SCSC 301 Chapter 11

  • What are the major pools of C in the carbon cycle?

    • Major pools of carbon include the atmosphere (as carbon dioxide), terrestrial biomass (plants), soils (as soil organic matter), oceans, and carbonate minerals. The most dynamic pools fluctuate seasonally or through biological activity, particularly in plants and soils.

  • Which ones are the most dynamic (active in cycling)?

    • The atmosphere and soil organic matter are considered the most dynamic, with atmospheric carbon dioxide rapidly changing due to photosynthesis and respiration, while soil organic matter is influenced by decomposition and microbial processes.

  • Where does C in plants come from?

    • Carbon in plants originates from atmospheric carbon dioxide (CO₂), which is absorbed during photosynthesis, where it is then fixed into carbohydrates and other organic compounds.

  • What compounds are found in plants?

    • Plants contain a variety of compounds, including carbohydrates (sugars, starches), proteins (amino acids), lipids (fats, oils), lignin, and polyphenols, all of which contribute to their structure and function.

  • What does mineralization mean?

    • Mineralization refers to the process by which organic matter is broken down into simpler inorganic forms, releasing nutrients such as nitrogen, phosphorus, and sulfur into the soil, making them available for plant uptake.

  • Which are most resistant to decomposition?

    • Compounds most resistant to decomposition include lignin, certain polyphenols, and complex organic matter, which degrade slowly due to their structural complexity and the resilience of their chemical bonds.

  • How does the presence/absence of oxygen affect organic matter decomposition?

    • Presence of oxygen facilitates aerobic decomposition, leading to faster breakdown and energy release; absence of oxygen causes anaerobic conditions, slowing decomposition and often generating methane and other by-products.

  • What factors affect organic matter decomposition?

    • Factors include soil moisture (adequate water enhances microbial activity), temperature (warm conditions accelerate processes), the quality of the organic material (lignin content affects decay rates), and oxygen availability.

  • What is C/N ratio and how does it affect SOM?

    • The carbon-to-nitrogen (C/N) ratio represents the relative amounts of carbon and nitrogen in organic matter. A lower C/N ratio (e.g., 10:1 to 25:1) is generally more favorable for microbial growth, enhancing decomposition. Higher ratios may result in nitrogen immobilization, reducing its availability for plants.

  • What compounds/particles can affect SOM degradation?

    • Factors affecting soil organic matter (SOM) degradation include soil texture, microbial community composition, residue quality, and environmental conditions such as moisture and temperature.

  • Why is there a difference among plant tissues in their degradability?

    • Differences in degradability are influenced by biochemical composition; for example, the presence of lignin and cellulose in woody tissues make them more resistant, while herbaceous tissues that are softer and contain less lignin decompose more readily.

  • What does immobilization mean?

    • Immobilization is the process where available nutrients (like nitrogen, phosphorus, and sulfur) are re-assimilated into microbial biomass or organic forms, which temporarily reduces their availability to plants.

  • What are some physical, chemical, and biological properties of humus/SOM?

    • Humus possesses high nutrient-holding capacity, supports microbial life, improves soil structure (aggregates), enhances water retention, and has a negative charge that can attract cations like calcium and magnesium.

  • What is compost?

    • Compost is a product of the aerobic decomposition of organic materials, such as plant matter, kitchen scraps, and yard waste, enriched into a nutrient-rich, humus-like material that improves soil health.

  • What factors result in increasing SOM?

    • Factors that lead to increased SOM include adding organic residues (manures, compost), enhancing plant biomass (through cover crops), practicing reduced tillage, and implementing conservation agriculture techniques.

  • What factors result in decreasing SOM?

    • Factors leading to decreased SOM include intensive tillage, erosion, over-extraction of organic resources, repeated cropping without replenishment of organic matter, and poor moisture management.

  • How would you manage soil to improve or maintain SOM?

    • Management strategies include keeping the soil covered with vegetation, applying organic amendments (like compost), rotating crops to enhance biomass diversity, minimizing soil disturbance, and promoting perennial plants to sustain soil health.

Extra Stuff:

Lignin & Polyphenol Content of Residues

  • Lignin: A structural polymer providing rigidity to plant cell walls, lignin is highly resistant to decomposition, affecting the overall rate of organic matter breakdown. Its presence in residues slows decomposition, contributing to the stability of soil organic matter (SOM) over time.

