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Learning Objectives
• List the major groups of soil organisms …• Identify the roles of organisms • Draw a simplified soil food web ...• Describe the conditions affecting growth…• Discuss the beneficial functions …
Classification – A means to make sense of the diversity
• Taxonomic groups (plants, animals, bacteria, fungi, protista)
• Functional groups & size (microflora vs. macroflora/fauna) – Table 11.1
• Carbon/energy source (detritivores vs. fungivores; autotrophic vs. heterotrophic) – Fig 11.1, Table 11.3
• Environmental tolerance (thermophiles, anaerobes, etc.)
Eukaryotes• Protists• Fungi• Plants• Animals
Prokaryotes• Bacteria• Archaea
Taxonomic groups:
(Pace 1997)
Phylogenic Tree of Life
Important Groups of Soil Organisms
• Macroflora
• Microflora
• Macrofauna
Vascular plants, mosses (autotrophs)
Vascular plants (root hairs), algae, actinomycetes, bacteria, and fungi (auto- and heterotrophs)
Vertebrates, arthropods, earthworms, snails… (herbivores, detritivores, predators)
Size, functional groups:
• Mesofauna
• Microfauna
Arthropods, worms (detritivores, predators)
Nematodes, protazoa… (detritivores, fungivores, bacterivores, predators)
A cup of soil contains...
Bacteria
Fungi
Protozoa
Nematodes
Arthropods
Earthworms
{
{
{
Microfauna
Macro- and mesofauna
See text Table 11.1
200 billion
100,000 meters
20 million
100,000
50,000
<1
Microflora, or “microbes”
Immobile organisms all primarily found in the rhizosphere, the zone of soil closest to plant roots
Size:
Relative Sizes
http://commtechlab.msu.edu/sites/dlc-me/zoo/zdrr0101.html
Animated gif – view in slideshow mode
Note ruler for scale
• heterotrophs (bacteria, fungi) & autotrophs (algae, cyanobacteria)
• the primary decomposers• release plant available nutrients
• stabilize soil aggregates
Microflora
Soil fungiSoil bacteria
Aggregates held together by:– Fungal hyphae– Bacterial “glues”– Organic matter
sand
silt
hyphaeclay
bacteria
Microflora –
Fungi
• The major agent of decay in acid environs• Network of hyphae: improves soil structure• Decomposition of cellulose!!!• Can compete with higher plants for N
N.B. – Fungi are in their own separate kingdom from plants: they are non-photosynthetic, and their RNA is actually more like animals, than like plants.
Microflora –
Bacteria
• Exist in both forest and grassland soils• Aerobic, anaerobic, and facultative forms• Autotrophic and heterotrophic forms• Most do best under high Ca2+, high pH• Do best when soil temp 20-40C (68-100F) but
seldom killed by temperature extremes
Microflora –
Fungi vs. bacteriaFungi Bacteria
Tube-like body; hyphae
Aerobic only
Generally slower growth rate
Single-celled, can form colonies
Aerobic, anaerobic, and facultative species
Rapid regeneration time (hours); can respond quickly to nutrient additions
Microflora –
Amoebae
CiliateFlagellate
Nematode
• heterotrophs; some parasitic
• feed on bacteria and fungi
• release plant nutrients – protozoa KEY for N
Nematode
Microfauna
eeee!
– Widely distributed in forest soils– Saprophytic and parasitic groups– Some predatory species attack tree roots and
cause damage
Nematodes (non-segmented, round worms)
Microfauna –
– Most abundant of all soil fauna– One-celled– Feed on bacteria– Up to 30% of all mineralized N from protozoa
Protozoa
Microfauna –
Collembola (springtails)
Fungus feeding mite
• heterotrophs (detritivores, predators)• feed on fungi, protozoa, nematodes, mites• important in regulating populations of everything smaller
Nematode feeding mite
Mesofauna
Photo by Suzanne Paisley
• heterotrophs
• shred plant material
• feed on bacteria and fungi associated with organic matter
Macrofauna
Earthworms
• Probably the most important component of soil fauna (not in acid soils, not in very dry soils)
• Pass as much as 30 tons/ha of soil through their bodies each year
• Excreted casts higher in N, P, K, Ca, Mg, pH, & CEC• Promote good soil structure and aeration
Macrofauna –
Earthworm casts vs. soil
Characteristic Earthworm casts Soils
silt & clay (%) 22.2
Bulk density (g/cm3) 1.28
Structural stability 65
CEC (cmolc/kg) 3.5
From text Table 11.6
38.8
1.11
849
13.8
Macrofauna –
Addo National Park, South Africa
Tembe Elephant ReserveKwaZulu Natal, South Africa
Dung Beetles
Amboseli National Park, Kenya
Macrofauna –
Key disposer of elephant dung and so a protected species!(you can imagine the ‘or else’…)
Influence of soil biota on soil processesNutrient cycling Soil structure
Microflora
Microfauna
Mesofauna
Macrofauna
Break up O.M., mineralize and immobilize nutrients
Bind aggregates, hyphae entangle particles
Regulate bacterial and fungal populations
Indirectly affect structure
Regulate above pops.; fragment plant tissue
Fecal pellets, pores
Fragment plant tissue Mix O.M. and mineral soil; pores; feces
Ecosystem Function –
Recall: Rate of decomposition depends on –
• Physical and chemical nature of the litter material
• Temperature and moisture of the soil environment
• Aeration (vs. anaerobic)
• The kinds and numbers of soil fauna
More bugs, and more different kinds of bugs, means more decomposition
Ecosystem Function –