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SOIL BIOLOGY AND ECOLOGY
Introduction
Importance
of Soil Organisms
Diversity of Soil Organisms
Relationships between Organisms
Characteristics of Soil Organisms
Abundance and Biomass of Organisms
Methods for Quantifying Organisms
Introduction
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Soil organisms are the organisms that spend all or part
of their lives in the soil environment.
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Soil animals (fauna) range in size from macrofauna
(moles, earthworms, millipedes) through mesofauna
(springtails and mites) to microfauna (such as
nematodes and protozoan)
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Soil plants (flora) include roots of higher plants, algae,
fungi, bacteria, and actinomycetes
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Soil is an ecosystem in which these different creatures
interact and contribute to the global cycles that make
all life possible
Interactions of Organisms
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Herbivores- organisms that subsist on living plants
Detritivores- organisms that subsist on detritus
Fungivores- organisms that feed on fungi
Bacterivores- organisms that feed on bacteria
Predators- organisms that consume other animals
Parasites- organisms that live off of other organisms
but not consume them.
Every handful of soil is likely to contain billions of
organisms with representatives from every phylum
of living things (See table on next slide).
Diversity of Organisms in Ecosystems
Importance of Ecosystem
Species Diversity
 A high species diversity indicates that organisms
present are fairly evenly distributed among a
large number of species.
 Species diversity usually indicates a high degree
of functional diversity.
 Capacity to carry out a wide array of processes.
 Functional redundancy
 This leads to ecosystem stability and resilience.
 High biological diversity is an indicator of soil
quality.
Importance of Soil Organisms
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Decomposition of plant and animal residues.
Release of nutrients in forms available to plants.
Synthesis of new organic compounds.
Humus formation causing increased nutrient holding
capacity, better soil structure and water holding
capacity.
Pollution and environmental aspects: decomposition
of sewage, manure, and pesticides.
Nitrogen fixation.
Soil Fauna
Classes of Soil Fauna
Soil fauna belong to one of three size
limit classes:
Macrofauna (>2mm, width)
 Mesofauna (0.1-2mm, width)
 Microfauna (<0.1mm, width)
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Macrofauana
Major specific groups of
macrofauna are:
 Vertebrates
–gophers, moles, mice,
 Arthropods –ants, beetles,
termites, centipedes, etc
 Annelids -earthworms
 Mollusks –snails, slugs
Functions of Macrofauna
1.
Pulverize, mix, and granulate soil and
incorporate O.M. into the lower
horizons.
2.
Provide large channels through which
air and water can move freely.
3.
Partially digest organic residues and
leave their excrement for microbial
degradation.
Mesofauna
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Major specific groups of mesofauna
are:
Arthropods –mites, collembola (springtails), protura
 Annelids –enchytraeid (pot) worms
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Functions of Mesofauna
1. First line of attack on litter on the
forest floor
2. Important regulators of some
microfaunal biomass and composition
Microfauna
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Major specific groups of animals
in this class are:
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Nematodes
Rotifera
Protozoa
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Small, mostly microscopic
animals
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Soil microfauna is important for
the cycling of nutrients in
ecosystems
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Control populations of microflora
Nematodes
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very small (usually microscopic) thread-like
worms
most live on dead O.M.
Nematodes are predators of the microflora,
other nematodes, and protozoa, thereby
acting as biological control for those
organisms
Protozoa
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The smallest, simplest forms of animallike life.
Most numerous of soil fauna
Biomass 20-200 kg/ha
Some protozoa are predators on soil
bacteria, especially in the area
immediately around plant roots (the
rhizosphere).
Protozoa
Rotifers
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Microscopic animals that thrive in
fresh water environments, and
moist swampy soil conditions.
The diet of rotifers most consists
of dead or decomposing organic
materials, as well as unicellular
algae and other phytoplankton
Rotifers are in turn prey to
secondary consumers, including
shrimp and crabs
Rotifera
Soil Flora
Classes of Soil Flora
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Soil flora belong to one of two classes:
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Macroflora (>0.1mm, width)
 Vascular plants –feeder roots
 Bryophytes -mosses
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Microflora (<0.1mm, width)
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Vascular plants –root hairs
Algae –yellow, green, diatoms etc
Fungi –yeast, moulds, mushrooms etc
Actinomycetes
Bacteria –aerobes and anaerobes
Soil Microflora
Algae
They are microscopic (2-20µm in size)
Algae consist of eukaryotic cells (nuclear membrane)
many contain chlorophyll
10 - 500 kg/ha algae, almost all at surface,
because they need light
 perform best in moist conditions where they
swim
 3 most important groups are: green, yellowgreen, & diatoms (some form association with fungi ~lichens)
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 Green algae are the most prevalent in normal soils
 Diatoms are common in neutral to alkaline OM rich soils
 Algae add OM to soil, excrete polysaccharides
that favor aggregation and structure formation
Fungi
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Physical appearance looks more like plants, but
they do not contain chlorophyll
They are eukaryotes with a nuclear membrane
and cell walls
They are heterotrophs
Form net-like mycelial growth
Fungi biomass ~ 1,000-15,000 kg/ha in the top
soil
Tolerant of very dry soil conditions and grow
over a wide range of pH
Three main groups of fungi are yeasts, molds,
and mushrooms.
