soil

Unit 7: Carbon in Soil

Vasily Dokuchaev (1846-1903)

  • Known as the “father of soil science.”

  • Russian geologist and geographer who established the foundation of soil science.

  • Identified five factors for soil formation.

  • Introduced several Russian terms related to soil types (e.g., chernozem, podzol, gley, solonetz).

Soil Organic Matter (SOM)

  • Defined as a complex and varied mixture of organic substances.

  • Comprises about half of its mass from carbon.

  • World's soils hold 4-5 times more carbon than all vegetation combined.

Major Sources of Soil Organic Matter

  • Above Ground Plant Inputs: Leaves, needles, and wood from plants.

  • Below Ground Plant Inputs: Roots (both coarse and fine).

  • Microfauna: Organisms such as bacteria and fungi aid in decomposition.

  • Macrofauna: Includes larger organisms like arthropods and earthworms that contribute to SOM.

Characteristics of Soil Organic Matter

  • SOM accumulates primarily in the top layer of the soil.

  • Soil color can indicate SOM content; darker soil typically contains more SOM.

  • Generally, the SOM percentage is small in most soils.

Functions of Soil Organic Matter

  • Chemical:

    • Increases soil cation exchange capacity.

    • Buffers pH levels, maintaining soil acidity/alkalinity.

    • Serves as a carbon reservoir.

  • Physical:

    • Enhances water-holding capacity.

    • Contributes to formation and stabilization of soil aggregates.

    • Improves overall soil structure.

  • Biological:

    • Acts as a storehouse for essential nutrients (N, P, K, S, Ca, Zn).

    • Supports microbial biodiversity.

    • Provides energy sources for microbial populations.

  • Ecological:

    • Plays a key role in nitrogen (N) and carbon (C) cycles.

    • Influences soil health, productivity, and produce quality.

The Global Carbon Cycle

  1. Plant Uptake of CO2:

    • Photosynthesis captures CO2,

    • Some CO2 is released back into the atmosphere through respiration.

  2. Carbon Storage:

    • Remaining carbon is stored in plants and later contributes to soil organic matter through litter decomposition, including crop residues.

  3. Consumption by Soil Organisms:

    • Plant tissues are consumed by soil organisms, returning carbon as CO2.

  4. Contribution to Soil Organic Matter: All carbon sources contribute to SOM.

  5. Chemical Reactions in Soil:

    • Soil CO2 reacts to form carbonates and bicarbonates (Ca, K, Mg, Na).

  6. Protection of Organic Carbon:

    • Some organic carbon is absorbed by soil aggregates, protecting it from microbial decomposition, facilitating accumulation.

Conditions for SOM Accumulation

  • Inputs: Large contributions of organic carbon, particularly from grass roots (e.g., Chernozems).

  • Microbial Activity Limits: Decomposition restricted by cooler climates, high water saturation, and anaerobic conditions.

Carbon-Nitrogen Ratio (C:N Ratio)

  • The C:N ratio is crucial for determining the decomposition rate and the availability of nitrogen to plants.

  • Low nitrogen availability leads to competition among microbes, often causing nitrogen immobilization.

  • Adequate nitrogen leads to mineralization, turning organic nitrogen into inorganic forms available for plants.

Soil Organic Matter Composition

  • Historically, SOM was viewed as large macromolecules.

  • Present understanding acknowledges SOM as a mix of recognizable biomolecules undergoing different degradation stages.

    • Methods of extraction include using strong bases (NaOH) and acids (HCl).

    • Products include soluble (fluvic acids), humic substances, and complex non-extractable forms.

Conversions and Factors Affecting SOM Measurement

  • Conversion factor for SOM calculation: %SOM = 1.72 × %SOC.

  • SOM encompasses living biomass, plant residues, and dissolved organic biomolecules.

Pedogenic Processes Increasing Carbon Stocks

  • Pauldization: Accumulation of organic layers at the mineral surface forming organic horizons (Of, Om, Oh).

  • Humification: Accumulation of carbon in the topsoil forming humic substances from plant residues.

  • Other processes such as Podzolization, Gleization, and the carbon accumulation in Canadian soils give insights into various soil types and their carbon storages.

Loss of Soil Organic Matter

  • Conversion of native lands to agriculture leads to the loss of about 25% of the C in the upper soil layers.

  • Causes of losses: erosion, overgrazing, intensive tillage, and removal of residues.

Soil Carbon Dynamics and Climate Change

  • Increased microbial activity due to soil warming leads to faster organic matter decomposition.

  • Thawing permafrost exposes stored organic material, accelerating decomposition.

  • Fires contribute to SOM losses and affect soil capacity as a carbon sink.

Management Practices to Enhance Carbon Inputs

  • Reduced tillage and reduced use of summer fallow.

  • Increasing organic carbon inputs via manure, biochar, cover crops, and perennials.

Summary Messages

  • SOM is integral for biogeochemical cycles and supports biodiversity.

  • Carbon is fundamental to SOM.

  • SOM has key chemical, physical, biological, and ecological functions.

