SOIL BIOLOGY AND ECOLOGY Introduction Importance of Soil Organisms Diversity of Soil Organisms Relationships between Organisms Characteristics of Soil Organisms Abundance and Biomass of Organisms Methods for Quantifying Organisms
Introduction • Soil organisms are the organisms that spend all or part of their lives in the soil environment. • Soil animals (fauna) range in size from macrofauna (moles, earthworms, millipedes) through mesofauna (springtails and mites) to microfauna (such as nematodes and protozoan) • Soil plants (flora) include roots of higher plants, algae, fungi, bacteria, and actinomycetes • Soil is an ecosystem in which these different creatures interact and contribute to the global cycles that make all life possible
Interactions of Organisms • Herbivores- organisms that subsist on living plants • Detritivores- organisms that subsist on detritus • Fungivores- organisms that feed on fungi • Bacterivores- organisms that feed on bacteria • Predators- organisms that consume other animals • Parasites- organisms that live off of other organisms but not consume them. • Every handful of soil is likely to contain billions of organisms with representatives from every phylum of living things (See table on next slide).
Importance of Soil Organisms • Decomposition of plant and animal residues. • Release of nutrients in forms available to plants. • Synthesis of new organic compounds. • Humus formation causing increased nutrient holding capacity, better soil structure and water holding capacity. • Pollution and environmental aspects: decomposition of sewage, manure, and pesticides. • Nitrogen fixation.
Importance of Ecosystem Species Diversity • A high species diversity indicates that organisms present are fairly evenly distributed among a large number of species. • Species diversity usually indicates a high degree of functional diversity. • Capacity to carry out a wide array of processes. • Functional redundancy • This leads to ecosystem stability and resilience. • High biological diversity is an indicator of soil quality.
Soil FaunaClasses of Soil Fauna Soil fauna belong to one of three size limit classes: • Macrofauna (>2mm, width) • Mesofauna (0.1-2mm, width) • Microfauna (<0.1mm, width)
Macrofauana Major specific groups of macrofauna are: • Vertebrates –gophers, moles, mice, • Arthropods –ants, beetles, termites, centipedes, etc • Annelids -earthworms • Mollusks –snails, slugs
Functions of Macrofauna • Pulverize, mix, and granulate soil and incorporate O.M. into the lower horizons. • Provide large channels through which air and water can move freely. • Partially digest organic residues and leave their excrement for microbial degradation.
Mesofauna • Major specific groups of mesofauna are: • Arthropods –mites, collembola (springtails), protura • Annelids –enchytraeid (pot) worms
Functions of Mesofauna • First line of attack on litter on the forest floor • Important regulators of some microfaunal biomass and composition
Microfauna • Major specific groups of animals in this class are: • Nematodes • Rotifera • Protozoa • Small, mostly microscopic animals • Soil microfauna is important for the cycling of nutrients in ecosystems • Control populations of microflora
Nematodes • very small (usually microscopic) thread-like worms • most live on dead O.M. • Nematodes are predators of the microflora, other nematodes, and protozoa, thereby acting as biological control for those organisms
Protozoa • The smallest, simplest forms of animal-like life. • Most numerous of soil fauna • Biomass 20-200 kg/ha • Some protozoa are predators on soil bacteria, especially in the area immediately around plant roots (the rhizosphere). Protozoa
Rotifers • Microscopic animals that thrive in fresh water environments, and moist swampy soil conditions. • The diet of rotifers most consists of dead or decomposing organic materials, as well as unicellular algae and other phytoplankton • Rotifers are in turn prey to secondary consumers, including shrimp and crabs Rotifera
Soil FloraClasses of Soil Flora • Soil flora belong to one of two classes: • Macroflora (>0.1mm, width) • Vascular plants –feeder roots • Bryophytes -mosses • Microflora (<0.1mm, width) • Vascular plants –root hairs • Algae –yellow, green, diatoms etc • Fungi –yeast, moulds, mushrooms etc • Actinomycetes • Bacteria –aerobes and anaerobes
Soil Microflora Algae • They are microscopic (2-20µm in size) • Algae consist of eukaryotic cells (nuclear membrane) • many contain chlorophyll • 10 - 500 kg/ha algae, almost all at surface, because they need light • perform best in moist conditions where they swim • 3 most important groups are: green, yellow-green, & diatoms (some form association with fungi ~lichens) • Green algae are the most prevalent in normal soils • Diatoms are common in neutral to alkaline OM rich soils • Algae add OM to soil, excrete polysaccharides that favor aggregation and structure formation
Fungi • Physical appearance looks more like plants, but they do not contain chlorophyll • They are eukaryotes with a nuclear membrane and cell walls • They are heterotrophs • Form net-like mycelial growth • Fungi biomass ~ 1,000-15,000 kg/ha in the top soil • Tolerant of very dry soil conditions and grow over a wide range of pH • Three main groups of fungi are yeasts, molds, and mushrooms. • They play vital role in the decomposition of fresh plant residues, humus formation, aggregate stabilization, and nutrient cycling
Bacteria • Very small-single cell organisms that thrive in all soil conditions (considered plant because of spores) • They are prokaryotic (no cell membrane) • most are heterotrophic, some autotrophic e.g., nitrification, and sulfur oxidation • -exist as clumps, mats, and filaments called colonies • Biomass ~400-5000 kg/ha bacteria, but have ability to increase in numbers rapidly • Bacteria are involved in a broad range of enzymatic processes in soils • Participate in many transformations in soil (e.g., some bacteria ~mainly rhizobium in association with legumes, fix atmospheric nitrogen) • They specialize in the breakdown of hydrocarbons in soil
