Soil organic matter

• Organic matter is a key component of soil composition.
• This chapter covers soil organic matter, humic substances, soil properties, organic material transformation, and soil as a carbon sink.

Soil Constituents

• A typical soil composition includes a solid phase (45% mineral, 5% organic), and a pore system (20-30% gas, 20-30% liquid).

Organic Components in Soil

• Plants and organisms in soil undergo continuous transformation and synthesis.
• Organic matter is typically lower in quantity than mineral content.
• It plays a crucial part in soil evolution and properties.

Amount and Distribution

• Soil contains variable amounts of organic matter, with common values being 0.5-15% by weight.
• Organic matter mainly accumulates in the surface horizon.
• It decreases with depth, except in specific soil types like Podsols, Histosols, and Fluvisols.
• Soil organic matter is classified into living (macrobiota, mesobiota, microbiota) and non-living materials.
• Non-living materials include fresh organic remains and humus, which break down into decomposition byproducts and synthesis compounds.
• Decomposition byproducts consist of carbohydrates, lipids, and proteins.
• Synthesis compounds include fulvic acids, humic acids, and humin.
• The humus content in soil organic matter ranges from 70-90%.
• Soil microbial biomass consists of fungi (50%), bacteria (30%), and algae/yeast/protozoa (10%) and fauna (10%).

Humic Substances

• Humic substances are the most abundant and chemically active part of soil organic matter, comprising 70-90% of it.
• Their exact structure is not well defined.
• They are classified based on their solubility under varying chemical conditions.
• Soil can be separated based on solubility.
• Soil + NaOH = Soluble and Insoluble components
• The soluble elements + HCI= Fulvic acids and Humic acids
• The insoluble element = Humin
• The soluable Humic acids + Ca2+ = Grey Humic Acids and Brown Humic Acids

Soil Humic Substances

• Soil humic substances are classified into fulvic acids, humic acids, and humin, all of which are pigmented polymers.
• Fulvic acids are light yellow, while humic acids range from yellow-brown to dark brown, and humin is black.
• Fulvic acids have a molecular weight of ~2,000, carbon content of ~45%, oxygen content of ~48%, and exchange acidity of ~1,400.
• Humin acids have the inverse relationship with Fulvic, and increase to humins
• Humic Acids increase in intensity of color with a higher degree of polymerization

Fulvic Acids

• Fulvic acids are solid or semisolid, amorphous substances, and appear yellowish in color
• They exhibit colloidal behavior.
• They disperse in water and do not precipitate in an acidic medium.

Humic Acids

• Humic acids are amorphous solids with a dark brown color.
• They are poorly soluble in water and most non-polar solvents.
• Disperse easily in alkaline solutions of hydroxides and alkaline-metal salts because of their small mass
• Containing an aromatic nucleus and a cortical region that has alliphatic radicals.
• They flocculate with acid treatment.

Humin

• Humin is non-extractable with alkaline reagents.
• Humin is a combination of heterogenous residual and neoformation substances.
• Residual humin exists as particles with a density less than 1.8 g/cm3, trapped in soil aggregates, found in soils with vegetation that is hard to degrade, and weakly bound to the soil clay fraction
• Neoformation humin is similar in origin to fulvic and humic acids.
• Neoformation is irreversibly attached to the mineral fraction.
• Neoformation can only be destroyed in the laboratory by using chemical reagents.

Humic Substance Proportion

• Humic substance composition varies in different soil types.

Characteristics of Humic Substances

• Properties of humic substances include polifunctionality, resistance to biodegradation, non-rigid structure, variable charge, high ion exchange capacity, adsorption of organic molecules, pH buffering, high surface, amorphous structure, hydrophyllicity, hydrophobicity, and colloidal behavior.

Carbon to Nitrogen Ratio

• C/N ratio assesses soil organic matter quality.
• The ideal C/N ratio is around 10, which favors microbial activity.
• High values indicate nitrogen deficiency, thus slowing down microbial activity.
• Balance between carbon and nitrogen indicates the energy source and microbial growth.
• Ideal C/N (≈ 10) balances efficient microbial activity and decomposition.
• C/N (>10-15) indicates low nitrogen availability, slowing down microbial activity and decomposition.
• C/N (<10) suggests excess nitrogen, which causes rapid decomposition plus nitrogen loss through leaching or volatilization.
• Maintaining optimal C/N ratio ensures nutrient cycling and soil fertility.
• Higher the lignin, the slower the decomposition

Types of Humus

• Humus types include Mor, Moder, and Mull, which are defined by evolution, morphology, property, and mineral fraction binding as a global point of view.
• Mor: Characterized as weakly transformed organic matter, has a >60 C:N ratio, is found in acidic/nutrient-poor soils, and has a slow decomposition due to low microbial activity
• Moder: Characterized as more transformed organic matter, fulvic acids, contains precursors, a 30-45 C:N ratio, moderate fertility soils, and faster decomposition when compared to Mor
• Mull: Characterized as highly evolved organic matter, humine, humic acids, and dark soil horizons, a <25 C:N ratio, can be found in nutrient-rich soils, has rapid decomposition, active microbial community, and high nutrient availability