  • Polyphenols: A class of compounds contributing to plant structure that can inhibit microbial activity. Their accumulation in plant materials can slow down decomposition and impact nutrient cycling in soil.

Humic Substances

  • Humic substances are complex organic molecules formed during the decomposition of plant and animal residues. They play a crucial role in soil health by enhancing nutrient availability and supporting microbial life. They improve soil structure, water retention, and cation exchange capacity, making nutrients more accessible to plants.

Non-Humic Substances

  • Non-humic substances consist of easily decomposable organic compounds, such as sugars, proteins, and simple organic acids. They are typically less stable than humic substances but are quickly available for microbial use, contributing to short-term nutrient cycling in the soil.

Composting Process

  • Composting is a managed biological process that involves the aerobic decomposition of organic materials like plant matter, kitchen scraps, and yard waste, converting them into fertile compost.

    • Stages of Composting:

    • Mesophilic Stage: Characterized by moderate temperatures (20-45°C), where mesophilic microorganisms break down easily digestible materials.

    • Thermophilic Stage: As temperatures rise (45-70°C), thermophilic bacteria take over, rapidly breaking down organic matter and killing pathogens and weed seeds.

    • Curing Stage: The temperature drops as microbial activity slows down, allowing the material to stabilize and mature into compost.

Benefits of Composting

  • Composting offers numerous benefits for soil health and environmental sustainability.

    • Improves Soil Structure: Enhances soil aggregation, aeration, and porosity, facilitating root growth and water infiltration.

    • Nutrient Supply: Provides essential nutrients in a slow-release form, reducing the need for synthetic fertilizers and improving nutrient availability for plants.

    • Increases Soil Organic Matter: Adds humus to the soil, enhancing its capacity to retain moisture and nutrients.

    • Pest and Disease Suppression: High temperatures during composting can kill harmful pathogens and weed seeds, leading to healthier crops.

    • Reduces Waste: Transforms organic waste materials into valuable resources, contributing to waste reduction and environmental conservation.

Cheat Sheet

  • Major Pools of C: Atmosphere (CO₂), terrestrial biomass (plants), soils (soil organic matter), oceans, and carbonate minerals.

  • Dynamic Pools: Atmosphere and soil organic matter are the most dynamic, changing rapidly through biological processes.

  • Carbon in Plants: Derived from atmospheric CO₂ absorbed during photosynthesis and fixed into organic compounds.

  • Compounds in Plants: Include carbohydrates, proteins, lipids, lignin, and polyphenols.

  • Mineralization: Breakdown of organic matter into inorganic forms, releasing nutrients into the soil.

  • Resistance to Decomposition: Lignin, certain polyphenols, and complex organic matter decompose slowly.

  • Oxygen's Role: Presence speeds up aerobic decomposition; absence slows it, often producing methane.

  • Decomposition Factors: Soil moisture, temperature, organic material quality, and oxygen availability.

  • C/N Ratio: Lower ratios favor microbial growth; higher ratios can immobilize nitrogen.

  • SOM Degradation Factors: Soil texture, microbial community, residue quality, and environmental conditions.

  • Differential Degradability: Influenced by biochemical composition; lignin-rich tissues are more resistant.

  • Immobilization: Re-assimilating nutrients into microbial biomass, reducing plant availability.

  • Properties of Humus/SOM: High nutrient-holding capacity, supports microbes, enhances soil structure, and retains moisture.

  • Compost: Nutrient-rich material produced from aerobic decomposition of organic waste.

  • Increasing SOM: Adding organic residues, enhancing biomass, reduced tillage, and conservation practices.

  • Decreasing SOM: Intensive tillage, erosion, over-extraction, and poor moisture management.

  • Management Practices: Cover soil with vegetation, apply organic amendments, crop rotation, minimize disturbance, and promote perennial plants.

Extra Stuff:

  • Lignin & Polyphenol Content: Lignin supports plant structure and slows decomposition; polyphenols inhibit microbial activity, affecting nutrient cycling.

  • Humic Substances: Complex organic molecules enhancing soil health, nutrient availability, and microbial activity while improving soil structure and water retention.

