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

Organic portion composed of: 5% 10% 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 solar-powered Respiration: C6H12O6 + O2 ENERGY + CO2 + H2O

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

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

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

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 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 < 100 500-2000 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

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.

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 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