  • Soil acts as a carbon sink but is losing carbon faster than it can sequester it.

  • Most carbon is stored in Cryosols and organic soils in Canada due to favorable climate conditions.

Unit 8: Soil Chemistry – Soil Colloids

What are Soil Colloids?

  • Comprise clay and humus particles in soils, often referred to collectively as the colloidal fraction.

  • Clay is the smallest particle size in soil, while humus is dark, organic material from decayed plant and animal matter.

Properties of Soil Colloids

  • Size: Extremely small; visible only with an electron microscope.

  • High Specific Surface Area: Colloids have significantly larger external specific surface than sand.

  • Surface Charge: Internal and external surfaces carry different charges (positive and negative), attracting or repelling nearby substances.

    • Charge types can be permanent or pH-dependent.

Adsorption and Ion Exchange

  • Cations (positively charged ions) are attracted to negatively charged colloid surfaces.

  • Anions (negatively charged ions) can also be adsorbed if colloids possess positive charges.

  • Ion exchange capacity is key for understanding ion mobility in the soil.

Types of Clay

  • Silicate Clay: Dominant in many soils; exhibits structure from crystalline sheets of bonded atoms.

    • Includes both crystalline and non-crystalline species.

  • Non-Silicate Clay: Composed of iron and aluminum oxides; often found in highly weathered soils.

Weathering Effects on Clay Minerals

  • The weathering process shapes the distribution and type of clay present in the soil.

Organic Matter Characteristics

  • Comprised of non-crystalline heterogeneous organic substances.

  • High surface area absorbing large quantities of water; varies in chemical composition but largely consists of carbon, oxygen, hydrogen, and nitrogen.

Unit 8c: Soil Salinization

Occurrence of Salinization

  • Caused by weathering, salt transport via water, human activities, and geological formations.

Water Balance and Salinization

  • Salts accumulate where rainfall is insufficient to leach them away from surface layers.

  • Changes in vegetation and land use can exacerbate salinization risk.

Contribution of Irrigation to Salinization

  • Irrigation significantly alters water balance and can bring salts into the root zone.

  • It is necessary for food security but needs to be carefully managed.

Management Practices to Reduce Salinization

  • Implement soil covers, cultivate salt-tolerant crops, and manage irrigation practices for effective drainage and salt leaching.

Measurement of Salinization

  • Electrical conductivity (EC), Sodium adsorption ratio (SAR), and Exchangeable sodium percentage (ESP) are common assessments for salinity levels in soils.

Unit 9: Soil Biology – Organisms and Ecology

Soil Biota and Community Characteristics

  • Soil biota includes microorganisms, soil fauna, and plants living in or on the soil, contributing energy and nutrient flow.

  • Characteristics include high diversity, abundance, and biomass.

Classification of Soil Organisms

  1. Microflora/Microfauna

  2. Mesofauna

  3. Macrofauna

Functions of Bacteria

  • Crucial for organic matter decomposition, nutrient cycling, and disease suppression.

  • Various functional groups exist, including decomposers and mutualists.

Role of Fungi and Actinomycetes

  • Decomposers contributing to nutrient cycling and soil structure improvement.

Role of Arthropods and Earthworms

  • Diverse feeding groups (shredders, predators, herbivores, fungal feeders).

  • Enhance soil structure and aeration, contributing to nutrient cycling.

Importance of Soil Health

  • Soil organisms drive ecological processes, contribute to nutrient dynamics, and impact soil productivity.

Unit 10: Soil Nutrients

Key Nutrients

  • Nitrogen (N), Phosphorus (P), Potassium (K) are essential for plant productivity.

  • Management practices are crucial for maintaining nutrient availability and mitigating environmental impacts from fertilizers.

Unit 11: Soil Health

Responsibility for Soil Health

  • Emphasizes individual and collective responsibility in maintaining soils for ecosystem services and food security.

Unit 12: Fire and Soil

Fire Dynamics

  • Understand how fire interacts with soil and ecosystems.

  • Fire management practices and their effects on soil carbon dynamics and regeneration.

Concise Notes on Carbon in Soil

  • Vasily Dokuchaev: Father of soil science; identified five soil formation factors.

  • Soil Organic Matter (SOM): Complex mix of organic substances; ~50% carbon; crucial for soil function.

  • Sources of SOM: Above/below ground plant inputs, microfauna, macrofauna.

  • Functions of SOM: Enhances soil chemical, physical, biological, and ecological properties.

  • Global Carbon Cycle: Plants uptake CO2; decomposition returns carbon to soil.

  • Conditions for SOM Accumulation: Organic carbon inputs, limited microbial activity in certain climates.

  • C:N Ratio: Critical for decomposition rates and nitrogen availability.

  • Factors Affecting SOM Measurement: SOM composition changes and extraction methods.

  • Losses of SOM: Agriculture can lead to significant carbon loss; Erosion, overgrazing, and intensive tillage are major causes.

  • Climate Change Impact: Soil warming increases decomposition rates; fires and thawing permafrost exacerbate losses.

  • Management Practices: Reduced tillage, increasing organic inputs help enhance carbon retention.