This week we will study some examples of soil organisms and discuss organic matter, forest humus types and laboratory determination of organic matter. First we'll discuss various soil organisms and look at slides of these organisms. Then we'll discuss mycorrhiza, and see various slides of them. Finally we'll make some remarks about humus types and also some concerning laboratory work, and then you'll have some work to do.
The major soil organisms are listed in Table 8.1. You should notice that the smaller the organism, the greater he abundance or the number, the more specific its function, and the greater its influence on soil properties---these statements all comprise a Law of Soil Biology. The slide shows a soil Bacterium (Bacillus cereus). The bacteria include the spore-forming and non-spore-forming rods, cocci, vibrous and spirilla. They vary considerably in size, shape, oxygen requirements (aerobic or anaerobic), energy utilization (autotrophic and heterotrophic), slime formation, and their relation to plants and animals (saprophytic and/or parasitic). In regards to their functions, some do not decompose organic matter (the autotrophs) while at the other extreme, specific ones are involved in specific functions such as the heterotrophic nitrifying bacteria.
The actinomycetes are flora with characteristics of both. The actinomycetes vary greatly in their biochemical properties, in their relation to higher plants and animals (saprophytic versus parasitic), and in their effects upon soil bacteria (associative and antagonistic interrelations). Slide 01 shows an example of the Basidiomycetes, a soil-inhabiting fungus. The fungi produce extensive mycelia and spores in soils and composts. Their growth throughout the soil may be so extensive as to hold the mass of particles together by means of very fine microscopic network of mycelia and their excretion products. Fungi vary greatly in their relation to higher forms of life, notably plants (being saprophytic versus parasitic), to soil bacteria (formation of antibiotic compounds), and other constituents of soil population.
Slide 02shows a representative of the blue-green algae. Their ability to produce chlorophyll makes their life in the soil, especially at the surface, independent of the presence of organic matter. Other algae are represented in the soil population as well.
An example of protozoa is shown in Slide 3. These unicellular organisms comprise amoebae, flagellates, and ciliates. The vegetative versus cyst condition of protozoa has attracted considerable attention. Protozoa appear to control the bacterial population by predation.
Slide 4provides an example of nematodes (Turbatrix aceti). The soil invertebrates, which include the nematodes, also are comprised of the mites, and various insects including springtails, ground beetles, ants, etc., the snails and slugs, and the worms. The nematodes are found in almost all soils and sometimes are called thread worms. Some live in decaying organic matter, some are predatory on other nematodes, and on bacteria, algae, and protozoa. The third group has become a serious pest problem to the plant specialist, even being troublesome in greenhouses.
Slide 5is an example of a mite of the Orbatid (milleri) suborder. Mites are primarily associated with decomposition of litter. This is a handsome brute! Slide 6 and Slide 7 show other mites from different families. Notice the difference in body shape and length, which are characteristics used to identify different families. SLIDE 0807 Another mite family. Slide 8SLIDE 0808 is an example of the Ascidae family of the Mesostigmatid Suborder. These are primarily predator mites, feeding on nematodes and other mites. They also feed on blue-green algae and the green algae. Most mites generally feed on litter.
Slide 9illustrates the various soil invertebrates of a grassland community. Nematodes are most numerous with the isopods the least. Spiders and centipedes are wholly carnivorous and ants predominantly so; beetles, fly maggots, mites, and nematodes range widely in their diet according to the species; the remaining groups feed largely on decaying organic matter. Collembola are closely associated with fungi and other microorganisms. Only the invertebrates are shown in this slide and they are not drawn to scale.
Snails as a group feed on dead organic litter, while slugs feed on living tissue as well as dead material, and can be parasitic. Most of you are probably familiar with slug damage on tomatoes, carrots, etc. A springtail is gaily chumping away on a leaf, and a second one is illustrated in the next slide. Springtails feed on litter, but apparently only after fungi have infested the litter. The sowbug , or rolly poly, is a feeder on plant materials.
The ground beetle, a member of the Carabidae, is shown in Slide 15. This is the dominant predator in the soil litter, feeding on most other members of the soil fauna.
