Transcript Organisms
Soil is home to a lot of critters Soil Organisms Chapter 11 Classification Terms Energy source Autotrophs Heterotrophs Animal or plant Fauna or flora Size MacroMesoMicro- And the relationship among soil organisms may be discussed in terms of their position in the food web. Herbivore? Measures of Biologic Activity Number per mass of soil Biomass per volume or area Respiration CO2 Organisms log (# / g) kg / ha ___________________________________ Microflora Bacteria Actinomycetes Fungi Algae 8-9 7-8 5-6 4-5 log 104 = 4, right? 400 - 5000 400 - 5000 1000 - 20000 10 - 500 That’s a lots a bugs. Assuming a density of ~ 1 g cm-3, maybe up to 30 m3 per ha! Organisms log (# / g) kg / ha ___________________________________ Fauna Protozoa Nematodes Earthworms Others 4-5 1-2 20 - 200 10 - 100 100 - 2000 20 - 400 Fauna Benefits Aid decomposition of residue Better aeration and drainage Increased infiltration Harms Channels are preferential flow pathways that speed up contaminant transport The latter can be disputed –linings of channels are highly adsorptive of chemicals. Termites Common in tropical and subtropical areas Generally do not consume living plants but strip the soil of plant residues Do not aid nutrient availability Mostly a nuisance Nematodes Tiny roundworms Most harmless to plants but some attack roots Leads to invasion by pathogens and stunted root and shoot growth Protozoa Thrive in moist, aerated soil near surface Minor biological activity in soil Flora Roots Vary in size to root hairs (10 - 50 µm) Root residues Major source of organic matter Living root system also adds organic substances to rhizosphere Zone within 1 - 2 mm of root Biologically and chemically different from bulk soil due to rhizodeposition Organic compounds Cells lost from root Soil on uprooted plant approximates rhizosphere Algae Green, yellow green and diatoms Eucaryotic Photosynthetic Gotta be at surface, right? Live at or near the soil surface Fungi Eukaryotic, aerobic and non-photosynthetic Yeasts and filamentous fungi Molds and mushrooms Filaments are long, threadlike structures called hyphae Mass of hyphae called mycelium Filamentous fungi produce spores Fruiting bodies of molds or mushrooms? Important decomposers of organic matter Some produce antibiotics and toxins including aflatoxin (carcinogen) Others cause wilts and root rots Mycorrhizae Symbiotic association between certain fungi and plant roots (fungus root) Fungus utilizes photosynthates but increases availability of nutrients, especially P The two major types ectomycorrhizae and endomycorrhizae Ectomycorrhizae are associated with trees Form a mantle around plant roots Hyphae invade the intracellular space of roots but do not invade cortical cells Endomycorrhiza hyphae invade root cells Form branched arbuscules inside cells Common with agronomic / horticultural crops Benefit greatest in infertile soil Actinomycetes Filamentous but unlike fungi are prokaryotic Sensitive to low pH Bacteria Single-celled prokaryotes Small (0.5 - 5.0 µm) Coccus Bacillus Spirilla spherical rod spiral Some autotrophic but most heterotrophic Bacteria, fungi and actinomycetes major decomposers of organic matter So, release nutrients Important transformations of bacteria Nitrogen fixation N2 + 6H+ + 6e → 2NH3 Nitrification → NO3- + H2O + 2H+ NH4+ + 2O2 Denitrification → N2O → N2 NO → NO 3 Sulfur oxidation →2H2SO4 2S + 3O2 + 2H2O Benefits of Soil Organisms Organic matter decomposition Releases nutrients bound in organic matter mineralization Nutrient elements like N, S and P are released in simple inorganic (mineral) forms that are available to plants and other soil organisms, including other microorganisms. Adverse Effects Diseases Competition for nutrients Yes, microbes do compete with plants for same nutrients.