Bài giảng Soil ecology

SOIL
 ECOLOGY
 
The
 soil
 is
 the
 home
 of
 
innumerable
 forms
 of
 
plant,
 animal,
 and
 
microbial
 life.
 
 
Life
 in
 the
 soil
 is
 
amazingly
 diverse,
 

ranging
 from
 microscopic
 
single-­‐celled
 organisms
 
to
 large
 burrowing
 
animals.
 
 
As
 is
 true
 with
 organisms
 
above
 the
 ground,
 there
 
are
 well-­‐defined
 food
 
chains

 and
 compe??on
 
for
 survival.
 
 


All ecosystems have two types of organisms based
on carbon source:
(1) producers, and
(2) the consumers and decomposers.
The producers use (fix) inorganic carbon from carbon
dioxide, and are autotrophs.
The consumers and the decomposers use the carbon
fixed by the producers, such as glucose, and are
heterotrophs.


Producers
 
The
 major
 primary
 producers
 are
 vascular
 plants
 that

 use
 
solar
 energy
 to
 fix
 carbon
 from
 carbon
 dioxide
 during
 
photosynthesis.
 
 
The
 tops
 of
 plants
 provide
 food
 for
 animals
 above
 the
 soil-­‐
atmosphere
 interface.
 Plants
 produce

 roots,
 tubers,
 and
 
other
 underground
 organs
 within
 the
 soil
 that
 serve
 as
 food
 
for
 soil-­‐dwelling
 organisms.
 
A
 very
 small
 amount
 of
 carbon
 is
 fixed
 from
 carbon
 dioxide

 
by
 algae
 during
 photosynthesis
 that
 occurs
 at
 or
 near
 the
 
soil
 surface.
 Some
 bacteria
 obtain
 their
 energy
 from
 
chemical
 reac?ons,
 chemoautotrophs,
 and
 fix
 a
 ?ny
 amount
 

of
 carbon
 from
 carbon
 dioxide.
 The
 material
 produced
 by
 
the
 producers
 serves
 as
 food
 for
 the
 consumers
 and
 
decomposers.
 


Consumers
 and
 Decomposers
 

Consumers

 are,
 typically,
 animals
 that
 feed
 on
 plant
 material
 or
 on
 other
 
animals.
 For
 example,
 very
 small
 worms
 invade
 and
 eat
 living
 roots.
 
The
 worms
 might
 be
 eaten
 or

 consumed
 by
 mites
 which,
 in
 turn
 might
 be
 
consumed
 by
 cen?pedes.
 
All
 forms
 of
 dead
 organic
 materials
 are
 aFacked
 by
 the
 decomposers,
 
mainly
 by
 bacteria
 and
 fungi.

 Through
 enzyma?c
 diges?on
 (decomposi?on),
 
the
 carbon
 is
 returned
 to
 the
 atmosphere
 as
 CO2
 and
 energy
 is
 released
 as
 
heat.
 
 
The
 microorganisms
 are
 considered
 to
 be
 the

 major
 or
 ul?mate
 
decomposers.
 

 

 

 

 

 


MICROBIAL
 DECOMPOSERS
 

Bacteria
 
Bacteria
 are
 single
 celled,
 among
 the

 smallest
 
living
 organisms,
 and
 exceed
 all
 other
 soil
 
organisms
 in
 kinds
 and
 numbers.
 A
 gram
 of
 
fer?le
 soil
 commonly
 contains
 101
 to
 10
 10
 
bacteria.
 

 
Researchers have estimated that the live
weight of bacteria in soils may exceed
2,000 kilograms per hectare (2,000 pounds
per acre).
 

FIGURE
 
 Rod-­‐shaped
 bacteria
 magnified
 20,000
 
?mes.
 (Courtesy
 Dr.
 S.
 Flegler
 of
 Michigan
 
State
 University).
 


