soil bio exam 3

What is ecology? Who coined it?

• Study of interactions between organisms and their environment (matter & energy).

• Coined by Ernst Haeckel (1866)

What is an ecosystem? Who coined it?

• A community of organisms interacting with each other and their environment.

• Introduced by Arthur G. Tansley (1936)

What is ecosystem stability?

Ability to maintain functions despite environmental changes.

What is ecosystem resilience?

Ability to recover after disturbance (e.g., drought → flood).

Stability vs. resilience (key difference)?

• Stability = resist change

• Resilience = recover after change

What is species diversity?

Number and distribution of species in an ecosystem.

What is functional diversity?

Variety of functions/processes organisms can perform.

Why can high species diversity still fail to degrade pollutants?

If functional diversity is low, no organisms can perform the needed breakdown.

What is functional redundancy?

Multiple species perform the same role.

Why is functional redundancy important?

Allows ecosystem recovery if one species is lost.

Food chain vs. food web?

• Food chain = linear

• Food web = interconnected, multiple pathways

Why is soil a food web instead of a food chain?

Organisms have many feeding relationships, not just one path.

What are ecosystem engineers?

Organisms that physically alter the soil environment.

Examples of ecosystem engineers?

Earthworms, termites

What changes do ecosystem engineers make?

Soil structure, aeration, water flow, nutrients, habitats

What is a keystone species?

Species with a major impact on ecosystem function.

What happens if a keystone species is removed?

Important processes fail (e.g., nitrogen cycling disrupted)

What are photoautotrophs?

Use sunlight for energy

What are chemoautotrophs?

Use energy from oxidation of inorganic compounds (N, S, Fe)

What are autotrophs?

Use CO₂ as carbon source.

Energy sources of autotrophs?

• Photoautotrophs: sunlight

• Chemoautotrophs: inorganic chemical reactions

What are heterotrophs?

Use organic matter for carbon and energy.

Role of heterotrophs in soil?

Decomposition and nutrient cycling

What are primary producers?

• Organisms that produce organic matter from CO₂

• Examples: plants, algae

What are primary consumers?

• Eat plants or detritus

• Examples: nematodes, termites, earthworms

What are secondary consumers?

• Eat primary consumers

• Examples: mites, centipedes, predatory microbes

What are tertiary consumers?

• Eat secondary consumers

• Examples: ants, birds, moles

What are saprotrophic microorganisms?

Fungi, bacteria, archaea that decompose organic matter

Main role of saprotrophs?

Chemical decomposition of dead material

Why are saprotrophs important for plants?

Release nutrients back into soil for plant uptake

How does organism size relate to abundance in soil?

Smaller organisms are more numerous than larger ones (inverse relationship).

How does organism size relate to biomass?

Larger organisms contribute more per individual, but microorganisms often dominate total biomass due to huge numbers.

How does organism size relate to metabolic capacity?

Smaller organisms (especially microbes) have higher overall metabolic activity due to rapid processes and large populations.

Which group dominates metabolic capacity in soil?

Microorganisms (bacteria, fungi, archaea)

Why do microorganisms dominate soil processes?

They are extremely abundant and carry out most chemical transformations (e.g., decomposition, nutrient cycling).

How does soil pH affect species composition?

Different organisms prefer different pH levels:

• Acidic soils → favor some fungi and bacteria

• Alkaline soils → favor other microbial groups
  → pH determines which species can survive

How does soil compaction affect soil organisms?

• Reduces pore space → limits air, water, and movement

• Restricts larger organisms (earthworms, insects) more than microbes

Which organisms are most affected by soil compaction?

• Mesofauna → restricted to existing pore spaces

• Macrofauna → need space for burrowing

• Microfauna → less affected (live in water films)

How do environmental factors influence diversity?

They determine which organisms can survive, affecting species composition and diversity

What is the starting point of carbon flow in soil?

Primary producers (plants fix CO₂ into organic matter)

What happens when a tree falls (start of carbon flow)?

Becomes detritus (dead organic matter)

Who first uses carbon from a decaying tree?

Saprotrophic microorganisms (fungi, bacteria) begin decomposition

What is the role of saprotrophic microorganisms in carbon flow?

They decompose complex organic compounds into simpler forms, releasing nutrients and carbon (as CO₂ or biomass).

How do detritivores contribute to carbon flow?

They fragment organic matter, making it easier for microbes to decompose

Where does carbon go after microorganisms process it?

Into microbial biomass and released as CO₂ through respiration

How do primary consumers fit into carbon flow?

They consume detritus and microbes, transferring carbon to higher levels

How does carbon move to higher trophic levels?

Secondary and tertiary consumers eat lower-level organisms → carbon transfers upward

Final fate of carbon in soil food web?

Released as CO₂ or stored in soil organic matter

Why is microbial activity critical in carbon flow?

Without microbes, dead material would not decompose → nutrients and carbon would not cycle

What is the rhizosphere?

The rhizosphere is the narrow zone of soil (about 2–3 mm) directly influenced by plant roots, with high biological and chemical activity.

How does bulk soil differ from the rhizosphere?

Bulk soil is soil not directly influenced by roots, with lower microbial activity and fewer nutrient changes compared to the rhizosphere.

How do biological and chemical activities differ between rhizosphere and bulk soil?

The rhizosphere has higher microbial populations (2–10× more), more nutrient depletion, and altered pH due to root activity and exudates.

