Transcript Soil Food Web

Soil Food
Web
and “Biotic Regulation”
“Land, then, is not merely soil; it is a foundation of energy flowing through a
circuit of soils, plants and animals”. Aldo Leopold
Organic portion composed of:
10%
5%
85%
Humus & decomposing
organic litter
“organic”
 What
is special about Organic
compounds? They have ENERGY
 Food
chain passes energy along through
photosynthesis and respiration
Green plants can directly use sun
Photosynthesis:
CO2 + H2O + solar energy
C6H12O6 + O2
organic !
Rest of us are only indirectly solarpowered
Respiration:
C6H12O6 + O2
ENERGY + CO2 + H2O
Fourth-order consumer
Secondary consumer
heterotrophs
Tertiary consumer
Primary consumer
green plants;
photosynthetic
bacteria and algae
autotrophs
Primary Producer
Food Chain
Concept
Energy is passed from one
trophic level to the next.
 Healthy
soil
function depends
on interactions of
many organisms
Mite eating a springtail
A Complex Soil Food Web
in an old growth Douglas fir forest
Complexity refers
to number of species
& kinds of species
Higher complexity
means more energy
transfer
Complexity of the Soil Food Web in
Several Ecosystems
Soil food web complexity is desirable. Why?
1. What do plants do for soil? What does soil do for plants?
2. Nutrient cycling is more rapid in complex systems
3. More competition for organisms that cause disease
4. Biota can retain and “hold” nutrients
5. Improves “tilth”
structure = aggregate stability
provided by glues, worm burrows, castings
increases ability to hold water, allow drainage, move
nutrients, let roots in
Typical Numbers of Soil Organisms in
Healthy Ecosystems
Ag Land
Prairie
Forest
Organisms per gram (teaspoon) of soil
Bacteria 100 mil. -1 bil.
Fungi Several yards
Protozoa 1000’s
Nematodes 10-20
100 mil. -1 bil.
100 mil. -1 bil.
10s – 100’s of yds 1-40 miles
(in conifers)
1000’s
100,000’s
10’s – 100’s
100’s
Organisms per square foot
Arthropods < 100
Earthworms 5-30
500-2000
10,000-25,000
10-50
10-50
(0 in conifers)
Biomass of Soil Organisms
in Four Ecosystems
Where are soil biota?
 Litter:
fungi important here
 Most are in top several inches (A horizon)
 Rhizosphere
 Humus : fungi
 surfaces of soil aggregates (blocks,
crumbs, plates, etc)
 Pore spaces
rhizosphere
1/10 inch
Exudates: carbohydrates and proteins secreted by roots
attracts bacteria, fungi, nematodes, protozoa
Bacteria and fungi are like little fertilizer bags
Nematodes and protozoa eat bacteria & fungi and excrete the fertilizer
Rhizosphere
Where roots and soil meet
Cells, proteins, sugars
released by young root tip