Soil Flora, Fauna, and Ecology

Biodiversity and the Soil Food Web

• Importance of Biodiversity: Biodiversity is considered a more significant metric than the total number of individuals within a soil system. The sheer number of taxa in soil is vast and critical for ecosystem function.
• Generic Food Web Components:     * Producers: Utilize solar energy and $CO_2$ to create organic matter.     * Humus: Serves as a reservoir of organic material.     * Decomposers: Break down organic matter, releasing heat and $CO_2$.     * Consumers: Organisms that eat producers or other consumers.
• Comparison: Above-Ground vs. Soil Food Webs:     * Most plant matter is not eaten by herbivores above ground; instead, it is decomposed via the soil food web.     * Soil Food Web Characteristics:         * It is significantly more complex and diverse than above-ground systems.         * It contains more functional redundancy (multiple species performing the same role).         * It spans a much greater range of timescales.         * It is approximately 80-90% microbial in nature.

Soil Biology and Earth History

• Timeline of Biologic Evolution in Soil:     * Early Earth: Life on land was limited to single-celled organisms existing in crusts on soils.     * 470 Ma (Million Years Ago): Earliest evidence of "complex" life on land appears in the form of mosses or moss-like plants (Bryophytes). These are non-vascular, lack flowers or seeds, and rely on spore-based reproduction in humid settings (e.g., Mosses, Hornworts, Liverworts).     * Silurian to Devonian (443–358 Ma): Appearance of Prototaxites, described as the "Godzilla" of fungi or lichen.     * 400 Ma: Vine-like plants begin to cover moist soils.     * 370 Ma: Evolution of seed plants and forests. This shift changed channel patterns, hydraulic regimes, and weathering processes.
• Evolution of Soil Types: Changes in soil biology drove the evolution of earth and soils, leading to the development of specific soil orders like Alfisols, Mollisols, Histosols, and Entisols (Reference: Gibling & Davies 2012).

Ecological Classification and Vocabulary

• Energy Source Classification:     * Phototrophs: Obtain energy from solar radiation via light reactions (Photosynthesis is described as the "Mother of all redox reactions").     * Chemotrophs: Obtain energy through the oxidation of organic or inorganic compounds.
• Carbon Source Classification:     * Heterotrophs: Use pre-existing organic matter; use other organisms as food.     * Autotrophs: Use $CO_2$ or otherwise manufacture their own food.
• Trophic Categories:     * Herbivores: Consume living plant tissues.     * Detritivores: Eat dead tissues; usually obtain nutrients from the microflora within the detritus rather than the detritus itself.     * Saprophytic Microorganisms: Primarily fungi and bacteria; responsible for the majority of decomposition.     * Predators: Described as the ‘cats’ of the soil Serengeti, preying on other organisms.

Habitats and Soil Quality

• Microscale Habitats: Soil offers an incredible diversity of "niche space," capacity for isolation, and non-uniform population distributions (spatial, temporal/seasonal, and genetic diversity). Landscapes hide complex ecosystems under the surface.
• Soil Biology & Soil Quality:     * Managing a healthy food web is not about inoculating with a single key species; it is about creating environmental conditions that support a diverse community.     * Soil organisms influence organic matter, nitrate leaching, nutrient cycling, soil structure/aggregation, humification, and decomposition.
• Functional Diversity and Redundancy:     * Species Diversity: Even distribution across a range of species.     * Functional Diversity: The ability to utilize a wide range of substrates and carry out various processes.     * Functional Redundancy: Many organisms performing the same task, providing stability and resilience to the ecosystem.     * Keystone Species: Specific species that have a disproportionately large effect on their environment relative to their abundance.

Survey of Soil Organisms by Size

• Size Classifications:     * Macrofauna: Organisms greater than $2\,mm$ (e.g., Earthworms, large arthropods).     * Mesofauna: Organisms between $0.1\,mm$ and $2\,mm$ (e.g., Mites, Springtails).     * Microfauna: Organisms smaller than $0.1\,mm$ (e.g., Fungi, Bacteria, Nematodes, Archaea).
• Microfauna Roles: They run all levels of decomposition, act as primary producers, and can be pathogens.