Estimates of
Amount of
Organic

Matter and
Proportions,
Dry Weight,
and Number
of Living
Organisms
in a Hectare
of Soil to a
Depth of 15
Centimeters
in a Humid
Temperate
Region


Fungi
 
Fungi
 are
 heterotrophs
 that
 vary
 greatly
 in
 size
 
and
 structure.
 Fungi
 typically

 grow
 or
 
germinate
 from
 spores
 and
 form
 a
 threadlike
 
structure,
 called
 the
 mycelium.
 Whereas the
activity of bacteria is limited to surface
erosion in place, fungi readily extend their
tissue and penetrate into the surrounding
environment

 
The most common fungi are molds and
mushrooms. Mold mycelia are commonly
seen growing on bread, clothing, or leather
goods. Rhizopus is a common mold that
grows on bread and in soil
 
FIGURE
 

 A
 soil
 fungus
 showing
 mycelium
 and
 
reproduc?ve
 structures
 that
 contain
 spores.
 
(Photograph
 courtesy
 of
 Michigan
 State
 
University
 Pes?cide
 Research
 Electron
 
Microscope
 Laboratory.)
 


Mushroom fungi have an

underground mycelium that
absorbs nutrients and water,
and an above-ground
mushroom that contains
reproductive spores. Many
mushrooms are collected for
food, such as the shaggy-mane
mushroom shown in Figure:
Mushroom fungal caps that
contain spores-an edible type.
!


Ac?nomycetes
 
Ac?nomycetes
 refers
 to
 a
 group
 of
 bacteria
 with
 a
 superficial
 resemblance
 
to
 fungi.
 The

 ac?nomycetes
 resemble
 bacteria
 in
 that
 they
 have
 a
 very
 
simple
 cell
 structure
 and
 are
 about
 the
 same
 size
 in
 cross
 sec?on.
 
 
The actinomycetes resemble bacteria in that they have a very simple
cell structure and are about the same size in cross section.
They resemble filamentous fungi in that they produce a branched
filamentous network.
 
Actinomycetes are in great abundance in soils, as shown in Table.

They make up as much as 50 percent of the colonies that develop
on plates containing artificial media and inoculated with a soil
extract. The numbers of actinomycetes may vary from 1 to 36
million per gram of soil. Although there is evidence that
actinomycetes are abundant in soils, it is generally concluded they
that are not as important as bacteria and fungi as decomposers. It
appears that actinomycetes are much less competitive than the
bacteria and fungi when fresh additions of organic matter are
added to soils. Only when very resistant materials remain do
actinomycetes have good competitive ability.


Vertical Distribution of Decomposers in the Soil
The surface of the soil is the interface between the
lithosphere and the atmosphere. At or near this
interface, the quantity of living matter is greater than at
any region above or below. As a consequence, the A
horizon contains more organic debris or food sources
than do the B and C horizons. Although other factors
besides food supply influence activity and numbers of
microorganisms, the greatest abundance of
decomposers typically occurs in the A horizon


Distribution of microorganisms in the A, B, and C horizons of
a cultivated grassland soil. All values refer to the number of
organisms per gram of air-dry soil.


SOIL

 ANIMALS
 

Soil
 animals
 are
 numerous
 in
 soils
 (see
 Table).
 Soil
 animals
 can
 be
 
considered
 both
 consumers
 and
 decomposers
 because
 they
 feed
 on
 
or
 consume
 organic
 maFer

 and
 some
 decomposi?on
 occurs
 in
 the
 
diges?ve
 tract.
 Animals,
 however,
 play
 a
 minor
 consumer-­‐
decomposer
 role
 in
 organic
 maFer
 decomposi?on.
 Some
 animals
 
are
 parasi?c
 vegetarians
 that
 feed
 on

 roots,
 whereas
 others
 are
 
carnivores
 that
 prey
 on
 each
 other.
 
Worms
 
There
 are
 two
 important
 kinds
 of
 worms
 in
 soils.
 Microscopic
 
roundworms,
 nematodes,
 are
 very
 abundant

 soil
 animals.
 They
 
are
 of
 economic
 importance
 because
 they
 are
 parasites
 that
 
invade
 living
 roots.
 The
 other
 important
 worm
 is
 the
 ordinary
 
earthworm.
 


Earthworms

 are
 perhaps
 the
 best
 known
 
of
 the
 larger
 soil
 animals.
 
