Composition and soil biodiversity (5.1.2) | IB DP ESS SL

IB Syllabus focus:
‘Soils contain mineral particles, organic matter, water and air, forming habitats for diverse microorganisms, animals and fungi.’

Soil composition and biodiversity are central to understanding how ecosystems function. They determine fertility, water regulation, and energy flow, while supporting vast biological communities crucial for ecological balance.

Components of Soil

Mineral Particles

Soils consist of sand, silt, and clay, which form the mineral matrix. Their proportions determine texture, porosity, and nutrient-holding capacity.

Sand: Large particles; allow high drainage but low nutrient retention.
Silt: Medium-sized; balance between drainage and retention.
Clay: Small particles; high water retention and nutrient-holding capacity.

Soil Texture: The relative proportions of sand, silt, and clay that define soil’s physical properties and influence fertility.

Organic Matter

Also called humus, organic matter originates from decomposed plant and animal material. It improves soil structure, enhances nutrient supply, and aids water retention.

Water

Soil water, found in pores, supports plant uptake of nutrients in dissolved form. Its availability varies with texture, depth, and climatic conditions.

Air

Air in soil pores provides oxygen for root respiration and aerobic microorganisms. Excess compaction or waterlogging reduces aeration and threatens soil life.

Soils contain mineral particles, organic matter, water and air in dynamic proportions that underpin their physical and biological functions.

Pie chart showing the typical volumetric composition of mineral soils: ~45% inorganic mineral matter, ~5% organic matter (humus and living biota), and ~25% each of water and air. Actual proportions vary with texture, climate, vegetation and management. Source.

Soil Biodiversity

Microorganisms

Soil hosts bacteria, fungi, and archaea.

Bacteria: Drive nutrient cycling (e.g., nitrogen fixation, decomposition).
Fungi: Form mycorrhizal associations with roots, extending nutrient uptake.
Archaea: Contribute to processes like methane production in anaerobic zones.

Mycorrhizae: Symbiotic associations between fungi and plant roots that improve nutrient absorption, particularly phosphorus, while the fungus gains carbohydrates.

Mycorrhizae are mutualistic associations between plant roots and fungi that significantly increase nutrient and water uptake.

Fauna

Soils provide habitat for diverse animals:

Microfauna (protozoa, nematodes) regulate microbial populations.
Mesofauna (mites, springtails) aid decomposition and nutrient recycling.
Macrofauna (earthworms, termites) mix and aerate soils, enhancing fertility.

Flora

Root systems influence structure and stability by binding particles and aiding infiltration. Plant roots also secrete compounds that stimulate microbial communities.

Interactions Between Components and Biodiversity

Nutrient Cycling

Microorganisms decompose organic matter, releasing nutrients. This maintains fertility and supports plant growth.

Carbon cycle: Decomposers recycle organic carbon into CO₂.
Nitrogen cycle: Bacteria mediate nitrogen fixation, nitrification, and denitrification.

Soil Structure

Organic matter and root systems create aggregates, clusters of soil particles bound by biological activity. Aggregates improve porosity, aeration, and erosion resistance.

Water Retention and Movement

Clay and organic matter increase cation-exchange capacity (CEC), retaining nutrients for plant uptake. Earthworms and other fauna create channels, promoting infiltration and drainage.

Cation-Exchange Capacity (CEC): The ability of soil particles, especially clays and organic matter, to retain and exchange positively charged nutrient ions.

Habitat Provision

Soil layers provide microhabitats with distinct moisture, temperature, and nutrient conditions. Diversity of niches supports high species richness across scales.

Factors Influencing Soil Composition and Biodiversity

Climate

Temperature and precipitation regulate decomposition, mineral weathering, and organic matter accumulation. Warm, moist conditions promote high biodiversity.

Parent Material

The rock from which soil develops influences mineral composition and fertility. For example, basalt-derived soils often contain more nutrients than quartz-derived soils.

Topography

Slope and drainage patterns affect erosion, moisture availability, and biodiversity distribution. Steep slopes are more prone to nutrient loss.

Land Use

Agriculture, deforestation, and urbanisation disrupt soil biodiversity. Practices such as monoculture and overuse of fertilisers reduce microbial and faunal diversity.

Importance of Soil Biodiversity

Ecosystem Services: Soil organisms support provisioning (food, fibre), regulating (carbon sequestration, climate regulation), and supporting (nutrient cycling, soil formation) services.
Plant Productivity: Fertile soils rich in organic matter and biodiversity ensure sustainable crop yields.
Resilience: Biodiverse soils resist pests, diseases, and environmental stresses better than degraded soils.
Carbon Storage: Healthy soils act as sinks for carbon, reducing atmospheric greenhouse gases.

Human Impacts on Soil Composition and Biodiversity

Intensive Agriculture: Reduces organic matter and microbial diversity through over-cultivation, fertiliser dependency, and pesticide use.
Deforestation: Removes organic inputs and increases erosion, degrading soil communities.
Pollution: Heavy metals, plastics, and chemicals disrupt microbial and faunal populations.
Climate Change: Alters temperature and moisture regimes, threatening soil biodiversity and carbon storage capacity.

Conservation of Soil Biodiversity

Strategies to maintain composition and biodiversity include:

Adding organic amendments such as compost and manure.
Reducing tillage to protect soil structure and habitats.
Employing crop rotation and polyculture to maintain nutrient balance.
Encouraging agroforestry to enhance organic inputs and root diversity.
Preventing overgrazing and compaction by managing livestock.

FAQ

Soil porosity refers to the proportion of pore spaces between particles. Large pores in sandy soils promote rapid drainage but can limit water retention, while small pores in clay-rich soils hold more water but reduce air circulation.

A balanced porosity ensures that both water and oxygen are available for plant roots and soil organisms, maintaining soil biodiversity and productivity.

Earthworms create burrows that improve soil aeration and infiltration. Their feeding activity mixes organic and mineral components, enhancing soil structure.

They also excrete nutrient-rich casts, which increase fertility and provide microhabitats for microorganisms.

Protozoa feed on bacteria, regulating microbial populations and preventing dominance of any single species.

This grazing releases nutrients such as nitrogen in plant-available forms, linking microbial processes to soil fertility.

Fibrous roots create dense networks, stabilising soil and supporting microbial activity in surface layers.
Taproots penetrate deeper, improving vertical mixing and allowing organisms to inhabit lower horizons.

Diverse root systems increase habitat variety, promoting higher soil biodiversity.

Soil fungi are sensitive to pH, temperature, and moisture. Acidic or alkaline soils can inhibit fungal growth, while drought reduces hyphal networks.

Intensive pesticide use also diminishes fungal populations, disrupting mycorrhizal associations essential for plant nutrient uptake.

Practice Questions

Question 1 (2 marks)
Identify two main components of soil, other than mineral particles, that contribute to supporting soil biodiversity.

Mark scheme:

Organic matter (humus) – 1 mark
Water – 1 mark

Air – 1 mark (accept any two correct; maximum 2 marks)

Question 2 (5 marks)
Explain how soil biodiversity contributes to the maintenance of soil fertility and plant productivity.