Soil Biology – A Primer*Who is who

&

What do they do?

* primer isn’t a complete review of Ch 11

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

Biological Kingdoms

Taxonomic groups:

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’…)

– Mendenhall

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 –

Soil Food Web See also text Fig 11.1Ecosystem Function –

Some generalizations . . .

• Forested soils more biologically diverse• Forested soils dominated by fungi• Faunal biomass (and activity) greater per ha in grasslands• Cultivated soils least diverse, less biomass, fewer

organisms

cf. text Table 11.4 (p. 453)

Ecosystem geography –