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Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems

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Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems Robert E. Ricklefs The Economy of Nature, Fifth Edition – PowerPoint PPT presentation

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Title: Chapter 8: Nutritional Regeneration in Terrestrial and Aquatic Ecosystems


1
Chapter 8 Nutritional Regeneration in
Terrestrial and Aquatic Ecosystems
  • Robert E. Ricklefs
  • The Economy of Nature, Fifth Edition

2
Acid Rain and Forest Growth
  • Decline in forests, noted in northeastern US and
    central Europe in the 1960s, appeared correlated
    with acid rain.
  • The Clean Air Act of 1970 reduced emissions of
    sulfur oxides and particulates in the US.
  • Forests did not show signs of recovery. Why?

3
Slow Recovery of Forests from Effects of Acid Rain
  • Studies at Hubbard Brook Experimental Forest in
    New Hampshire showed why forests did not recover
    after passage of Clean Air Act
  • acidity of rain declined slowly
  • emissions of particulates declined, reducing an
    important source of calcium at Hubbard Brook
  • leaching of calcium and other nutrients by acid
    rain left lasting effects on soil fertility

4
Lessons from Hubbard Brook
  • Acidity itself is not the cause of tree death
  • long-term leaching of nutrients kills trees
  • Natural recovery will be slow on nutrient-poor
    soils
  • restoration of nutrients will require weathering
  • weathering is a slow process

5
More Lessons from Hubbard Brook
  • Effects of acid rain on soils may remain for
    years, even if causes of the problem are
    addressed.
  • Understanding nutrient cycling and regeneration
    is crucial to understanding ecosystem function.

6
Nutrient regeneration occurs in soils.
  • Nutrients are added to the soil through
    weathering of bedrock or other parent material.
  • How fast does such weathering occur?
  • estimates can be made for positive ions such as
    Ca2, K, Na, and Mg2
  • at equilibrium, net losses must be balanced by
    replenishment from weathering

7
Weathering of Ca2 at Hubbard Brook
  • Watershed budgets
  • precipitation inputs 2 kg ha-1 yr-1
  • streamflow losses 14 kg ha-1 yr-1
  • assimilation by vegetation 9 kg ha-1 yr-1
  • net removal thus 21 kg ha-1 yr-1
  • Total weathering of bedrock to offset Ca2 losses
    is 1,500 kg ha-1 yr-1 or 1 mm depth.
  • Later analyses showed this to be an overestimate
    the system was not in equilibrium.

8
Quality of detritus influences the rate of
nutrient regeneration.
  • Weathering is insufficient to supply plants with
    essential elements (Ca, Mg, K, Na, N, P, S, etc.)
    at the rates required.
  • Rapid regeneration of these elements from
    detritus is essential for ecosystem function.
  • In forests, detritus is abundant
  • includes plant debris, animal excreta, etc.
  • gt90 of plant biomass enters detritus pool

9
Breakdown of Leaf Litter
  • Breakdown is a complex process
  • leaching of soluble minerals
  • 10-30 of substances in leaves are water-soluble
  • consumption by large detritivores
  • assimilate 3-40 of energy
  • macerate detritus, speeding microbial activity
  • breakdown of woody components by fungi
  • decomposition of residue by bacteria

10
Quality of Plant Detritus
  • Litter of various species decays at different
    rates
  • weight loss in 1 yr for broadleaved species
    varied from 21 for beech to 64 for mulberry
  • needles of pines and other conifers decompose
    slowly
  • resistance to decay is largely a function of
    composition, especially lignins, which resist
    decay
  • Fungi play special roles in degrading resistant
    materials
  • fungi especially capable of degrading cellulose,
    lignins

11
Mycorrhizae
  • Mycorrhizae are mutualistic associations of fungi
    and plant roots
  • endomycorrhizae - fungus penetrates into root
    tissue
  • ectomycorrhizae - fungus forms sheath around root
  • Mycorrhizae facilitate nutrient extraction from
    nutrient-poor soils, enhancing plant production.

