Bot1320 Chapter 4 Secrets of the Soil
  1. Prairie plants form intricate relationships (such as mutualism) with the soil and its microbes.
    • • Prairie plants, especially grasses, often have fibrous roots that can maximize absorption of water and minerals from the soil.

      Modified underground stems may also function as absorbing organs.

    • • Plant roots can be largely divided into two basic types.

      • Fibrous roots are highly branched, spreading systems that can hold soil together, preventing erosion.

      • Taproots consist of one or a few large main roots with smaller side roots, and often serve as energy storage.

      Many plant roots are covered with root hairs, single-celled extensions that increase surface area for absorption.

      A single rye grass has been found to produce 7,000 miles of roots and root hairs in a four-month season.

    • Prairie Cord Grass (Spartina pectinata) has a highly branched root system that forms "sod" that pioneers used to make sod houses and roofs.

      The leaves can be bundled into fuel cords, giving this species its common name.

      A nickname is Rip Gut, due to its sharply serrated leaf blades that can cut skin.

    • Modified stems

      • Rhizomes (iris) are horizontal stems that grow just below the surface, and can produce shoots from buds along the stem.

      • Bulbs (onion) are underground shoots that store food in layers of modified leaves.

      • Stolons (strawberry) are horizontal stems ("runners") that grow along the surface. New shoots can form at buds along each runner.

      • Tubers (potatoes) are enlarged rhizomes specialized for storing food. The "eyes" on a potato are buds that can form new shoots.

    • • Organisms in an ecosystem often form complex, interdependent (symbiotic) relationships with other organisms, including:

      • Predator/prey: the predator eats the prey.

      • Parasitic: a parasite feeds off a host without killing the host.

      • Mutualistic: both organisms benefit from the interaction.

    • Mutualism is a relationship between 2 species of organisms where both sides benefit from the relationship.

      Plants called legumes often have mutualistic bacteria living in root nodules that help fix nitrogen.

      Most prairie plants form mycorrhizae with mutualistic fungi.

    • • Plants in the Leguminoceae (Fabaceae) family, such as Purple Prairie Clover (Petalostemum purpureum) have root nodules containing Rhizobium ("root living") bacteria.

      The bacteria convert atmospheric nitrogen (N2) into an organic form that plants can use in a process called nitrogen fixation.

      In turn, the bacteria obtain photosynthetic products from the plant.

    • Nitrogen fixation involves the bacterial conversion of atmospheric nitrogen (N2), which plants cannot use, into ammonia, which plants can use to produce proteins.

      The Rhizobium live within structures called bacteroids in the root nodules of the legume.

      Excess nitrogen from fertilizers may favor growth of Old World weeds at the expense of New World natives.

    • • Most plants form intricate relationships called mycorrhizae, consisting of plant roots united with mutualistic fungi.

      The fungus body is an extensive network called hyphae, that greatly increase the surface area for absorbing water and minerals, which are transported into plant root cells via specialized projections of fungal hyphae.

      In return, the fungus obtains photosynthetic carbohydrates from the plants.

    • Soil consists of multiple layers called horizons.

      The USDA classifies soils into types based on texture, moisture, and other properties.

      Most grassland soils are mollisols with a thick, dark A horizon.

      Illinois also has significant alfisols in wooded areas that yield a thinner A horizon and clay-rich B horizon.

      Warmer climate and faster decomposition in the southern plains result in less fertile aridisols with thinner A and B horizons.

    • • A profile of a typical grassland soil reveals vertical horizons.

      • A horizon is the topsoil rich in decayed organic material (humus).

      • B horizon is the subsoil that accumulates nutrients leached from above.

      • C horizon contains parent material (rock), low in organics.

      Sitting on top is a litter layer of organic matter that has not decomposed (detritus).

      After decomposition, this litter may be incoporated into the topsoil as humus.

    • Soil can be characterized by moisture content into 3 broad kinds:

      • Xeric soil is dry.

      • Mesic soil has moderate moisture.

      • Hydric soil is wet.

    • • Other propertues of soil include:

      • Aeration: plant roots and burrowing animals aerate the soil by creating space between sticky clay particles.

      • Color: These organisms also add organic matter to the topsoil; prairie soils thus have a rich topsoil that gives it a dark color - "black earth".

      • Acidity: The calcium-rich limestone bedrock in northern Illinois gives the soil a pH greater than 7; native plants are adapted to this basic/alkaline soil.

    • • A host of small organisms dwell in the soil, ranging from microscopic bacteria that decompose organic matter to larger arthropods.

      Fungi are mainly decomposers, though some are mutualistic mycorrhizae; some may even be predatory on nematodes (roundworms) by wrapping their hyphae around the victim.

      Most nematodes and springtails are scavengers, feeding on detritus (dead organic matter), starting the decomposition process that cycle nutrients through the ecosystem.

      Predators here include arachnids such as predatory mites and pseudoscorpions.

    • Arthropods have jointed legs and an exoskeleton that are molted as they grow.

      • Insects have 3 body segments:
        Head contains the brain, mouth, eyes, antennae. Thorax is where 6 legs and possibly wings are attached.
        Abdomen holds digestive and reproductive organs.

      • Arachnids such as mites and pseudoscorpions have 2 body segments:
        Cephalothorax is the fusion of head and thorax; 8 legs are attached here.
        Abdomen is similar to that of an insect.

      • Decapods such as crayfish have 20 segments grouped into 2 parts:
        Cephalothorax is the fusion of head and thorax; 10 legs are attached here.
        Abdomen contains appendages called swimmerets used for swimming and brooding eggs.

      Note the pseudoscorpion's pincer-like pedipalps should emerge from its mouth, and 8 legs should be attached to the cephalothorax.

  2. Other soil organisms include earthworms, beetles, and ants that tunnel through the matrix.
    • • In glaciated northern plains, few earthworms existed prior to European settlement.

      Old World earthworms from soil in potted plants quickly invaded the New World, forever changing the delicate balance under the soil.

      Invasive earthworms may speed up nutrient cycles: moving detritus down into the soil, where organic molecules can leach down, out of reach of trees.

      Such disturbance may aid the spread of Old World invasive plants such as Common Buckthorn (Rhamnus cathartica) and Garlic Mustard (Alliaria petiolata).

    • • A vast diversity of beetles inhabit the prairies.

      Dung beetles play an important role in cycling nutrients in this ecosystem: females lay an egg in each dung pile, and the hatched larva feeds on the nitrogen-rich excrement.

      The source of the dung can range from bison to rodents such as pocket gophers and Thirteen-lined Ground Squirrels.

    • • In the northern plains, most of the work of turning over soil was done by ants prior to European settlement.

      Some species form mutualistic relationships with aphids.

      The aphids feed on plant sap, secreting excess sugar (called "honeydew"), which are consumed by the ants.

      In turn the ants protect the aphids from predators and may even construct mud shelters for their domesticated herd.

      Some ants are also known to feed on elaiosomes of Common Blue Violet and Red Trillium (and dispersing the seeds).