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The soil is the home of innumerable forms of plant, animal, and microbial life. Life in the soil is amazingly diverse, ranging from microscopic single‐celled organisms to large burrowing animals.
SOIL ECOLOGY The soil is the home of innumerable forms of plant, animal, and microbial life Life in the soil is amazingly diverse, ranging from microscopic single-‐celled organisms to large burrowing animals As is true with organisms above the ground, there are well-‐defined food chains and compe??on for survival All ecosystems have two types of organisms based on carbon source: (1) producers, and (2) the consumers and decomposers The producers use (fix) inorganic carbon from carbon dioxide, and are autotrophs The consumers and the decomposers use the carbon fixed by the producers, such as glucose, and are heterotrophs Producers The major primary producers are vascular plants that use solar energy to fix carbon from carbon dioxide during photosynthesis The tops of plants provide food for animals above the soil-‐ atmosphere interface Plants produce roots, tubers, and other underground organs within the soil that serve as food for soil-‐dwelling organisms A very small amount of carbon is fixed from carbon dioxide by algae during photosynthesis that occurs at or near the soil surface Some bacteria obtain their energy from chemical reac?ons, chemoautotrophs, and fix a ?ny amount of carbon from carbon dioxide The material produced by the producers serves as food for the consumers and decomposers Consumers and Decomposers Consumers are, typically, animals that feed on plant material or on other animals For example, very small worms invade and eat living roots The worms might be eaten or consumed by mites which, in turn might be consumed by cen?pedes All forms of dead organic materials are aFacked by the decomposers, mainly by bacteria and fungi Through enzyma?c diges?on (decomposi?on), the carbon is returned to the atmosphere as CO2 and energy is released as heat The microorganisms are considered to be the major or ul?mate decomposers MICROBIAL DECOMPOSERS Bacteria Bacteria are single celled, among the smallest living organisms, and exceed all other soil organisms in kinds and numbers A gram of fer?le soil commonly contains 101 to 10 10 bacteria Researchers have estimated that the live weight of bacteria in soils may exceed 2,000 kilograms per hectare (2,000 pounds per acre) FIGURE Rod-‐shaped bacteria magnified 20,000 ?mes (Courtesy Dr S Flegler of Michigan State University) Estimates of Amount of Organic Matter and Proportions, Dry Weight, and Number of Living Organisms in a Hectare of Soil to a Depth of 15 Centimeters in a Humid Temperate Region Fungi Fungi are heterotrophs that vary greatly in size and structure Fungi typically grow or germinate from spores and form a threadlike structure, called the mycelium Whereas the activity of bacteria is limited to surface erosion in place, fungi readily extend their tissue and penetrate into the surrounding environment The most common fungi are molds and mushrooms Mold mycelia are commonly seen growing on bread, clothing, or leather goods Rhizopus is a common mold that grows on bread and in soil FIGURE A soil fungus showing mycelium and reproduc?ve structures that contain spores (Photograph courtesy of Michigan State University Pes?cide Research Electron Microscope Laboratory.) Mushroom fungi have an underground mycelium that absorbs nutrients and water, and an above-ground mushroom that contains reproductive spores Many mushrooms are collected for food, such as the shaggy-mane mushroom shown in Figure: Mushroom fungal caps that contain spores-an edible type ! Ac?nomycetes Ac?nomycetes refers to a group of bacteria with a superficial resemblance to fungi The ac?nomycetes resemble bacteria in that they have a very simple cell structure and are about the same size in cross sec?on The actinomycetes resemble bacteria in that they have a very simple cell structure and are about the same size in cross section They resemble filamentous fungi in that they produce a branched filamentous network Actinomycetes are in great abundance in soils, as shown in Table They make up as much as 50 percent of the colonies that develop on plates containing artificial media and inoculated with a soil extract The numbers of actinomycetes may vary from to 36 million per gram of soil Although there is evidence that actinomycetes are abundant in soils, it is generally concluded they that are not as important as bacteria and fungi as decomposers It appears that actinomycetes are much less competitive than the bacteria and fungi when fresh additions of organic matter are added to soils Only when very resistant materials remain actinomycetes have good competitive ability Vertical Distribution of Decomposers in the Soil The surface of the soil is the interface between the lithosphere and the atmosphere At or near this interface, the quantity of living matter is greater than at any region above or below As a consequence, the A horizon contains more organic debris or food sources than the B and C horizons Although other factors besides food supply influence activity and numbers of microorganisms, the greatest abundance of decomposers typically occurs in the A horizon Ant (Formica cinera) in a Prairie soil in southwestern Wisconsin The sketch shows soil horizons and location of ant channels; numbers refer to the number of channels observed at the depth indicated SUMMARY Higher plants are the major producers contribut- ing to the supply of soil organic matter The microorganisms (bacteria and fungi) are the major decomposers and are mainly responsible for the cycling of nutrients and energy in soil ecosystems Soil animals play a minor role in the cycling of nutrients and energy, but play an important role in earth-moving activities Nutrient cyling results in reuse of the nutrients in an ecosystem Nutrients are efficiently recycled in natural ecosystems Interference of the cycle, such as cropping and removal of nutrients in food, results in reduced soil fertility Manures and fertilizers are used to maintain soil fertility in agriculture Soil organisms and higher plants engage in many interactions related to disease, mycorrhiza, and nitrogen fixation, and soil organisms and higher plants compete for the same growth factors A zone adjacent to plant roots with a high population of microorganisms is the rhizosphere Microorganisms play important environmental quality roles, such as detoxification of chemicals and decomposition of oil from spills Earthworms, ants, termites, and rodents move large quantities of soil and may greatly alter the nature of soil horizons Soil as a Microbial Habitat Major things a microorganism needs in soil: 1) Nutrients a) Substrate (electron donor) – Organic -‐ sugars, amino acids, organic acids Inorganic -‐ S=, So, H2, NH4+ b) Electron acceptor if substrate is nonfermentable e.g., O2, NO3-‐, Fe(III), SO4= c) Minerals and trace nutrients Soil as a Microbial Habitat Major things a microorganism needs in soil: 1) Nutrients 2) Water Soil as a Microbial Habitat Major things a microorganism needs in soil: 1) Nutrients 2) Water 3) Favorable microenvironment -‐ Important physicochemical factors include: soil atmosphere pH temperature ionic strength redox poten?al light Soil is a discon?nuous environment Very diverse condi?ons exist over small distances which allow a diverse popula?on to coexist Nutrient Sources in Soil A) Role of Soil Minerals Dissolu?on of minerals releases inorganic ions needed by biota e.g., Fe2+, Mg2+, Ca2+ CEC of inorganic frac?on serves as reservoir of ca?ons, e.g., Ca2+, NH4+ Clay frac?on can bind organic molecules and biochemicals -‐ bound enzymes can have enzyma?c ac?vity B) Role of Soil Organic MaZer Major source of microbial nutri?on = soil organic maZer Soil organic maZer = plant, animal, and microbial residues in various stages of decay Microorganisms grow on the surface of soil par?cles or in pores Paul E and Clark F Soil Microbiology and Biochemistry Academic Press, Inc San Diego, 1989, p15 Microorganisms and Soil Atmosphere Kilham K Soil Ecology Cambridge University Press, 1994 Microorganisms and Soil Atmosphere Coexistence of water and air in soil pores allows good diffusive movement of both gases and soluble nutrients Too dry -‐ movement of solutes is reduced Too wet -‐diffusion of gases is reduced Maximum biological ac?vity -‐-‐-‐-‐> about 40% of satura?on Microorganisms and Soil Atmosphere Coexistence of water and air in soil pores allows good diffusive movement of both gases and soluble nutrients Too dry -‐ movement of solutes is reduced Too wet -‐diffusion of gases is reduced Maximum biological ac?vity -‐-‐-‐-‐> about 40% of satura?on Anaerobic condi?ons: -‐ when rate of O2-‐ u?liza?on > rate of diffusion Anaerobic microsites can exist in a bulk aerobic soil NUTRIENT CYCLING Nutrient cycling is the exchange of nutrient elements between the living and nonliving parts of the ecosystem Plants and microbes absorb nutrients and incorporate them into organic maFer, and the microbes (with aid of animals) digest the organic maFer and release the nutrients in mineral form Nutrient cycling conserves the nutrient supply and results in repeated use of the nutrients in an ecosystem Nutrient Cycling Processes Two simultaneous processes, mineraliza5on and immobiliza5on, are involved in nutrient cycling Immobiliza?on is the uptake of inorganic elements (nutrients) from the soil by organisms and conversion of the elements into microbial and plant ?ssues Mineraliza?on is the conversion of the elements in organic maFer into mineral or ionic forms such as NH3 , H2P04-‐, S042-‐, and K+ These ions then exist in the soil solu?on and are available for another cycle of immobiliza?on and mineraliza?on SOIL MICROBE AND ORGANISM INTERACTIONS The role of soil organisms in nutrient cycling has been stressed, and it has been shown that some organisms are parasites that feed on plant roots In this sec?on the specialized roles of microorganisms living near and adjacent to plant roots will be considered The Rhizosphere Plant roots leak or exude a large number of organic substances into the soil Sloughing of root caps, and other root cells, provide much new organic maFer More than 25 percent of the photosynthate produced by plants may be lost from the roots to the soil These substances are food for microorganisms, and they create a zone of intense biological ac?vity near the roots in an area called the rhizosphere The rhizosphere is the zone of soil immediately adjacent to plant roots in which the kinds, numbers, or ac?vi?es of microorganisms differ greatly from that of the bulk soil Disease Soils may contain organisms that cause both plant and animal disease Bacteria are responsible for wilt of tomatoes and potatoes, soc rots of a number of vegetables, leaf spots, and galls Some fungi cause damping-‐off of seedlings, cabbage yellows, mildews, blight, and certain rusts Mycorrhiza Some fungi infect the roots of most plants Fortunately, most of the fungi form a symbio?c rela?onship-‐one that benefits both fungi and higher plants Nitrogen Fixa?on We live in a "sea" of