Detailed Taxonomy: Bacteria, Archaea, and Earthworms

• Prokaryotes (Bacteria & Archaea):     * Cells lack an organized nucleus.     * Size range is approximately $0.5$ to $5\,μm$ (similar to the size of clay particles).     * Archaea: Distinct from bacteria evolutionarily; once thought to be only extremophiles but are found everywhere and may comprise approximately $10\%$ of soil biomass.
• Earthworms (Macrofauna):     * ~7,000 species worldwide.     * Targeted Functional Groups:         1. Epigeic: Live on/near the surface; feed on litter; no soil mixing.         2. Endogeic: Live in the upper $30\,cm$; horizontal burrows; "eat their way" through the soil, mixing it.         3. Anecic: Vertical burrows; haul litter into holes; Soil Organic Matter (SOM) eaters (Example: Lumbricus terrestris).     * Ecosystem Services:         * Comminution: Physical breakdown of soil and organic matter.         * Nutrient Cycling: High-nutrient bodies; casts are loaded with nutrients.         * Soil Structure: Casts are responsible for granular structure; increase porosity and aeration.     * Negative Impact: invasive worms can destroy leaf litter habitats for salamanders (e.g., the ringed salamander Ambystoma annulatum) and other arthropods. Introduction methods include discarded bait or transplanted soil in tree balls.

Arthropods, Nematodes, and Soil Management

• Arthropods (Meso to Macro):     * Major Groups: Ants (approx. 9,000 species), mites, springtails, spiders, termites.     * Functions: Shredding organic material, stimulating microbial activity via grazing, mixing microbes with food/soil, mineralizing nutrients in waste, enhancing aggregation (via saliva/waste), and burrowing.     * Ants: Walter Tschinkel (FSU) studied nest architecture using casts to show size and complexity. Ant nests are often enriched in nutrients.     * Termites: More efficient digesters than earthworms; nutrients are concentrated near mounds rather than being uniformly distributed, which can lead to poor plant growth on mounds.
• Nematodes (Microfauna):     * Threadworms or roundworms; approx. 20,000 species.     * Size: $4$ to $100\,μm$ in diameter.     * Can be parasitic or predatory; used for pest control or act as the pest themselves.     * Example: Root-knot nematodes cause stunting in tobacco plants. Marigolds can be used to control plant-parasitic nematodes.

Roots and the Rhizosphere

• Root Biomass percentages:     * Grassland: $50-60\%$     * Forest: $40-70\%$     * Crops (annual): $15-40\%$
• Root Dynamics: Roots occupy approx. $1\%$ of soil volume but account for $20-30\%$ of soil respiration.
• Rhizosphere: The zone of soil within $1-2\,mm$ of live roots. It is enriched in organic compounds via Rhizodeposition:     1. Low molecular weight compounds: Phenols (important for allelopathy).     2. High molecular weight mucilages: Mucigel (plant exudate + microbes + clay) provides lubrication and protection.     3. Root cells: Sloughed-off tissues.

Soil Fungi

• Fungal Characteristics: Heterotrophic eukaryotes with filamentous morphology (hyphae). Individuals are hard to count; biomass is often measured by hyphal length per area ($m^2$).
• Morphological Subdivisions:     1. Yeasts: Live in waterlogged, anaerobic soils.     2. Molds: Simplest spores; filamentous; long branching chains.     3. Mushroom Fungi: Basidiomycetes and Ascomycetes. Mushrooms are best at breaking down woody tissue, but molds are more important for the decay process overall.
• Key terms: Hyphae (filaments), Mycelia (mats of hyphae), Mantle (sheath-like growth).
• Roles: Production of humus, SOM accumulation, nutrient availability (releasing excess N), binding soil particles, and pest control (e.g., Arthrobotrys anchonia trapping nematodes with hyphal loops).

Mycorrhizae: Symbiotic Relationships

• Definition: Symbiotic association between roots and certain fungi.
• Nutrient Exchange: Fungi receive sugars from the plant (costing the plant $5-30\%$ of its energy budget). In return, plants gain extended reach for phosphorus ($P$) and water ($H_2O$), and protection from metals and pests.
• Types:     1. Ectomycorrhizae: Occur in temperate shrubs and trees; hyphae remain outside cortical cell walls, creating a mantle.     2. Endomycorrhizae (Arbuscular Mycorrhizae/AM or VAM): Form branched structures within cortical cell walls; can link multiple plants together in a 4-way relationship involving mycorrhizal fungi, rhizobial bacteria, legumes, and other plants.