Ants, predatory as well as scavengers, play an important role of incorporation of organic matter into the mineral soil and the converse process of bringing subsoil material to the surface, thus mixing the upper portion of the soil profile. The importance of ants as mixers is generally greatly underestimated. This ant hill of the Allegheny hill ants illustrates how they can disturb the soil surface. In some parts of the southern United States fire ants from South America have badly damaged hay and pasture fields by their predation of the crops.
Finally we have our cultivator extraordinaire, the lowly earthworm, another European import. This creature probably does more than any other animal to incorporate organic matter, mix materials, enhance granulation through excretion of slime mucilage, loosening the soil and creating aerating channels for air and water movement. They feed on organic matter, create a home for digestive bacteria, and are preyed upon by moles. Enchytraeid worms have similar functions to the earthworm, but they are smaller in size.
Larger animals have an effect on the mixing and stirring of the soil also. These vole tunnels have been exposed by the removal of an oak forest floor. SLIDE 0821 A further example of soil disturbance. This groundhog was rudely awakened during the excavation of this soil pit in southwest Pennsylvania. Note how he has pulled soybean straw into his hole for bedding and food, thus contributing organic matter to the subsoil. The poor, cold groundhog is removed from his warm bed so that the soil examination and sampling could continue. Mr. Groundhog was replaced in his warm bed as we didn't have the heart to shoot him at this time (he may not have fared so well if he auditioned for Caddyshack). By the way, groundhog holes are dangerous to cattle and horses (and golfers) as they can easily break a leg if the hoof should slip into the hole
.
SLIDE 23 is a diagram of organisms involved in the decomposition of compost. The decomposition of compost has been studied closely because experimental conditions can be easily controlled. First, notice the flow of energy in the direction of the arrows and that the 1st, 2nd, and 3rd level of consumers are indicated by 10, 20, and 30, respectively. Thus, the flow is from organic matter to 10 - actinomycetes, fungi, bacteria, and the various worms, beetles, mites, snails, and nematodes; then to Collembola or springtails and mold mites, and beetle mites on fungi with rotifers, protozoa and nematodes feeding on bacteria; these in turn are fed on by ground beetles, centipedes, predatory mites and ants. These same general relationships hold for the decomposition of forest litter. Many of the slides shown are from the collection of Dr. Dindal of the Faculty of Forest Biology here at ESF.
These organisms must be capable of living in an environment of only limited nutrients. Competition is comparatively small. The autotrophic bacteria are of prime importance since they are highly specialized, capable of using as energy sources the traces of NH3 brought down by rainfall or the traces of H and CH3 found in the atmosphere. Microbial life is thus at a minimum and competition limited since the carbon source for cell synthesis, CO2, is plentiful. Only upon the death of the microbes, when they themselves become nutrients for other organisms, does competition set in. A certain amount of association is possible, as when nitrate-forming bacteria utilize the nitrate produced by ammonia-oxidizing forms (the latter may also include some general-purpose fungi).
The next step in the microbial population development occurs when organic materials become available. To simplify the complex reactions involved, let's consider the three groups of constituents of organic matter: glucose, cellulose, and lignin. These three groups make up 80-90 percent of the organic matter in the soil. The simple carbohydrates can be attacked by many types of organisms. When the nitrogen supply is low, only organisms capable of fixing nitrogen of the atmosphere will be able to grow and utilize glucose. Under these conditions there is little competition, since the two groups of bacteria capable of bringing the process about, in the absence of green plants, are aerobic forms (Azotobacter) and anaerobic types (Clostridium).
The decomposition of cellulose requires development of totally different groups of organisms, since it cannot be utilized directly by nitrogen-fixing bacteria. Its decomposition is therefore controlled by the amount of available nitrogen. The abundance and nature of the nitrogen and the nature of the environment influence greatly the kind of organisms developing at the expense of the cellulose. A variety of associative and competitive phenomena may result. The first is manifested when the cellulose is broken down by some organisms to dextrin-like compounds or to simple carbohydrates. These are transformed by other organisms to organic acids, which are finally broken down by still other organisms to CO2 and water, or CO2 and methane. Competitive processes result when the cellulose is attacked by bacteria, lower or higher fungi, actinimycetes, or even the invertebrates. Whether one group or another becomes dominant depends upon the reaction (acidity) of the soil, nature and amount of available nitrogen, oxygen supply, and temperature.