Mycorrhizae • Mycorrhizae"fungus root" - an economically important association between numerous fungi and the roots of higher plants. • The plant roots exude sugars to feed the fungi and the fungi carry in soil nutrients which might otherwise by deficient in the plant. Types of Mycorrhizae • Endomycorrhizae - penetrate plant cells • Ectomycorrhizae - grow into intercellular spaces but do not penetrate cells. • They enhance plant uptake of nutrients • They enhance water uptake by plants • They protect plants from uptake of toxins
Actinomycetes • Actinomycetes are filamentous like fungi but threads are smaller in size. • They are like bacteria in that they are single celled and about same size, and have no nuclear membrane. • They are heterotrophs that live mostly on decaying organic matter. • They develop best in moist, well aerated soil (do poorly below pH 6). • -biomass ~ 5000 kg/ha • Actinomycetes produce antibiotic compounds that kill other microorganisms ( actinomycin, neomycin, streptomycin) • They participate in atmospheric N-fixation
Relationships between organisms The food web • The activities of soil organisms (flora and fauna) are related in the food chain or food web. • As one organism eats another, nutrients and energy are passed from one trophic level to a higher one (from the primary producers through the primary and secondary consumers to the tertiary consumers). • The food web or food chain illustrates how various soil organisms are involved in the breakdown of plant tissue, the formation of humus, and the cycling of carbon and nutrients.
Primary Producers • Plants play principal role as primary producers • They produce organic matter (C, chemical energy that other organisms depend upon). • Primary producers form the food base for the entire food chain. • Photosynthesize, (CO2 water, sunlight) • Other primary producers are mosses, algae, lichens, and photosynthetic bacteria. • Some primary producers can use inorganic chemical reactions to obtain energy for making living cells
Primary Consumers • Primary consumers are the macro & mesofauna, and microflora that use the energy produced and stored in the plant residues (primary producers). • The macro- and mesofaunachew up big pieces into little pieces for microflora to follow in the attack. • Some primary consumers are Herbivores: • termites, ants, beetle larvae, woodchucks, moles, nematodes mice that eat above ground plant parts, earthworms, other insects • Some primary consumers are Detritivore • Source of food is debris of dead tissues (detritus) left in soil. • The detritus contains microorganisms that are the source of nutrients. • Some primary consumers are Saprophytic (feeding on decaying OM eg, bacteria, fungi) • Feed on detritus, dead animals, feces by forming colonies which may then provide food for secondary consumers
Secondary Consumers • The bodies of primary consumers become food sources for secondary consumers (predators and parasites in the soil). • Secondary consumers include microflora such as bacteria, fungi, actinomycetes that eat leftovers and waste from primary consumers. • Secondary consumers also include carnivores such as centipedes and mites, spiders, snails (usually prey on primary consumers, small insects or nematodes) • Secondary consumers include Microphytic feeders (organisms that feed on microflora as their food) eg., collembola, mites, termites, nematodes, protozoa, eat fungi and bacteria, thereby checking the activities and growth of their populations.
Tertiary Consumers • Secondary consumers are prey for other carnivores- tertiary consumers. • E.g., ants consume centipedes, spiders, mites and scorpions, all of which can prey on primary and secondary consumers. • Some species of birds (Robins) specialize in eating soil animals such as beetles and earthworms from their burrows. • Microflora is involved at every level of the process (ultimate decomposers) • directly attacking plant tissues and bodies of dead animals • active in the digestive tracts of soil animals • Decomposing animal feces
Abundance and Biomass of Soil Organisms • Soil biomass is the living fraction of the soil. • This living fraction is generally related to the total organic matter in the soil (1-8% of organic matter). • Soil organism numbers in general depend on the types of organism as well as the environment of the organisms.
Factors affecting Organism Abundance and Biomass • Amount and quality of food available • Physical factors • Moisture and temperature • Biotic factors • Predation and competition • Soil chemical factors • Acidity, salinity, dissolved nutrients
Sources of Carbon and Energy • Heterotrophic - obtain their energy and carbon from the breakdown of organic materials. • Autotrophic - obtain their energy and carbon from other sources, such as energy from the sun and carbon from CO2. Examples: • Photoautotrophs - use solar energy. • Chemoautotrophs – obtain C from CO2 and get energy from oxidation of inorganic compounds such as iron (FeII), sulfur (H2S), nitrogen (NH4 & NO3).
Methods for quantifying soil organisms: Different methods are used (Each method have their value depending on objective) • Organism Numbers - plate counts; get a sample, place on agar plate by streaking, incubate and count colonies (e.g., Bacteria = 108-109 g-1 soil). • Biomass - Weight of organisms per unit area or volume (by fumigation); e.g., Bacteria = 400-5000 kg/ha • Metabolic Activity - Measurement of some end product given off by the organism; e.g., • measure CO2 evolution for respiration,