🌱Intro to Soil Science Unit 6 – Soil Biology: Organisms and Processes

Key Concepts and Definitions

• Soil biology encompasses the study of organisms living in the soil and their interactions with each other and the soil environment
• Soil ecosystem consists of both biotic (living) and abiotic (non-living) components that interact to support life and nutrient cycling
• Soil organisms include bacteria, fungi, protozoa, nematodes, arthropods, and earthworms among others
• Soil food web describes the complex feeding relationships between soil organisms and their role in energy transfer and nutrient cycling
• Nutrient cycling involves the transformation and movement of essential nutrients (carbon, nitrogen, phosphorus) through the soil ecosystem
• Organic matter decomposition is the breakdown of dead plant and animal material by soil organisms releasing nutrients back into the soil
• Soil-plant interactions involve the symbiotic relationships between soil organisms and plant roots (mycorrhizae, nitrogen-fixing bacteria)
• Environmental factors affecting soil biology include temperature, moisture, pH, and soil structure which influence organism activity and diversity

Soil Ecosystem Overview

• Soil ecosystems are complex and dynamic environments that support a diverse array of life
• Composed of both biotic components (soil organisms) and abiotic components (soil particles, water, air)
• Soil organisms play critical roles in decomposition, nutrient cycling, and soil structure formation
• Interactions between soil organisms create complex food webs and energy transfer pathways
• Soil ecosystems are influenced by factors such as climate, vegetation, and land management practices
  • Temperature and moisture affect organism activity and decomposition rates
  • Vegetation provides organic matter inputs and influences soil properties
• Healthy soil ecosystems maintain soil fertility, support plant growth, and provide ecosystem services (water filtration, carbon sequestration)

Major Soil Organisms

• Bacteria are the most abundant soil organisms and play key roles in decomposition and nutrient cycling
  • Decompose organic matter and release nutrients (carbon, nitrogen, phosphorus)
  • Some bacteria fix atmospheric nitrogen making it available to plants (rhizobia)
• Fungi are important decomposers and form symbiotic relationships with plant roots (mycorrhizae)
  • Break down complex organic compounds (lignin, cellulose)
  • Mycorrhizal fungi improve plant nutrient and water uptake in exchange for carbon
• Protozoa are single-celled organisms that feed on bacteria and release nutrients in plant-available forms
• Nematodes are microscopic roundworms that feed on bacteria, fungi, and other soil organisms
  • Help regulate soil food web dynamics and nutrient cycling
• Arthropods (insects, mites, spiders) shred and consume organic matter aiding in decomposition
• Earthworms are important ecosystem engineers that improve soil structure and fertility
  • Burrow through soil creating channels for air and water movement
  • Consume organic matter and excrete nutrient-rich casts

Soil Food Web Dynamics

• Soil food webs describe the complex feeding relationships between soil organisms
• Energy and nutrients flow through the food web from primary producers (plants) to various consumer levels
• Decomposers (bacteria, fungi) break down dead organic matter releasing nutrients for plant uptake
• Predator-prey interactions (protozoa eating bacteria, nematodes eating fungi) regulate population dynamics and nutrient cycling
• Soil food webs are influenced by factors such as resource availability, environmental conditions, and disturbance events
  • Organic matter inputs from plants fuel the food web and support organism diversity
  • Drought or tillage can disrupt soil food webs and alter nutrient cycling processes
• Understanding soil food web dynamics is important for managing soil health and fertility in agricultural and natural ecosystems

