Chapter5 Soil Organisms

Soil Organisms

• Soil organisms – creatures that spend all/part of their lives in the soil environment.

Diversity of Soil Organisms

• Size of Organisms

  • Macrofauna – Moles, mice, ants, beetles, termites, grubs, millipedes, earthworms,

  • Mesofauna – Springtails and mites

  • Microfauna – Nematodes and single-celled protozoans

  • Flora – Plant roots, algae, diatoms

  • Microorganisms – Fungi, bacteria, and actinomycetes

Types of Diversity

• Species Diversity – the mix of species present.

• Functional Diversity – the capacity to use a wide variety of substrates and provide a range of ecosystem services.

Ecosystem Dynamics

• Functional Redundancy – several organisms can carry out each ecosystem service.

  • leads to ecosystem stability and resilience

• Keystone Species – a species that is unique in filling an ecological niche and is therefore integral to the functioning of that system.

• Global Biodiversity – great genetic resource of soil organisms.

Stability & Resilience

• Stability – the ability of soils, even in the face of wide variations in environmental conditions and inputs, to continue to perform such functions as:

  • cycling of nutrients

  • assimilation of organic wastes

  • maintenance of soil structure

• Resilience – the ability of the soil to bounce back to functional health after a severe disturbance has disrupted normal processes.

Organisms in the food web

Autotrophs

• Organisms that can feed themselves by harnessing light energy to make organic molecules such as carbohydrates, proteins, lipids, and nucleic acids out of inorganic raw materials (such as CO₂, H₂O, N compounds, etc.)

  • Autotrophs = Primary Producers - the first link in the food web/chain.

Organisms in the food web

Heterotrophs

• Organisms capable of deriving energy for life processes only from the decomposition of organic compounds and incapable of using inorganic compounds as sole sources of energy or for organic synthesis.

Soil Food Web

• The community of organisms living all/part of their lives in the soil.

• Fueled by primary producers: – plants, lichens, moss, photosynthetic bacteria, and algae.

Photosynthesizers

First trophic level

• Algae

• Bacteria

Role:

• Capture solar energy to fix CO₂

• Add OM to soil

Decomposers

Second trophic level

• Bacteria

• Fungi

• Protozoa

Role:

• Breakdown residue

• Immobilize nutrients in their biomass

• Create new organic compounds

• Bind soil aggregates

Mutualists

Second trophic level

• Two organisms living in beneficial association

  • Bacteria

  • Fungi

Role:

• Enhance plant growth

• Fix N

Symbiosis with Mycorrhiza

• Facilitates the uptake of P, other nutrients, and water uptake.

• Glomus spp. Acaulospora, P. tinctorius, etc.

Biological Nitrogen Fixation (BNF)

• The process by which microorganisms in the soil 'fix' atmospheric N and make it available for assimilation by plants.

• Nitrogen-fixing bacteria are Rhizobium, Azospirillum, etc.

Pathogens/Parasites

Second trophic level

• Bacteria

• Fungi

• Nematodes

• Arthropods

Role:

• Promote disease

• Consume roots

• Parasitize nematodes or insects

Root-feeders

Second trophic level

• Nematodes

• Arthropods

Role:

• Consume plant roots

• Crop yield losses

Shredders

Third trophic level

• Earthworms

• Arthropods

Role:

• Breakdown residue

• Enhance soil structure

• Provide habitat for bacteria in gut

Organism Abundance

• Soil organism numbers are influenced primarily by amount and quality of food

  available.

• Other factors include:

  • Physical factors = Moisture, temperature

  • Biotic factors = Predation and competition

  • Chemical characteristics of soil = Acidity, dissolved nutrients, and salinity

Food Web Structures

• Fungi to bacteria ratios

  • Grasslands/Ag soils 1:1

  • Deciduous 5:1 to 10:1

  • Conifer 100:1 to 1000:1

• Organism communities reflect their food source

• Management practices change food webs

How Soil Organisms Affect Soil Properties

1. Earthworms’

• Digest organic and mineral matter passing their bodies - 30 tons/ha/year.

• Excreta/cast has high amounts of nutrients; high pH, % base saturation, and CEC.

• Through their passage in the soil, channels are created - facilitate aeration and drainage.

• Increases size and stability of soil aggregates.

How Soil Organisms Affect Soil Properties

2. Nematodes

• Help in the decomposition of OM;

  • known as harmful parasites

    - they invade plant roots and cause the death of the roots

How Soil Organisms Affect Soil Properties

3. Protozoa

• They ingest other soil organisms, particularly bacteria.

  • help hasten the release of some nutrients that have been immobilized by bacteria.

How Soil Organisms Affect Soil Properties?

4. Bacteria

• Involved in OM decomposition as well as in various transformations of soil nutrients such as:

  • ammonification: organic N —> NH₃

  • nitrification: NH_4⁺ —> NO_3^-

  • denitrification: NO_3^- —> N₂

  • immobilization of N: available —> unavailable form

  • biological N fixation: N₂ to NH₃ in root nodules

  • sulfur oxidation and reduction

• other chemical processes

How Soil Organisms Affect Soil Properties?

5. Fungi

• The most adaptable and versatile of soil organisms.

• Able to thrive even in extreme soil acidity and alkalinity.

• Able to decompose even the resistant organic compounds such as lignins, cellulose, and gums.

• Mycorrhizae - help plants in solubilization of P and its absorption.

How Soil Organisms Affect Soil Properties?

6. Actinomycetes

• These organisms also attack and simplify complex and resistant organic compounds such as:

  • cellulose

  • chitin

  • phospholipids

Autotrophic & Heterotrophic Bacteria

• Autotrophic bacteria – use CO₂ as their source of C and derive energy for their metabolism from the oxidation of simple inorganic compounds such as:

  • Ammonium = NH_4⁺

  • Sulfur = H₂S

  • Fe = Fe²⁺

• Heterotrophic bacteria – oxidize organic matter to get C and energy.

Chemoautotrophic & Photoautotrophic Bacteria

• Chemoautotrophic – use inorganic chemical compounds.

• Photoautotrophic – use sunlight as their source of energy.

Aerobic, Anaerobic, & Facultative Bacteria

• Aerobic bacteria – thrive only in the presence of free oxygen.

• Anaerobic bacteria – can live even in complete absence of oxygen.

• Facultative bacteria – can grow in the presence/absence of free oxygen.

  • can use oxidized compounds such as nitrates and sulfates as electron acceptors.

  • in the process of reduction, NO_3^- is transformed to N₂ or N₂O and SO₄ = to H₂S.

Mesophilic, Thermophilic, & Psychrophilic Bacteria

• Mesophilic: 25^oC– 35^oC

• Thermophilic: 45^oC– 65^oC

• Psycrophilic: <20^oC

  • Optimum temperature for growth of most bacteria: 21^oC - 38^oC

  • Most soil organisms are mesophiles.

Measurement of Microbial Activity

• Counting

  • Direct counts

  • Plate counts

• Activity levels

  • Respiration

  • Nitrification rates

  • Decomposition rates

• Cellular constituents

  • Biomass C, N, or P

  • DNA fingerprinting