12
Function of Mycorrhizae
  • Mycorrhizae are effective at extracting
    nutrients
  • penetrate larger volume of soil than roots alone
  • secrete enzymes and acids, which extract
    nutrients
  • Endomycorrhizae are associated with most plants
  • apparently an ancient association
  • fungi are specialists at extracting phosphorus
  • Ectomycorrhizae are also widespread
  • sheath stores nutrients and carbon compounds
  • fungi consume substantial amount of net production

13
Climate and Nutrient Regeneration
  • Nutrient cycling is affected by climate
  • temperate and tropical ecosystems differ because
    of effects of climate on
  • weathering
  • soil properties
  • decomposition of detritus
  • In temperate soils, organic matter provides a
    persistent supply of mineral elements released
    slowly by decomposition.

14
A Tropical Paradox
  • Tropical forests are highly productive in spite
    of infertile soils
  • tropical soils are typically
  • deeply weathered
  • have little clay
  • do not retain nutrients well
  • high productivity is supported by
  • rapid regeneration of nutrients form detritus
  • rapid uptake of nutrients
  • efficient retention of nutrients by
    plants/mycorrhizae

15
Slash-and-Burn Agriculture
  • Cutting and burning of vegetation initiates the
    cycle
  • nutrients are released from felled and burned
    vegetation
  • 2-3 years of crop growth possible
  • fertility rapidly declines as nutrients are
    leached
  • upward movement of water draws iron and aluminum
    oxides upward, resulting in laterite

16
Is Slash-and-burn sustainable?
  • Traditional agriculture is sustainable
  • 2-3 years of cropping depletes soil
  • 50-100 years of forest regeneration rebuilds soil
    quality
  • Population pressures lead to acceleration of the
    cycle
  • soils are insufficiently replenished
  • soils deteriorate rapidly, requiring expensive
    fertilizer subsidies

17
Vegetation and Soil Fertility
  • Vegetation is critical to development and
    maintenance of soil fertility
  • clear-cutting of an experimental watershed at
    Hubbard Brook, NH resulted in
  • several-fold increase in stream flow
  • 3- to 20-fold increase in cation losses
  • shift from nitrogen storage to massive nitrogen
    loss
  • uncut system gained 1-3 kg N ha-1 yr-1
  • clear-cut system lost 54 kg N ha-1 yr-1

18
Soil versus Vegetation Stocks of Nutrients
  • Litter and other detritus do not form a large
    reserve of nutrients in the tropics
  • forest floor litter as percentage of vegetation
    plus detritus
  • 20 in temperate needle-leaved forests
  • 5 in temperate hardwood forests
  • 1-2 in tropical forests
  • soil to biomass ratio for phosphorus in forests
    is 23.1 in Belgium, 0.1 in Ghana

19
Eutrophic and Oligotrophic Soils
  • Tropics have both rich and poor soils
  • eutrophic (rich) soils develop in geologically
    active areas with young soils where
  • erosion is high
  • rapid weathering of bedrock adds nutrients
  • oligotrophic (poor) soils develop in old,
    geologically stable areas with old soils where
  • intense weathering of soils removes clay and
    reduces storage capacity for nutrients

20
Nutrient Retention by Vegetation
  • Retention of nutrients by vegetation is crucial
    to sustained productivity in tropics.
  • Plants retain nutrients by
  • retaining leaves
  • withdrawing nutrients before leaves are dropped
  • developing dense root mats near soil surface

21
Nutrients are regenerated from aquatic sediments.
  • Soils and aquatic sediments share similar
    regenerative processes (both processes occur in
    aqueous medium).
  • Soils and aquatic sediments differ in two
    profound ways
  • release of nutrients in soils occurs near plant
    roots in soils, far from roots in sediments
  • release of nutrients is aerobic in soils,
    anaerobic in aquatic sediments

22
Nutrients and Aquatic Productivity
  • Productivity in aquatic systems is stimulated
    when nutrients are in the photic zone, resulting
    from
  • proximity to bottom sediments
  • upwelling of nutrient-rich water
  • Regeneration of nutrients by excretion and
    decomposition may take place within the water
    column.
  • Sedimentation represents a continual drain on
    nutrients within the water column.