 

Nematodes
 
(roundworms)
 are
 
microscopic
 worms
 and
 
are
 the
 most
 abundant
 
animals
 in

 soils.
 They
 
are
 round
 shaped
 with
 
a
 pointed
 posterior.
 
 


Arthropods
 
A
 high
 propor?on
 of
 soil
 animals
 is
 arthropods;
 they
 have
 an
 
exoskeleton

 and
 jointed
 legs.
 Most
 have
 a
 kind
 of
 heart
 and
 blood
 
system,
 and
 usually
 a
 developed
 nervous
 system.
 The
 most
 abundant
 
arthropods
 are
 mites
 and
 springtails.
 Other
 important

 soil
 arthropods
 
include
 spiders,
 insects
 (including
 larvae),
 millipedes,
 cen?pedes,
 wood
 
lice,
 snails,
 and
 slugs.
 

FIGURE
 Electron
 microscope
 photograph
 
of
 a
 mite
 on
 the
 top
 of

 a
 pinhead;
 
magnified
 about
 50
 to
 100
 ?mes.
 
 

FIGURE
 Springtails
 and
 eggs
 photographed
 
through
 a
 light
 microscope.
 


Millipedes
 and
 Cen?pedes
 
 

Millipedes
 and
 cen?pedes
 are
 elongate,
 fairly
 large
 soil
 animals,
 with
 
many
 pairs
 of
 legs.
 They
 are
 common
 in
 forests,
 and
 overturning
 almost
 
any
 log
 or
 stone
 will
 send

 them
 running
 for
 cover.
 Millipedes
 have
 
many
 pairs
 of
 legs
 and
 are
 mainly
 vegetarians.
 
They
 feed
 mostly
 on
 dead
 organic
 maFer,
 but
 some
 browse
 on
 fungal
 
mycelia.

 Cen?pedes
 typically
 have
 fewer
 pairs
 of
 legs
 than
 millipedes,
 
and
 are
 mainly
 carnivorous
 consumers.
 Cen?pedes
 will
 aFack
 and
 
consume
 almost
 any-­‐sized
 animal
 
that
 they
 can
 master.
 Worms

 are
 a
 favorite
 food
 of
 some
 cen?pedes.
 
The
 data
 in
 previous
 Table
 show
 that
 the
 numbers
 of
 millipedes
 and
 
cen?pedes
 are
 small
 compared
 with
 springtails
 and
 mites,
 but

 that
 their
 
biomass
 may
 be
 larger.
 

FIGURE
 
 A
 cen?pede,
 a
 common
 
carnivorous
 soil
 animal.
 


EARTH MOVING BY SOIL ANIMALS

All soil animals participate as consumers and play a
minor role in the cycling of nutrients and energy. Many of
the larger animals move soil to such an extent that they
affect soil formation

Earthworm Activity

Earthworms are probably the best known earth
movers. This activity produces thicker than normal,
dark-colored surface layers in soils
As a result of their earth-moving activities, earthworms
leave channels. Where these channels are open at the
soil surface, they can tranport water very rapidly into
and through the soil


Ants and Termites
The activities of ants and termites are, perhaps, more
important than the activities of earthworms. Ants
transport large quantities of material from within the
soil, depositing it on the surface. Some of the largest
ant mounds are about 1 meter tall and more than 1
meter in diameter. The effect of this transport is
comparable to that of earthworms in creating thick A
horizons and in burying objects lying on the soil
surface.
Rodents
Many rodents, including mice, ground
squirrels, marmots, gophers, and prairie dogs
inhabit the soil.


