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Soil Biology – A Primer* Who is who & What do they do? * primer isn’t a complete review of Ch 11 Learning Objectives • • • • • List the major groups of soil organisms … Identify the roles of organisms Draw a simplified soil food web ... Describe the conditions affecting growth… Discuss the beneficial functions … Classification – A means to make sense of the diversity • Taxonomic groups (plants, animals, bacteria, fungi, protista) • Functional groups & size (microflora vs. macroflora/fauna) – Table 11.1 • Carbon/energy source (detritivores vs. fungivores; autotrophic vs. heterotrophic) – Fig 11.1, Table 11.3 • Environmental tolerance (thermophiles, anaerobes, etc.) Taxonomic groups: Phylogenic Tree of Life Prokaryotes • Bacteria • Archaea Eukaryotes • • • • Protists Fungi Plants Animals (Pace 1997) Taxonomic groups: Biological Kingdoms Size, functional groups: Important Groups of Soil Organisms • Macroflora Vascular plants, mosses (autotrophs) • Microflora Vascular plants (root hairs), algae, actinomycetes, bacteria, and fungi (auto- and heterotrophs) • Macrofauna Vertebrates, arthropods, earthworms, snails… (herbivores, detritivores, predators) • Mesofauna Arthropods, worms (detritivores, predators) • Microfauna Nematodes, protazoa… (detritivores, fungivores, bacterivores, predators) Size: A cup of soil contains... Microflora, or “microbes” Microfauna Macro- and mesofauna Bacteria 200 billion { Fungi 100,000 meters See text Table 11.1 Protozoa 20 million { Nematodes 100,000 Arthropods { Earthworms 50,000 <1 Immobile organisms all primarily found in the rhizosphere, the zone of soil closest to plant roots Relative Sizes Animated gif – view in slideshow mode Note ruler for scale http://commtechlab.msu.edu/sites/dlc-me/zoo/zdrr0101.html Microflora • heterotrophs (bacteria, fungi) & autotrophs (algae, cyanobacteria) • the primary decomposers • release plant available nutrients • stabilize soil aggregates Soil bacteria Soil fungi Microflora – hyphae clay Aggregates held together by: sand – Fungal hyphae – Bacterial “glues” – Organic matter silt bacteria Microflora – Fungi • • • • The major agent of decay in acid environs Network of hyphae: improves soil structure Decomposition of cellulose!!! Can compete with higher plants for N N.B. – Fungi are in their own separate kingdom from plants: they are nonphotosynthetic, and their RNA is actually more like animals, than like plants. Microflora – Bacteria • • • • • Exist in both forest and grassland soils Aerobic, anaerobic, and facultative forms Autotrophic and heterotrophic forms Most do best under high Ca2+, high pH Do best when soil temp 20-40C (68-100F) but seldom killed by temperature extremes Microflora – Fungi vs. bacteria Fungi Bacteria Tube-like body; hyphae Single-celled, can form colonies Aerobic only Aerobic, anaerobic, and facultative species Generally slower growth rate Rapid regeneration time (hours); can respond quickly to nutrient additions Microfauna • heterotrophs; some parasitic • feed on bacteria and fungi Ciliate Flagellate • release plant nutrients – protozoa KEY for N Amoebae Nematode Nematode eeee! Microfauna – Nematodes (non-segmented, round worms) – Widely distributed in forest soils – Saprophytic and parasitic groups – Some predatory species attack tree roots and cause damage Microfauna – Protozoa – – – – Most abundant of all soil fauna One-celled Feed on bacteria Up to 30% of all mineralized N from protozoa Mesofauna • heterotrophs (detritivores, predators) • feed on fungi, protozoa, nematodes, mites • important in regulating populations of everything smaller Fungus feeding mite Nematode feeding mite Collembola (springtails) Macrofauna • heterotrophs • shred plant material • feed on bacteria and fungi associated with organic matter Photo by Suzanne Paisley Macrofauna – Earthworms • Probably the most important component of soil fauna (not in acid soils, not in very dry soils) • Pass as much as 30 tons/ha of soil through their bodies each year • Excreted casts higher in N, P, K, Ca, Mg, pH, & CEC • Promote good soil structure and aeration Macrofauna – Earthworm casts vs. soil Characteristic Earthworm casts Soils silt & clay (%) 38.8 22.2 Bulk density (g/cm3) 1.11 Structural stability 849 CEC (cmolc/kg) From text Table 11.6 13.8 1.28 65 3.5 Macrofauna – Dung Beetles Key disposer of elephant dung and so a protected species! (you can imagine the ‘or else’…) Amboseli National Park, Kenya Tembe Elephant Reserve KwaZulu Natal, South Africa Addo National Park, South Africa – Mendenhall Ecosystem Function – Influence of soil biota on soil processes Nutrient cycling Soil structure Microflora Break up O.M., mineralize and immobilize nutrients Bind aggregates, hyphae entangle particles Microfauna Regulate bacterial and fungal populations Indirectly affect structure Mesofauna Regulate above pops.; fragment plant tissue Fecal pellets, pores Macrofauna Fragment plant tissue Mix O.M. and mineral soil; pores; feces Ecosystem Function – Recall: Rate of decomposition depends on – • • • Physical and chemical nature of the litter material Temperature and moisture of the soil environment Aeration (vs. anaerobic) • The kinds and numbers of soil fauna More bugs, and more different kinds of bugs, means more decomposition Ecosystem Function – Soil Food Web See also text Fig 11.1 Ecosystem geography – Some generalizations . . . • • • • Forested soils more biologically diverse Forested soils dominated by fungi Faunal biomass (and activity) greater per ha in grasslands Cultivated soils least diverse, less biomass, fewer organisms cf. text Table 11.4 (p. 453)