Finally, lignin presents a different problem in microbial development. This substance, the chemical nature of which is still a dispute, is more resistant to decomposition than most other organic compounds synthesized by plant or animal life. Although it is known that various fungi, such as certain basidiomycetes, certain actinomycetes, and certain bacteria, are capable of decomposing lignin, very little is known about the mechanism of its breakdown. Thus, in summary, the first organisms are the autotrophic and the photosynthesizing groups, then the heterotrophs, either saprophytic or parasitic forms, and finally a definite community develops.
Carrying the discussion a little further, the associative influences among
microorganisms are numerous. Therefore, the following just summarizes several classes:
1) The aerobic forms have effect upon the growth of anaerobes
2) Preparation of an essential nutrient or of an appropriate substrate by one organism for
the growth of another.
3) Production by certain organisms of specific substances which are essential for the
growth of other organisms.
4) Utilization and destruction by various microorganisms of the metabolic waste products
of other organisms.
5) Dependence of certain organisms upon others for carrying out life activities; this
association becomes one of symbiosis.
There are also some very common antagonistic interrelationships among soil organisms.
These can be summarized as:
1) Competition for available nutrients.
2) Creation by one organism of conditions which are unfavorable for the growth of another,
such as changing the soil reaction.
3) Production by one organism of specific substances which are harmful to the growth of
other organisms.
4) Direct parasitism of one organism on another.
5) Predaceous effects, or the feeding of one organism upon another.
Now, we need to consider for a moment, the special relationship between certain fungi and roots, called mycorrhiza. The fungi infect or invade certain roots, feeding upon the organic constituents therefrom, whereas certain soil nutrients are made more readily available by the fungus to the host plant. At least, the trees appear to benefit from the association. Mycorrhiza on red pine roots is shown. Note how they appear to encase the root and are themselves branched. This is a pitch pine root with mycorrhiza . SLIDE 26 shows the association on white pine roots. The mycorrhizal fungi can be divided into three groups depending on the form of invasion of the plant root. The ectotropic group have hyphae that penetrate between the cortex cells of the root but do not enter the cells.
This is a longitudinal section of a red pine root with the hyphae stained green and confined to a sheath on the root surface and the hyphae in the intercellular spaces. Here is a root tip with ectotrphic mycorrhiza . This is the close-up of the cap cells of the growing tip with small hyphae in the intercellular spaces. The hyphae are within the cell walls here. This is representative of the second group, the endotropic mycorrhizal fungi. This type does not usually form a sheath on the surface of the roots. It is also a type difficult to isolate in the laboratory. SLIDE 0831 A third group of mycorrhizal fungi is the ectendotropic, which invades both intercellular and cellular spaces and has an external sheath. This form is very common. In each case the fungi secrete appropriate enzymes which permit the penetration of the plant tissue to which the fungus is adapted.
The wide-spread occurrence of the mycorrhiza is of great practical importance since it makes possible the growth of certain plants in areas nearly devoid of available nutrients, or where there is a lack of one or two critical elements. Dr. Hugh Wilcox of the Faculty of Forest Biology graciously supplied these slides.
Please refer to Part IV of the laboratory exercise entitled the Forest Humus Types. Here is given a key for the humus types found in the northeastern United States. This key is not in your textbook. You were briefly exposed to the mor and mull humus types in the first laboratory exercise. The contrast between the mor and mull type, and the intermediates, indicates variation in organism activity and in turn nutrient cycling amount and rate, as influenced by the various environmental factors of climate and vegetation. The physical and chemical properties of the soil are also influencing factors. Some tree species may be well-adapted to the low nutrient cycling and acid conditions indicated by the mor humus type, while others may be just as well suited or adapted to the more active system and lower acidity of the mull condition. Therefore, mor humus type is not necessarily bad, and mull is not always good!
Finally, The Laboratory Exercise!!! Most of the work has been done for you and appropriate data are provided. If you were required to do all the work by yourself, several weeks of the semester would be consumed and this would be a 5-credit course. Fortunately, you only have to blow off an hour to read about it. Look for the data in the laboratory. The exercise is self explanatory and describes the techniques involved in the analysis and outlines the procedures. Check your work carefully before turning in your answers!