What is rhizodeposition?

The release of organic compounds from roots into the soil, supplying carbon and energy to soil organisms.

What types of compounds are released during rhizodeposition?

• Mucigel

• Sugars

• Amino acids

• Organic acids

• Vitamins

• Phenolic compounds

• Sloughed root cells

How does rhizodeposition benefit plants and the soil food web?

It feeds microorganisms, enhances microbial activity, stabilizes soil aggregates, and improves nutrient cycling.

What is the meristematic zone?

Region of rapid cell division near the root tip.

What is the elongation zone?

Region where cells increase in length and size, contributing to root growth.

What is the maturation zone?

Region where cells differentiate into specialized types; root hairs develop here.

Where do root hairs develop?

In the maturation (root hair) zone.

What is the origin of root hairs (trichoblasts)?

They are extensions of epidermal cells.

What is the primary function of root hairs?

Increase surface area for water and nutrient absorption.

How does soil compaction affect root hairs?

It reduces pore space, limiting root hair growth and reducing access to water and nutrients.

Root Anatomy — What is the epidermis?

The outermost root layer (rhizodermis) that includes root hairs and absorbs water and nutrients.

Root Anatomy — What is the cortex?

The region between the epidermis and vascular cylinder, composed mainly of parenchyma cells.

Root Anatomy — What is the endodermis?

The innermost layer of the cortex that regulates movement of water and solutes into the vascular system.

Root Anatomy — What is the stele (vascular cylinder)?

The central part of the root containing xylem, phloem, and pericycle.

What is the role of the pericycle?

It forms lateral roots and contributes to vascular and cork cambium.

Casparian Strip — Where is it located?

In the endodermis.

What is the Casparian strip made of?

Suberin (waterproof, waxy material).

What is the function of the Casparian strip?

Acts as a gatekeeper, forcing water and solutes to pass through cell membranes before entering the xylem.

Transport Pathways — What is the apoplastic pathway?

Movement of water/solutes through cell walls and spaces outside membranes.

Transport Pathways — What is the symplastic pathway?

Movement through the cytoplasm via plasmodesmata.

Transport Pathways — What is the transmembrane pathway?

Movement where water repeatedly crosses cell membranes from cell to cell.

Which transport pathway is blocked by the Casparian strip?

The apoplastic pathway.

What is the nutrient depletion zone?

A region around roots where nutrients are reduced due to continuous uptake by root hairs.

Why does the nutrient depletion zone form?

Because roots absorb nutrients faster than they can be replenished in the surrounding soil.

Contact Mechanisms — What is mass flow?

Movement of nutrients to roots via water flow driven by plant transpiration.

Contact Mechanisms — What is diffusion?

Movement of nutrients from high concentration to low concentration toward the root.

Contact Mechanisms — What is root interception?

Direct contact of roots with nutrients as roots grow through soil.

Arnon & Stout Criteria — What is the first criterion?

The plant cannot complete its life cycle without the element.

Arnon & Stout Criteria — Second criterion?

The element’s function cannot be replaced by another element.

Arnon & Stout Criteria — Third criterion?

The element must be directly involved in plant metabolism or required for a metabolic process.

Why are CO₂ emission rates higher near living plant roots than in bulk soil?

Because of high root respiration (roots take in O₂ and release CO₂) and increased microbial activity fueled by root exudates.

What percentage of soil respiration is attributed to plant roots?

Approximately 25–75% of total soil respiration comes from plant roots.

How do root exudates contribute to higher CO₂ emissions?

They provide carbon and energy to microorganisms, increasing microbial respiration and CO₂ release.

Lateral Roots — From which tissue do they originate?

From the pericycle, located inside the root within the vascular cylinder.

Why is the pericycle important for root development?

It is a meristematic tissue that gives rise to lateral roots and contributes to secondary growth.

How do lateral roots respond to environmental cues like soil moisture?

They grow and branch more in areas with favorable conditions (e.g., higher moisture and nutrient availability), optimizing resource uptake.

Plant Influence on Soil — How do roots physically alter soil?

• Create channels (macropores)

• Increase soil aggregation

• Cause shrinkage and cracking by removing water

How do roots chemically alter soil?

• Uptake nutrients and water

• Release exudates (organic acids, sugars, etc.)

• Change soil pH and nutrient availability

How do roots biologically alter soil?

• Supply carbon and energy to soil organisms

• Increase microbial populations in the rhizosphere

• Support the soil food web

Why are plant roots considered central to soil ecosystems?

They connect physical, chemical, and biological processes, driving nutrient cycling, microbial activity, and soil structure formation.

What are macrofauna?

Macrofauna (>2 mm) are large soil organisms (e.g., earthworms, moles) that create their own burrows and strongly influence soil structure.

What are mesofauna?

Mesofauna (0.1–2 mm) (e.g., mites, collembola) live in existing air-filled pore spaces and cannot create their own.

What are microfauna?

Microfauna (<0.1 mm) (e.g., nematodes, protozoa) live in water films surrounding soil particles.

What are transient soil fauna?

Organisms present in soil only briefly due to short-term conditions (e.g., hibernating insects).

What are temporary soil fauna?

Species that live in soil during a specific life stage (e.g., larvae or eggs).

What are periodic soil fauna?

Organisms that spend part of their life cycle in soil, often emerging to reproduce.

What are permanent soil fauna?

Organisms that live their entire life cycle in soil (e.g., collembola).