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TEDSPEDS
an in depth study of soils
Soils are intricate, living, dynamic, ecosystems, found deep down in the dark, hiding many unsolved mysteries, haunting unsuspecting generations of failed ancient civilizations and giving succour to those who love and obey the laws of our mother earth.
SOIL CARBON CYCLE
Carbon is the natural building block of all living organisms. In natural eco-systems carbon is continually cycled from the atmosphere to plants and then to animals and microorganisms and back to the atmosphere. All living creatures contain carbon and all organic matter found in soils contains carbon.
Soils play a very important role in the carbon cycle. There is more living material in the soil than in living organisms above the soil surface. The mass of plant roots is often greater than the mass of stems and leaves growing above ground level. Bacteria, actinomycetes, algae, protozoa and fungi are microorganisms found in all soils. Insects, mites, worms and many more small animals live in the dark under the soil surface. Rabbits and wombats are larger animals who make their home underground.
Soil organic matter is the decaying remains of plants and animals and consists of carbon compounds. Plants are made from sugars, starches, cellulose, fats, oils, proteins and lignins.
In our modern world fossil fuels are burnt to make electricity and to propel cars. Coal and oil from deep down in the earths crust and are not a part of the natural carbon cycle. When coal and oil are burnt CO2 is given off into the atmosphere and this CO2 is an addition to the total atmospheric carbon. Carbon dioxide in the atmosphere increases atmospheric temperature and the burning of fossil fuels is increasing global warming.
The energy cycle in natural eco-systems is closely coupled to the carbon cycle. Organic compounds contain energy captured by plants during photosynthesis and released during respiration by plants, animals and microorganisms.
Photosynthesis
In green chlorophyll of plant leaves, organic compounds are synthesised using energy from sunlight, water from soils and atmospheric carbon dioxide CO2.
Photosynthesis is the basic process in plants producing food and capturing energy. Animals eat plants and each other and their food and energy is originally synthesized by photosynthesis.
Root Exudates
Plant roots exude water and many organic chemicals including sugars, amino acids, organic acids, vitamins, plant hormones, growth substances, mucilage and proteins. Exudates encourage microbial growth and the microbes improves uptake of nutrients by plants.
Up to 20% of carbon fixed by photosynthesis in plants is transfered to the soil as root exudates.
Respiration by Animals
Plants and animals use oxygen from atmosphere and organic compounds to produce energy and exhale carbon dioxide CO2 to atmosphere. All living organisms respire, including plants, animals and microorganisms.
Soil Organic Matter
Soil organic matter provides food and energy to microorganisms and small soil animals. Decomposing organic matter releases Carbon Dioxide CO2 into soil air spaces and the CO2 slowly diffuses to the soil surface and into the atmosphere.
Microorganisms
Soils contain millions of small microbes. Bacteria, like to eat sugars and fungi are able to feed on woody material containing cellulose and lignin.
Most microbes live in surface soils, within decaying organic matter and in rhyzosphere surrounding growing tips of plant roots.
Respiration by Soil Organisms
Carbon dioxide CO2 is expired by plant roots, small soil animals and microorganisms into the soil air and the CO2 diffuses up into the atmosphere and completes the carbon cycle.
Microbes
Invisible organisms
Soils are an ideal medium for microbes to grow in. Bacteria prefer to grow in the thin layer of moisture surrounding clay particles and fungi grow better in large soil pores and can survive in dry conditions. Bacteria have a diameter of approximately 0.001 mm and fungi filaments about 0.005 mm. In comparison clay particles are less than 0.002 mm.
Microbes need a continual supply of food from plants and animals. Different microbes eat different plant materials. Bacteria prefer to eat smaller soluble compounds, especially sugars and starch. Fungi feed on hard to decompose plant fibers and woody material including cellulose, lignin and plant fibers.
Most microbes are aerobic and need oxygen to grow. Some microbes are anaerobic and are able to grow in the absence of oxygen. Anaerobic bacteria growing in waterlogged soils give off methane and carbon dioxide. Marsh gas is produced by anaerobic bacteria and the methane in marsh gas can catch alight.
