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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