Nutrient Cycling in Soil

• Nutrient cycling involves the transformation and movement of essential nutrients through the soil ecosystem
• Key nutrient cycles in soil include carbon, nitrogen, and phosphorus
• Carbon cycle involves the fixation of atmospheric CO2 by plants, decomposition of organic matter, and release of CO2 back to the atmosphere
  • Soil organisms play a critical role in decomposing organic matter and releasing carbon
• Nitrogen cycle involves the fixation of atmospheric N2 by bacteria, mineralization of organic nitrogen, and plant uptake of inorganic nitrogen (ammonium, nitrate)
  • Nitrogen-fixing bacteria (rhizobia) form symbiotic relationships with legume plant roots
• Phosphorus cycle involves the weathering of rock minerals, mineralization of organic phosphorus, and plant uptake of inorganic phosphorus
• Soil organisms mediate nutrient transformations through their metabolic activities and interactions
  • Bacteria and fungi decompose organic matter releasing nutrients in plant-available forms
  • Mycorrhizal fungi improve plant phosphorus uptake by extending root surface area
• Nutrient cycling is influenced by factors such as soil pH, moisture, and temperature which affect organism activity and nutrient availability

Organic Matter Decomposition

• Organic matter decomposition is the breakdown of dead plant and animal material by soil organisms
• Decomposition releases nutrients back into the soil for plant uptake and supports soil structure formation
• Decomposition occurs in stages with different organisms playing key roles at each stage
  • Initial rapid decomposition of easily degradable compounds (sugars, proteins) by bacteria and fungi
  • Slower decomposition of more recalcitrant compounds (lignin, cellulose) by specialized fungi and actinobacteria
• Factors affecting decomposition rates include temperature, moisture, and substrate quality
  • Warm, moist conditions promote decomposer activity and faster decomposition rates
  • High-quality substrates (low C:N ratio) decompose more quickly than low-quality substrates (high C:N ratio)
• Organic matter decomposition contributes to soil fertility by releasing nutrients and improving soil structure
  • Decomposition products (humus) help bind soil particles together creating stable aggregates
  • Stable aggregates improve soil porosity, water retention, and root growth

Soil-Plant Interactions

• Soil-plant interactions involve the symbiotic relationships between soil organisms and plant roots
• Mycorrhizal fungi form symbiotic associations with plant roots improving nutrient and water uptake
  • Fungal hyphae extend into the soil increasing root surface area and access to resources
  • Plants provide fungi with carbon in exchange for improved nutrient and water uptake
• Nitrogen-fixing bacteria (rhizobia) form symbiotic associations with legume plant roots
  • Bacteria convert atmospheric N2 into plant-available forms (ammonium) in root nodules
  • Legumes provide bacteria with carbon and a protected environment for N fixation
• Plant roots release exudates (sugars, amino acids) that stimulate microbial activity in the rhizosphere
  • Rhizosphere is the zone of soil directly influenced by root secretions and associated microorganisms
  • Increased microbial activity in the rhizosphere can improve nutrient availability and plant growth
• Soil organisms can also have negative impacts on plant growth through pathogenic interactions
  • Some fungi and nematodes can cause plant diseases and reduce crop yields
  • Understanding soil-plant interactions is important for managing crop health and productivity

Environmental Factors Affecting Soil Biology

• Soil biology is influenced by a range of environmental factors that affect organism activity and diversity
• Temperature influences microbial activity and decomposition rates
  • Higher temperatures (up to a point) promote faster microbial growth and organic matter breakdown
  • Extreme temperatures (too hot or cold) can limit microbial activity and slow decomposition
• Moisture affects organism activity and nutrient availability
  • Adequate moisture is needed for microbial growth and nutrient transport
  • Dry conditions can limit microbial activity and slow decomposition rates
  • Saturated conditions can create anaerobic environments and alter microbial community composition
• Soil pH influences nutrient availability and organism diversity
  • Most soil organisms prefer neutral to slightly acidic conditions (pH 6-7)
  • Acidic soils (pH < 5.5) can limit nutrient availability and reduce microbial diversity
  • Alkaline soils (pH > 7.5) can also limit nutrient availability and alter microbial communities
• Soil structure and texture affect porosity, water retention, and organism habitat
  • Well-structured soils with a mix of pore sizes support diverse microbial communities
  • Sandy soils have lower water retention and may limit microbial activity during dry periods
  • Clayey soils have higher water retention but may lack oxygen for some organisms
• Understanding environmental factors affecting soil biology is important for managing soil health and fertility in different ecosystems and land use contexts