They play vital role in the decomposition of
fresh plant residues, humus formation,
aggregate stabilization, and nutrient cycling
Bacteria
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Very small-single cell organisms that thrive in all soil
conditions (considered plant because of spores)
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They are prokaryotic (no cell membrane)
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most are heterotrophic, some autotrophic e.g.,
nitrification, and sulfur oxidation
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-exist as clumps, mats, and filaments called colonies
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Biomass ~400-5000 kg/ha bacteria, but have ability to
increase in numbers rapidly
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Bacteria are involved in a broad range of enzymatic
processes in soils
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Participate in many transformations in soil (e.g., some
bacteria ~mainly rhizobium in association with
legumes, fix atmospheric nitrogen)
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They specialize in the breakdown of hydrocarbons in soil
Mycorrhizae
Mycorrhizae "fungus root" - an
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economically important association
between numerous fungi and the roots of
higher plants.
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The plant roots exude sugars to feed the
fungi and the fungi carry in soil nutrients
which might otherwise by deficient in the
plant.
Types of Mycorrhizae
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Endomycorrhizae - penetrate plant cells
Ectomycorrhizae - grow into intercellular
spaces but do not penetrate cells.
They enhance plant uptake of nutrients
They enhance water uptake by plants
They protect plants from uptake of toxins
Actinomycetes
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Actinomycetes are filamentous like
fungi but threads are smaller in size.
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They are like bacteria in that they are
single celled and about same size,
and have no nuclear membrane.
They are heterotrophs that live mostly on decaying
organic matter.
They develop best in moist, well aerated soil (do
poorly below pH 6).
-biomass ~ 5000 kg/ha
Actinomycetes produce antibiotic compounds that kill
other microorganisms ( actinomycin, neomycin,
streptomycin)
They participate in atmospheric N-fixation
Relationships between
organisms
The food web
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The activities of soil organisms (flora and fauna) are
related in the food chain or food web.
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As one organism eats another, nutrients and energy are passed
from one trophic level to a higher one (from the primary
producers through the primary and secondary consumers to the
tertiary consumers).
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The food web or food chain illustrates how various soil
organisms are involved in the breakdown of plant tissue, the
formation of humus, and the cycling of carbon and nutrients.
Primary Producers
 Plants play principal role as primary
producers
 They produce organic matter (C, chemical energy
that other organisms depend upon).
 Primary producers form the food base for the
entire food chain.
 Photosynthesize, (CO2 water, sunlight)
 Other primary producers are mosses, algae,
lichens, and photosynthetic bacteria.
 Some primary producers can use inorganic
chemical reactions to obtain energy for making
living cells
Primary Consumers
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Primary consumers are the macro & mesofauna, and
microflora that use the energy produced and stored in
the plant residues (primary producers).
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Some primary consumers are Herbivores:
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termites, ants, beetle larvae, woodchucks, moles, nematodes mice
that eat above ground plant parts, earthworms, other insects
Some primary consumers are Detritivore
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The macro- and mesofauna chew up big pieces into little pieces for
microflora to follow in the attack.
Source of food is debris of dead tissues (detritus) left in soil.
The detritus contains microorganisms that are the source of
nutrients.
Some primary consumers are Saprophytic (feeding on
decaying OM eg, bacteria, fungi)
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Feed on detritus, dead animals, feces by forming colonies which
may then provide food for secondary consumers
Secondary Consumers
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The bodies of primary consumers become food sources for
secondary consumers (predators and parasites in the soil).
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Secondary consumers include microflora such as bacteria,
fungi, actinomycetes that eat leftovers and waste from
primary consumers.
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Secondary consumers also include carnivores such as
centipedes and mites, spiders, snails (usually prey on primary
consumers, small insects or nematodes)
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Secondary consumers include Microphytic feeders (organisms
that feed on microflora as their food) eg., collembola, mites,
termites, nematodes, protozoa, eat fungi and bacteria, thereby
checking the activities and growth of their populations.
Tertiary Consumers
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Secondary consumers are prey for other
carnivores- tertiary consumers.
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E.g., ants consume centipedes, spiders, mites and scorpions, all of
which can prey on primary and secondary consumers.
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Some species of birds (Robins) specialize in
eating soil animals such as beetles and
earthworms from their burrows.
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Microflora is involved at every level of the
process (ultimate decomposers)
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directly attacking plant tissues and bodies of dead animals
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active in the digestive tracts of soil animals
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Decomposing animal feces
Abundance and Biomass of Soil
Organisms
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Soil biomass is the living fraction of the soil.
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This living fraction is generally related to the
total organic matter in the soil (1-8% of
organic matter).
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Soil organism numbers in general depend on
the types of organism as well as the
environment of the organisms.
Factors affecting Organism
Abundance and Biomass
Amount and quality of food available
Physical factors
 Moisture and temperature
Biotic factors
 Predation and competition
Soil chemical factors
 Acidity, salinity, dissolved nutrients
Sources of Carbon and Energy
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Heterotrophic - obtain their energy and
carbon from the breakdown of organic
materials.
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Autotrophic - obtain their energy and carbon
from other sources, such as energy from the
sun and carbon from CO2. Examples:
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Photoautotrophs - use solar energy.
Chemoautotrophs – obtain C from CO2 and get
energy from oxidation of inorganic compounds such
as iron (FeII), sulfur (H2S), nitrogen (NH4 & NO3).
Methods for quantifying soil
organisms:
Different methods are used (Each method have their value
depending on objective)
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Organism Numbers - plate counts; get a sample, place
on agar plate by streaking, incubate and count colonies
(e.g., Bacteria = 108-109 g-1 soil).
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Biomass - Weight of organisms per unit area or volume
(by fumigation); e.g., Bacteria = 400-5000 kg/ha
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Metabolic Activity - Measurement of some end product
given off by the organism; e.g.,
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measure CO2 evolution for respiration,