1. and “Biotic Regulation”
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

2. Organic portion composed of:5%
10%
85%
Humus & decomposing
organic litter

3. “organic” What is special about Organic compounds? They have ENERGY
Food chain passes energy along through photosynthesis and respiration

4. Green plants can directly use sun
Photosynthesis:
CO2 + H2O + solar energy C6H12O6 + O2
organic !

5. Rest of us are only indirectly solar-powered
Respiration:
C6H12O6 + O2 ENERGY + CO2 + H2O

6. Fourth-order consumer
Tertiary consumer
heterotrophs
Secondary consumer
Primary consumer
Primary Producer
green plants;
photosynthetic
bacteria and algae
autotrophs

7. Food Chain Concept Energy is passed from one
trophic level to the next.

9. Healthy soil function depends on interactions of many organisms
Mite eating a springtail

10. 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
Image courtesy of USDA-NRCS.
File name: CompFW.jpg, 509K. (Also: CompFWa.jpg at 478K, and ComplexWeb.eps at 272K.)

11. Complexity of the Soil Food Web in Several Ecosystems
File name: Funct.jpg 382K
Image courtesy of USDA-NRCS

12. 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

13. 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
10s – 100’s of yds
1-40 miles
(in conifers)
Protozoa
1000’s
100,000’s
Nematodes
10-20
10’s – 100’s
100’s
Organisms per square foot
Arthropods
< 100
10,000-25,000
Earthworms
5-30
10-50
(0 in conifers)
Each type of ecosystem has a characteristic food web structure.
Credit: USDA Natural Resources Conservation Service

14. Biomass of Soil Organisms in Four Ecosystems
File name: biomassB.jpg, 237K (Also: Biomass.jpg at 299K and B-1 at 740KB)
Image courtesy of USDA-NRCS.

15. 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

16. 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

17. Rhizosphere Where roots and soil meet Cells, proteins, sugars
released by young root tip
The rhizosphere is the narrow region where roots and soil meet. Soil biological activity is concentrated here. This photo shows cells, proteins, and sugars being released by the young root tip.
Credit: No. 53 from Soil Microbiology and Biochemistry Slide Set.1976 J.P. Martin, et al., eds. SSSA, Madison, WI.
File name: SST