A gram of healthy soil contains up to 3,000 million bacteria and 500,000 fungi plus actinomycetes, algae and protozoa.
Microbes generally grow faster in fertile soils similar to the ideal conditions for plant growth. The soil needs to have adequate moisture, aeration and good drainage. Most microbes need similar inorganic nutrients to plants. Nearly all microbes differ from plants and cannot manufacture organic material by photosynthesis and need a supply of organic matter to grow on.
Healthy soils with dynamic microbial ecosystems are able to break down many organic pollutants.
Soil Microorganisms
Bacteria
Actinomycetes
Fungi
Protozoa
Algae
Lichens Algae +Fungi
Rhizosphere
The rhizosphere is the zone of soil surrounding roots containing a large, very active population of microbes. This microbial active zone can extend up to 1mm from the root.
Root exudates & secretion of border cells
Plant roots exude water and many organic chemicals including sugars, amino acids, organic acids, vitamins, plant hormones, growth substances, mucilage and proteins. Special chemicals can inhibit growth of pathogens and competitor plants. Beneficial microbes are encouraged by exudates.
Up to 20% of carbon fixed by photosynthesis in plants is transfered to the soil as root exudates.
Exudates regulate microbiological activity and encourage beneficial symbiosis. Microbiological activity encouraged by exudates improves the uptake of nutrients by roots. Exudates defend roots from pathogens, reduce diseases and inhibit growth of competing plants.
Rhizosphere and exudates improve water conditions in soils encouraging plant growth. Fungi filaments and mucilage stabilize soil aggregates and improve soil structure.
Activity of soil organisms
Nearly all the microbial activity in soils occurs in the surface leaf litter, the top 30cm of soils and the rhizosphere. The CO2 content of soil air increases with depth and oxygen decreases deep down in soils.
Respiration in soils and the decomposition of organic matter is faster at higher temperatures and in well aerated soils. Tropical soils often have a lower carbon content. Waterlogged soils have a high carbon content because the activity of microbes is slower when oxygen is lacking.
Peats containing up to 90% organic matter can form in waterlogged swamp soils and in cold climates.
Small animals, especially worms can mix soils and bury organic matter deeper down in the soil profile.
Exchange zone surrounding roots
At the growing tip of plant roots materials are moving from the soil into the root and also materials move from the root into the soil. Water and plant nutrients are absorbed into the root mainly by root hairs and epidermal cells growing just behind the growing tip.
The tip of the plant root also deposits into the soil a considerable amount of material. The root cap is slothed off into the soil as the growing tip moves forward. Often epidermal cells on the surface of roots are also slothed off. Root hairs stop functioning and are incorporated into the soil. Root exudates are transferred into the soil near the growing tip of plant roots.
Surrounding the growing tip of roots is the rhizosphere. In the rhizosphere there is a very active population of microbes feeding on the root exudates. Sometimes mycorrhiza fungi grow into the roots and form a symbiotic relation between the plant and the fungi. If the fungi harms the plant root it is a parasite and becomes a disease.
This microbial activity in the rhizosphere is an important part of the carbon cycle in soils. Carbon compounds from the plant are transferred into the soil and microbes feed on these compounds. Respiration by the microbes break down sugars and other organic compounds into carbon dioxide and the CO2 returns to the atmosphere and completes the carbon cycle.
Microorganisms are sometimes more active in the rhizosphere than in decomposing organic matter.
Mycorrhiza fungi
Mycorrhiza fungi form a symbiotic relation with plant roots. Fungi hypha enter into roots near the growing tip and do no harm to the roots. The fungi and roots are beneficial to each other and this symbiotic relation is healthy and increases the growth of both plants and fungi. The fungi obtain sugar from the plant roots and the fungi provides phosphorus and other inorganic nutrients to the plant. Each fungi species only forms a symbiotic relation with one specific plant species.
Nitrogen cycle
Nitrogen gas occurs in the atmosphere as N2. Most plants, microbes and animals can not use this gas. The gas has to be converted into nitrates or ammonium ions, the inorganic form of nitrogen in soils, before it can be absorbed into plant roots.
