and decomposition.
 In terrestrial and shallow-water environments,
plants directly link production (in canopy or
leaves) and decomposition (at soil surface).
88
Fig. 21.22 Comparison of the vertical zones of production and
decomposition in (a) a terrestrial (forest) and (b) an open-water (lake)
ecosystem.
In the terrestrial ecosystem, the two zones are linked by the vegetation
(trees). However, this is not the case in the lake ecosystem.
89
21.10 Nutrient Cycling Differs between
Terrestrial and Open-Water Aquatic Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
 Vertical structure of open-water ecosystems
 Epilimnion — surface water is relatively warm,
relatively high concentration of oxygen
 Hypolimnion — deep water is cold and
relatively low in oxygen
 Thermocline — the transition zone between
surface and deep waters is characterized by a
steep temperature gradient
90
21.10 Nutrient Cycling Differs between
Terrestrial and Open-Water Aquatic Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
 The vertical structure and physical separation of
the epilimnion and hypolimnion have
important influence on the distribution of
nutrients and patterns of primary productivity in
aquatic ecosystems.
91
http://myweb.nutn.edu.tw/~hycheng/
Fig. 21.23 Seasonal dynamics in the vertical structure of an openwater aquatic ecosystem in the temperate zone.
(a) Winds mix the waters within the epilimnion during summer, but
the thermocline isolates this mixing to the surface waters.
92
21.10 Nutrient Cycling Differs between Terrestrial
and Open-Water Aquatic Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
Nutrients are transported vertically from
deeper waters to the surface where
temperature and light conditions allow for
primary productivity



Wind is not adequate to fully mix the
epilimnion and hypolimnion
Turnover (mixing) of the waters occurs as
autumn and winter approach in the temperate
and polar zones  the thermocline breaks
down
The thermocline is reestablished in spring
93
Fig. 21.23 (b) With the breakdown of the thermocline during the fall
and spring months, turnover occurs, allowing the entire water
column to become mixed. This mixing allows nutrients in the
epilimnion to be brought up to the surface water.
94
21.10 Nutrient Cycling Differs between
Terrestrial and Open-Water Aquatic Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
 The annual cycle of productivity in open-water
ecosystems is a direct result of thermocline
behavior and the consequent behavior of the
vertical distribution of nutrients.
95
96
Fig. 21.24 Seasonal dynamics of (a) the thermocline and
associated changes in (b) the availability of light and
nutrients, and (c) net primary productivity of the surface
waters.
97
Fig. 21.24 Seasonal dynamics of (a) the thermocline and
associated changes in (b) the availability of light and
nutrients, and (c) net primary productivity of the surface
waters.
98
Fig. 21.24 Seasonal dynamics of (a) the thermocline and
associated changes in (b) the availability of light and nutrients,
and (c) net primary productivity of the surface waters.
99
21.11 Water Flow Influences Nutrient
Cycling in Streams and Rivers
http://myweb.nutn.edu.tw/~hycheng/
Stream inputs of nutrients



Dead organic matter from adjacent terrestrial ecosystems
Rainwater
Subsurface seepage
The continuous, directional movement of water
affects nutrient cycling in streams


Nutrient spiraling occurs because nutrients are
continuously being transported downstream
A time and spatial element to cycling
100
21.11 Water Flow Influences Nutrient
Cycling in Streams and Rivers
http://myweb.nutn.edu.tw/~hycheng/
How quickly material is moved downstream depends
on water velocity and the degree of physical and
biological retention of organic matter.
Spiraling is measured as the distance needed to
complete one cycle — the longer the distance required,
the more open the spiral.

Spiraling patterns are different for different areas of a strea.
101
Fig. 21.25 Nutrient spiraling between organic matter and the water column
in a stream ecosystem. Uptake and turnover take place as nutrients flow
downstream. The tighter the spiraling, the longer the nutrients remain in
place. (a) Tight spiraling; (b) open spiraling.
102
Fig. 21.25 Nutrient spiraling between organic matter and the water column
in a stream ecosystem. Uptake and turnover take place as nutrients flow
downstream. The tighter the spiraling, the longer the nutrients remain in
place. (a) Tight spiraling; (b) open spiraling.
103
21.12 Land and Marine Environments Influence
Nutrient Cycling in Coastal Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
 Coastal ecosystems are among the most
productive environments.
 Estuaries are semi-enclosed parts of the coastal
ocean where seawater is diluted and partially
mixed with water coming from the land.
 Nutrients and oxygen are carried into the
estuary by the tides.
104
21.12 Land and Marine Environments Influence
Nutrient Cycling in Coastal Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
As freshwater rivers meet the ocean and current
velocity drops, sediments are deposited within a short
distance (sediment trap)
 A "salt wedge" of intruding (more dense) seawater
on the bottom and less dense inflowing freshwater
on the surface

The pycnocline is the zone of maximum vertical difference
in water density — functions similarly to the thermocline
105
A pycnocline is the cline or
layer where
the density gradient is
greatest within a body of
water. The physical
properties in a pycnocline
driven by density gradients
also affect the flows and
vertical profiles in the ocean.
These changes can be
connected to the transport of
heat, salt, and nutrients
through the ocean, and the
pycnocline diffusion
controls upwelling.
106
Fig. 21.26 Circulation of freshwater and saltwater in an estuary
functions to trap nutrients. A salty wedge of intruding seawater
on the bottom produces a surface flow of lighter freshwater and
a counterflow of heavier brackish water. These layers are
physically separated by variations in water density arising from
both salt concentration and temperature differences.
107
21.12 Land and Marine Environments Influence
Nutrient Cycling in Coastal Ecosystems
http://myweb.nutn.edu.tw/~hycheng/
 Nutrients are carried to coastal marshes by precipitation,
groundwater, and surface water.
 Coastal marshes are also affected by the tidal cycle which
serves to flush out salts and other toxins.
 The tidal subsidy supplies nutrients brought to coastal
marshes.
 The salt marsh is a detrital system with three-quarters of the
detritus broken down by bacteria and fungi.
 ~50 percent of total NPP is lost through respiration via
microbial breakdown.
 20 to 40 percent of NPP is exported to adjacent estuaries.
108
21.13 Surface Ocean Currents Bring
About Vertical Transport of Nutrients
http://myweb.nutn.edu.tw/~hycheng/
The global pattern of ocean surface currents
influences patterns of surface water temperature,
productivity, and nutrient cycling.
The lateral movement of water is limited to the upper
100 m.

In certain regions, the lateral movements can bring about a
vertical circulation or upwelling of water. (湧升流)
Regions of nutrient-rich waters are highly productive
and support some of the world's most important
fisheries.
109
Chap.21 Decomposition and nutrient cycling
Ayo NUTN website:
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