Ant (Formica
cinera) in a
Prairie soil in
southwestern
Wisconsin. The

sketch shows
soil horizons and
location of ant
channels;
numbers refer to
the number of
channels
observed at the
depth indicated


SUMMARY
Higher plants are the major producers contribut- ing to the supply of soil
organic matter. The microorganisms (bacteria and fungi) are the major
decomposers and are mainly responsible for the cycling of nutrients and
energy in soil ecosystems.
Soil animals play a minor role in the cycling of nutrients and energy, but
play an important role in earth-moving activities.
Nutrient cyling results in reuse of the nutrients in an ecosystem. Nutrients
are efficiently recycled in natural ecosystems. Interference of the cycle,
such as cropping and removal of nutrients in food, results in reduced soil
fertility. Manures and fertilizers are used to maintain soil fertility in
agriculture.
Soil organisms and higher plants engage in many interactions related to
disease, mycorrhiza, and nitrogen fixation, and soil organisms and higher
plants compete for the same growth factors.
A zone adjacent to plant roots with a high population of microorganisms is
the rhizosphere
Microorganisms play important environmental quality roles, such as
detoxification of chemicals and decomposition of oil from spills.

Earthworms, ants, termites, and rodents move large quantities of soil and
may greatly alter the nature of soil horizons.


Soil
 as
 a
 Microbial
 Habitat
 
Major
 things
 a
 microorganism
 needs
 in
 soil:
 

 
1) Nutrients
 

 

 

 

 a)
 Substrate
 (electron
 donor)
 –
 
 

 

 


 

 


 Organic
 -­‐
 sugars,
 amino
 acids,
 organic
 acids.
 

 Inorganic
 -­‐
 S=,

 So,
 H2,
 NH4+
 


 b)
 Electron
 acceptor
 if
 substrate
 is
 nonfermentable
 

 

 

 e.g.,
 O2,
 NO3-­‐,
 Fe(III),
 SO4=.
 

 c)
 Minerals
 and
 trace

 nutrients
 


Soil
 as
 a
 Microbial
 Habitat
 
Major
 things
 a
 microorganism
 needs
 in
 soil:
 

 
1)  Nutrients
 

 
 
2)
 Water
 

 

 

 

 
 


Soil
 as
 a
 Microbial
 Habitat
 
Major
 things
 a
 microorganism
 needs
 in
 soil:
 

 
1) Nutrients
 

 
 
2)

 Water
 

 
3)
 Favorable
 microenvironment
 

 
 
 
 
 
 -­‐
 Important
 physicochemical
 factors
 include:
 

 

 

 soil
 atmosphere
 

 

 

 pH
 

 

 

 temperature
 

 

 

 ionic
 strength
 

 

 

 redox
 poten?al
 

 

 

 light
 

 
 
Soil
 is
 a
 discon?nuous
 environment.
 
 Very
 diverse
 condi?ons
 exist
 over
 
small
 distances
 which
 allow
 a
 diverse
 popula?on
 to
 coexist.
 

Nutrient
 Sources
 in
 Soil
 
A)  Role
 of
 Soil
 Minerals
 

 Dissolu?on
 of
 minerals
 releases
 inorganic
 ions
 needed
 by
 biota
 

 

 

 e.g.,
 Fe2+,

 Mg2+,
 Ca2+
 

 
 

 
 

 
 


 CEC
 of
 inorganic
 frac?on
 serves
 as
 reservoir
 of
 ca?ons,
 

 

 

 e.g.,

 Ca2+,
 NH4+
 

 Clay
 frac?on
 can
 bind
 organic
 molecules
 and
 biochemicals.
 

 

 

 -­‐
 bound
 enzymes
 can
 have
 enzyma?c
 ac?vity.
 

B)
 Role
 of

 Soil
 Organic
 MaZer
 

 
 

 
 

 

 


 Major
 source
 of
 microbial
 nutri?on
 =
 soil
 organic
 maZer.
 

 Soil
 organic
 maZer

 =
 plant,
 animal,
 and
 microbial
 residues
 in
 various
 stages
 of
 

 

 

 decay.
 


Microorganisms
 grow
 on
 the
 surface
 
of
 soil
 par?cles
 or

 in
 pores
 

Paul
 E
 and
 Clark
 F.
 Soil
 Microbiology
 and
 Biochemistry.
 Academic
 Press,
 Inc.
 San
 Diego,
 1989,
 p15
 


Microorganisms
 and
 Soil
 Atmosphere
 

Kilham

 K.
 Soil
 Ecology.
 Cambridge
 University
 Press,
 1994.