Nitrogen fixation
Rhizobian bacteria form a symbiotic relation with legumes in nodules on plant roots. N fixation occurs when Rhizobian bacteria extract gaseous nitrogen from the soil atmosphere and incorporate it into the host legume plant. The nitrogen in the legume is incorporated into the soil when the plant dies and becomes soil organic matter. Nitrogen becomes available to other plants when the organic matter from legumes is broken down.
Common legumes are clovers, Lucerne, wattle, peas and beans.
Ammonification
Soil organic matter is eaten by microbes and nitrogen is converted to ammonia. Plants absorb small amounts of ammonia.
Nitrification
Most ammonium ions (NH4) in soils is rapidly converted into nitrate ions (NO3). There are two steps in nitrification, initially ammonia is converted to nitrites (NO2) then nitrates (NO3) are formed. Different species of microorganisms carry out these two reactions.
Nitrogen fixation, ammonification and nitrification is carried out by microorganisms. A chain of reactions occur with each different step carried out by a different species of microorganism. Sometimes denitrification occurs when NO3 is converted to N2. Denitrification is more common when soils are poorly aerated or poorly drained. Denitrification reduces available soil nitrogen and fertility.
The nitrogen cycle in soils depends on many different species of microorganisms. The operation of the cycle is more successful in healthy soils where microorganisms flourish. A supply of organic matter is required, stable soil structure and good aeration encourages microbial growth.
The above description is a simplified model of the nitrogen cycle. There are many more steps in the cycle and the above model includes the more important steps.
Lightening and rain in thunderstorms add nitrates to soils.
Nitrates lost to soils in drainage water.
Immobilization of nitrogen in humus.
Non-symbiotic nitrogen fixation eg Azotobacter and Clostridium bacteria.
waterlogged swamp soils and in cold climates.
During respiration, animals use oxygen from atmosphere and organic compounds to produce energy and exhale carbon dioxide CO2 to atmosphere.
Most soil animals are saprophites and eat dead and decaying plant material. Carnivors eat other small animals eg centipeads and spiders. Very small mites often graze on bacteria. A few soil animals eat the excreta of other animals.
Animals are not able to readily digest cellulose and lignin. Bacteria in the gut is able to break down resistant plant material into forms available to the host animal eg termites. Soil fungi can break down cellulose and lignin into forms available to animals.
Often the decomposition of plant material occurs by a chain of different animals and microrganisms. Each animal undergoes one step in the chain and the partly decomposed plant material is then eaten by the next animal or microrganism in the decomposition chain. In each step of decomposition, carbon dioxide is given off during respiration. In a typical decay chain the resitant plant material is eaten by fungi enabling the softer material to be eaten by small animals. The undigested material is excreated and forms food suply for a new chain of animals.
Class Oligochaeta, Order Lumbricus
European earthworms – Oligochaeta
Australian earthworms – Lumbricidae ?
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Class Arachnida
small and nonpoisonous, eight legsAcari,
Soil mites, 0.1 to 1mm, ticksAraneae,
Spiders
2 pairs legs a segment, eat decaying plant litterChilopoda, Centipedes –
1 pair legs segment, are carnivorous, eat small soil animals and have poison fangs.
Springtails – Collembola, Wingless insects, 0.5 to 2mm
Beetles – Coleoptera, Eggs, Grub (larva) & pupa (chrysalis)
Fly larvae – Diptera
Cutworms – Lepidoptera moths and buterflies
Wireworms – Elateridae larvae
Ants
Termites, Common in tropical soils
Cockroach
Thrips – Thysanoptera
Agricultural land is now destroyed faster than new land becomes available for farms.
Beating raindrops
Buried by silt
Wind erosion
Landslides
Nutrient erosion
Nutrient harvesting
Organic matter decay
Acidification
Salting
Scalds
Desertification
Nutrient leaching
Urban spread and
Increase in impermeable soils
Mining
Water storages
Roads and railways
Structure degradation
Land degradation
Soil contamination
Soil compaction
Surface seals
Plant materials fall onto the ground and are incorporated into the soil. Soil organic matter is a renewable resource containing enargy from the sun.