  • Non-Humic Substances: Easily decomposable compounds like sugars and proteins that promote short-term nutrient cycling through rapid microbial use.

  • Composting Process: Aerobic decomposition of organic materials, with stages including mesophilic (moderate temps), thermophilic (high temps for rapid breakdown), and curing (stabilization).

  • Benefits of Composting: Enhances soil structure, provides slow-release nutrients, increases soil organic matter, suppresses pests and diseases, and reduces waste.

  • Carbon Forms in Soil

    • Organic vs inorganic forms

    • Effect on soil properties:

    • Contributes to:

      • soil darkening

      • Cation exchange capacity (CEC)

      • Slow release of nutrients

      • Increased water holding capacity

      • Stabilization of soil aggregates

      • Enhanced infiltration

      • Serves as food for soil organisms

      • Reduced plasticity, allowing better manipulation of clayey soils

      • Stabilizes pH levels

      • Alleviates certain toxicities

      • Improves the availability of various nutrients

  • Types of Organic Compounds

    • Carbohydrates

    • Hemicellulose and cellulose

    • Proteins

    • Fats, waxes, and oils

    • Lignin

    • Polyphenols and tannins

  • Rate of Degradation

    • Aerobic Decomposition:

    1. Mineralization of Carbon

      • Involves enzymatic oxidation reactions

    2. Nutrients

      • Mineralization of nitrogen (N), phosphorus (P), and sulfur (S)

      • Nutrient immobilization occurs when N is limiting

    3. Formation of humus

    • Anaerobic Decomposition:

    • Occurs under flooded or water-saturated conditions

    • Decomposition rate decreases significantly

    • Produces partially decomposed organic compounds and residues

    • Generates organic acids, alcohols, and methane

  • Factors Promoting Rapid Decomposition

    • Near neutral pH

    • Sufficient soil moisture

    • Good aeration

    • Relatively warm temperatures

    • High residue quality

    • High nitrogen content

    • Easily decomposable materials

  • C/N Ratio

    • High ratio leads to competition for N, resulting in immobilization

    • Low ratio provides ample N for mineralization and availability to plants

  • Lignin and Degradation

    • Lignin can slow the degradation rate

    • Reduces immobilization in high C/N scenarios

    • Delays mineralization in low C/N situations, useful for synchronizing N release with crop needs

  • Characteristics of Humus

    • Colloidal nature

    • Negatively charged

    • Contains functional groups

    • pH-dependent charge

    • High water holding capacity

    • Dark coloration

  • Classes of Organic Carbon

    • Biomass: living organic matter

    • Detritus: dead but identifiable organic matter

    • Humic substances: not easily defined, non-inorganic

    • Non-humic substances: chemically classified but without identifiable source

  • Management Practices for Improving Soil Organic Matter

    • Add organic carbon: residues, compost, manure, green manures

    • Increase plant biomass

    • Reduce carbon losses: limit harvested biomass, decrease tillage, minimize erosion

    • Understand that overall cultivation usually decreases SOM

    • Intensive tillage and harvesting can contribute to this decrease

    • Crop rotation and cover cropping can help mitigate these losses

  • Factors Affecting Soil Carbon Levels

    • Climate and vegetation:

    • Warmer temperatures increase microbial degradation and plant growth

    • Water levels: excess or insufficient water can inhibit microbial activity

    • Temperature and moisture interplay affects SOM levels

      • Warm and dry conditions generally correlate with low SOM

      • Cool and wet conditions usually lead to high SOM

    • Vegetation type impacts residue degradability

    • Deposition of carbon can vary (surface vs subsurface support)

    • Soil texture: clay can enhance fertility and reduce decomposition

    • Poor drainage hinders organic matter decomposition due to low oxygen

  • Strategies for Enhancing Soil Organic Matter

    1. Keep soil vegetated and supply organic residues (manures, composts, cover crops)

    2. Set target SOM levels based on natural soil-plant-climate systems (e.g., 1.5% in sandy warm soils)

    3. Maintain adequate nitrogen levels (include legumes, use N fertilizers)

    4. Support maximum plant growth (manage nutrient and water levels)

    5. Limit tillage to reduce organic matter losses

    6. Encourage perennial vegetation to maintain natural ecosystems.