23
Thermal stratification hinders vertical mixing.
  • Vertical mixing is critical to replenishment of
    surface waters with nutrients from below
  • results from turbulent mixing driven by wind
  • impeded by vertical density stratification
  • may be caused by thermal stratification
  • also occurs when fresh water floats over denser
    salt water
  • Vertical mixing has positive and negative effects
    on productivity
  • nutrients brought from depths stimulate
    productivity
  • phytoplankton may be carried below photic zone

24
Stratification inhibits production.
  • Thermal stratification in temperate lakes
  • nutrients regenerated in deeper waters cannot
    reach the surface
  • vertical mixing in fall brings nutrient-rich
    water to the surface
  • Stratification in other aquatic systems
  • arctic/subarctic and tropical lakes are not
    thermally stratified and mix freely
  • in marine systems, stratified and non-stratified
    water bodies may meet, stimulating production

25
Nutrients limit production in the oceans.
  • Primary production of marine ecosystems is tied
    closely to nutrient supplies
  • nitrogen is especially limiting
  • shallow seas and areas of upwelling are
    especially productive
  • some areas of open ocean are unproductive,
    despite adequate nitrogen and phosphorus
  • iron may be limiting in some areas of open ocean
  • silicon may also be limiting, especially for
    diatoms

26
Oxygen depletion facilitates nutrient
regeneration.
  • Nutrient regeneration is facilitated as anoxic
    conditions develop in hypolimnion and sediments
    of stratified temperate lakes
  • nitrification ceases, leading to accumulation of
    ammonia
  • iron is reduced from Fe3 to Fe2
  • insoluble iron-phosphorus complexes are
    solubilized, releasing iron and phosphorus
  • These processes reverse when oxidizing conditions
    return during fall overturn.

27
Phosphorus and Trophic Status in Lakes
  • Phosphorus typically limits productivity in
    freshwater systems
  • P is especially scarce in well-oxygenated surface
    waters
  • Natural lakes exhibit a wide range of
    fertilities
  • productivity depends on
  • external nutrient inputs
  • internal regeneration of nutrients

28
Temperate lakes exhibit varied degrees of mixing.
  • Productivity depends in part on degree of mixing
    of surface and deeper waters
  • shallow lakes may lack hypolimnion and circulate
    continuously
  • somewhat deeper lakes stratify sporadically, with
    periods of mixing caused by
  • strong winds
  • occasional cold weather in summer
  • deepest lakes rarely mix completely, so
    productivity depends on external nutrient sources

29
Productivity varies in temperate lakes.
  • Lakes may be classified on a continuum from
    oligotrophic to eutrophic.
  • oligotrophic lakes are nutrient-limited and
    unproductive
  • naturally eutrophic lakes exist in a
    well-nourished and productive dynamic
    steady-state
  • human activities can lead to inappropriate
    nutrient loading resulting from
  • inputs of sewage
  • drainage from fertilized agricultural lands

30
Cultural eutrophication of lakes is harmful.
  • Nutrients stimulate primary production.
  • Production is not inherently harmful, but
  • biomass accumulates, overwhelming natural
    regenerative processes
  • untreated sewage also increases the amount of
    organic material in water
  • increased biological oxygen demand depletes
    oxygen, killing fish and other obligate aerobes

31
Estuaries and marshes are highly productive.
  • Shallow estuaries and salt marshes are among the
    most productive ecosystems on earth.
  • High production in these systems results from
  • rapid and local regeneration of nutrients
  • external loading of nutrients

32
Marshes and estuaries export their production.
  • Adjacent marine ecosystems benefit from export of
    production from marshes and estuaries. For
    example,
  • a Georgia salt marsh exported nearly 50 of its
    net primary production to surrounding marine
    systems in the form of
  • organisms
  • particulate detritus
  • dissolved organic material

33
Marshes and estuaries are critical to functioning
of marine ecosystems.
  • Marshes and estuaries are important feeding areas
    for larval and immature stages of fish and
    invertebrates, providing
  • hiding places
  • high productivity
  • These organisms later complete their life cycles
    in the sea.

34
Summary
  • Chemical and biochemical transformations are
    modified by physical and chemical conditions in
    each type of ecosystem.
  • Pathways of elements in ecosystems reflect
    patterns of nutrient cycling.

35
Summary Terrestrial and Aquatic Systems
  • In terrestrial systems
  • ecosystem metabolism is mostly aerobic
  • production is limited by regeneration of
    nutrients from soils
  • In aquatic systems
  • anaerobic respiration and regeneration of
    nutrients occurs in sediments, far from producers
  • local regeneration of nutrients occurs in water
    column
  • productivity is ultimately limited by
    regeneration of nutrients from deeper waters
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