Transformation of Organic Matter

• Humification describes humus formation and organic matter transformation.
• Mineralization describes breakdown of organic compounds, generating inorganic compounds.
• The organic fraction is more rapidly transformed when compared to a mineral fraction
• A horizons are formed faster than B horizons due to transformation of organic fraction.
• Fate of organic compounds is to be mineralized.
• Stability in compounds result in balanced mineralization paired with additions.
• Humic compounds can last for hundreds or thousands of years.
• Humification occurs when organic matter is transformed into humus, leading to soil accumulation, thus stabilizing organic material, forming complex humic compounds for thousands of years.
• Mineralization is when organic compounds completely breakdown into simple inorganic substances, making available nutrients for plants and microorganisms.
• Organic residues decompose quicker than minerals which result in faster A horizon development than the B horizon.
• While organic compounds decompose and stabilize, more humic substances remain, creating a balance between new input and decomposition, which is essential for soil fertility, carbon storage, and microbial activity.
• Microbial soil activity develops when organic residues are incorporated into the soil, where a lot of organic compounds that are intensely degradable are produced.
• Rate of activity lessens after the initial phase because the most stable compounds remain.
• Plant type/C:N ratio influences short-term residue decomposition.
• There is a fast initial microbial activity shaping carbon cycling/humus formation followed by a slowdown of resistant material breakdown.
• Decomposition rate of organic and inorganic compounds depends on the type of compound. Sugars will quickly decompose compared to waxes/phenols.

Stages of Organic Matter Decomposition

• Decomposition is initiated with chemical transformations, before even reaching the soil, which can be leaves “attacked” by microorganisms, which degrade its structure and composition through mineral loss.
• It ends with a catalyzation process that has many interactions, such as integration with the mineral fraction

Humification Processes

• Chemical reactions occurs such as oxidation, condensation, or other results depending on the source.
• Possible sources are enzymatic activity, clay acting as a catalizer, or living organisms.
• Results are consequential. It may lead to new formations, oxidation, molecules merging, and formation of stable compounds.

Phases of Humification

• Humification involves molecule degradation. During this process, macromolecules are broken down into simpler forms such as polymers become monomers though enzymes or direct humification.
• Oxidation of aromatic compounds and formation of quinones.
• Condensation, polymerization, and N fixation occurs to produce humic compounds, bacterial activity will make new polymers, or use polymerization to create humification.
• Bacterial growth may be affected because of deficits or external factors.

Quinone Formation

• Quinone proceeds with polyphenol with the help of lignin in the form of aromatic compounds.
• Quinone and amino acids can combine.

Factors Affecting Organic Matter Degradation

• The factors that affect organic matter degradation: the quality of organic matter, the type of soil, the climate, abiotic and antropic, and microorganisms.

Factors Affecting Soil Organic Matter

• Factors that influence soil organic matter content are vegetation, climate, soil composition, topographic position, drainage, texture, clay mineralogy, estructure, and tillage.
• Humid and cold climates show high soil organic matter content because of slow decomposition.
• Hot and dry climates show lower soil organic matter content because of rapid decomposition.
• Tropical climates see rapid decomposition.
• High wetlands due to anaerobic conditions slowing break down.
• Soil depth is proportional to its organic carbon content.
• SOM includes soil’s components that have interacted with humification and mineralization, whereas OM includes the soils’s total amount of organic components.
• There is more surface soil on flood plains.
• There is more organic matter in grassland soils than grasslands converted to torest, which declines.
• ↑ OM with ↓ slope.
• No-till is the best strategy for maintaining high productivity and carbon.

Properties of Soil Organic Matter

• Soil organic matter affects color of soil, promotes structure, maintain high porosity, creates permeability, retains water, raises the temperature of the soil, and protects against pollution and erosion
• Organic amendments such as straw leaves improve organic content.

Chemical and Physico-Chemical Properties

• Colloidal properties, (absorb water, swelling-shrinking, dispersion and flocculation).
• Formation of organo-mineral complexes.
• Exchange capacity: Organic matter exchange capacity is 3-5 times higher than clays particles
• OM is considered to be a good nutrient reservoir
• It is responsible for the modification of pH which leads to acidifying the soil
• Organic Matter influences the soil's dispersion
• Weathering is based on the acidity of the soil
• It helps buffer the soil's pH against changes

Type of Charge

• The charge found on a OM stems from carboxly groups
• Colloids have charges which can be measured.
• Organic matter measured more than montmorillonite at 200 cmolc/kg, versus it's 100

Organo-Mineral Complexes

• Types of organo-mineral complex: clay-organic compounds, and organic compounds-metal.
• Humic-clay complexes are the result of an organic molecule and clay particles binding.
• Consisting of stabilized structures which are insoluble with with large particle size.
• Complex interactions with a biological active soil and humificiation of that soil is directly proportionate.
• When there is an acid imbalance in the soil, clay and organic matter activity is lower

Organo-Metallic Complexes

• Organo-metallic complexes are the result of a metal and organic compound union
• These occur from interactions between methyl and carboxyl groups
• Variable with a functional molecule structure
• Soluble from metals and environmental conditions
• Smaller, at is relevant for translocation