nitrogen, because the atmosphere is 79 percent nitrogen In spite of this, nitrogen is generally considered the most limi?ng nutrient for plant growth Nitrogen exists in the air as N2 and, as such, is unavailable to higher plants and most soil microbes There are some species of bacteria (nitrogen fixers) that absorb N 2 gas from the air and convert the nitrogen into ammonia that they and the host plant can use This process of nitrogen fixa?on is symbio?c The bacteria obtain food from the host plant and the host plant benefits from the nitrogen fixed FIGURE Symbio?c nitrogen-‐fixing nodules on the roots of soybean roots (lec), and on sweet clover roots (right) EARTH MOVING BY SOIL ANIMALS All soil animals par?cipate as consumers and play a minor role in the cycling of nutrients and energy Many of the larger animals move soil to such an extent that they affect soil forma?on Earthworm Ac?vity Earthworms are probably the best known earth movers As a result of their earth-‐moving ac?vi?es, earthworms leave channels Where these channels are open at the soil surface, they can tranport water very rapidly into and through the soil Ants and Termites The ac?vi?es of ants and termites are, perhaps, more important than the ac?vi?es of earthworms Ants transport large quan??es of material from within the soil, deposi?ng it on the surface Some of the largest ant mounds are about 1 meter tall and more than 1 meter in diameter The effect of this transport is comparable to that of earthworms in crea?ng thick A horizons and in burying objects lying on the soil surface Rodents Many rodents, including mice, ground squirrels, marmots, gophers, and prairie dogs inhabit the soil SUMMARY Higher plants are the major producers contribu?ng to the supply of soil organic maFer The microorganisms (bacteria and fungi) are the major decomposers and are mainly responsible for the cycling of nutrients and energy in soil ecosystems Soil animals play a minor role in the cycling of nutrients and energy, but play an important role in earth-‐moving ac?vi?es [...]...Distribution of microorganisms in the A, B, and C horizons of a cultivated grassland soil All values refer to the number of organisms per gram of air-dry soil SOIL ANIMALS Soil animals are numerous in soils (see Table) Soil animals can be considered both consumers and decomposers because they feed on or consume organic maFer and... carnivorous soil animal EARTH MOVING BY SOIL ANIMALS All soil animals participate as consumers and play a minor role in the cycling of nutrients and energy Many of the larger animals move soil to such an extent that they affect soil formation Earthworm Activity Earthworms are probably the best known earth movers This activity produces thicker than normal, dark-colored surface layers in soils As a... the soil surface Rodents Many rodents, including mice, ground squirrels, marmots, gophers, and prairie dogs inhabit the soil Ant (Formica cinera) in a Prairie soil in southwestern Wisconsin The sketch shows soil horizons and location of ant channels; numbers refer to the number of channels observed at the depth indicated SUMMARY Higher plants are the major producers contribut- ing to the supply of soil. .. and trace nutrients Soil as a Microbial Habitat Major things a microorganism needs in soil: 1) Nutrients 2) Water Soil as a Microbial Habitat Major things a microorganism needs in soil: 1) Nutrients 2) Water 3) Favorable microenvironment -‐ Important physicochemical factors include: soil atmosphere pH... can have enzyma?c ac?vity B) Role of Soil Organic MaZer Major source of microbial nutri?on = soil organic maZer Soil organic maZer = plant, animal, and microbial residues in various stages of decay Microorganisms grow on the surface of soil par?cles or in pores Paul E and Clark F Soil Microbiology and Biochemistry Academic... Clark F Soil Microbiology and Biochemistry Academic Press, Inc San Diego, 1989, p15 Microorganisms and Soil Atmosphere Kilham K Soil Ecology Cambridge University Press, 1994 Microorganisms and Soil Atmosphere Coexistence of water and air in soil pores allows good diffusive movement of both gases and soluble nutrients Too dry -‐ movement... as detoxification of chemicals and decomposition of oil from spills Earthworms, ants, termites, and rodents move large quantities of soil and may greatly alter the nature of soil horizons Soil as a Microbial Habitat Major things a microorganism needs in soil: 1) Nutrients a) Substrate (electron donor) – Organic -‐ sugars, amino acids, organic acids... the soil These substances are food for microorganisms, and they create a zone of intense biological ac?vity near the roots in an area called the rhizosphere The rhizosphere is the zone of soil immediately adjacent to plant roots in which the kinds, numbers, or ac?vi?es of microorganisms differ greatly from that of the bulk soil Disease Soils... soybean roots (lec), and on sweet clover roots (right) EARTH MOVING BY SOIL ANIMALS All soil animals par?cipate as consumers and play a minor role in the cycling of nutrients and energy Many of the larger animals move soil to such an extent that they affect soil forma?on Earthworm Ac?vity Earthworms are probably the best known earth... activities, earthworms leave channels Where these channels are open at the soil surface, they can tranport water very rapidly into and through the soil Ants and Termites The activities of ants and termites are, perhaps, more important than the activities of earthworms Ants transport large quantities of material from within the soil, depositing it on the surface Some of the largest ant mounds are about