Ted Floyd Creekcare

Urban Soils

Carbon Cycle in Soils

diagram with numbers of a tree with roots below ground with kangaroo
  1. Photosynthesis
  2. Sugar Transport
  3. Root Exudates
  4. Respiration by Animals
  5. Leaf Litter
  6. Soil Organic Matter
  7. Microorganisms
  8. Respiration by Soil Organisms
  9. Plant Nutrients

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.

  1. masses of fungi in leaf litter


    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.

  2. Sugar Transport

    Sugars synthesized during photosynthesis are transported to all parts of the plant. In stems and roots phloem cells form small capillaries where sugars are transported to the roots, seeds, storage organs and other parts of the plant.

  3. Root Exudates

    fungi in sunlight between two trees 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.

  4. 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.

  5. fungi in leaf litter with wall mural in backgroundLeaf Litter

    Dead leaves and stems fall from plants forming a leaf litter on the soil surface. The leaf litter is natures perfect mulch where small soil animals and microbes eat the plant material expiring CO2 and producing small organic compounds which are deposited deeper in the soil.

  6. 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.

  7. 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.

  8. 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.

  9. Plant Nutrients

    When soil organic matter is broken down by microbes essential plant nutrients are released into the soil solution in soluble forms available to be absorbed by plant roots. Important nutrients are nitrogen, phosphorus, potassium and sulphur.

    single red-orange capped fungi in leaf litterThe absorption of some nutrients is facilitated by microorganisms living in the rhyzosphere and feeding on root exudates. Often a very close symbiotic relationship forms between microbes and plant roots, especially when mychoriza fungi grow inside plant roots.

Loss of Soil Carbon

Land Clearing and Forestry

Cutting down trees and selling timber reduces soil carbon.


Water and wind erosion often remove plant material and carbon from soils.

Farm Products

Wheat, meat, wool and other farm products contain carbon which is removed from a farm when products are sold.


Ploughing often reduces soil organic matter.

Stubble Burning

After the harvesting of grain crops, especially wheat and oats, the remaining straw in the stubble is burnt so as to enable easy seedbed preparation for the next crop. Stubble burning reduces soil carbon and many farmers now retain the straw and manage the soil by stubble mulching.


During bushfires many tons of carbon dioxide go up with the smoke and this decreases the addition of plant material to soils.

Addition Carbon to Soils

Composts, mulches and organic fertilizers

Gardeners add composts, mulches and organic fertilizers to soils containing carbon.


During floods, silt is deposited on flood plains forming alluvial soils. Silt carried by rivers often contain plant material and carbon compounds.

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.

Mycorrhiza Fungi Hyphae growing between root cells Hyphae growing into soil Rhizospher Root Hair Hyphae grwoing in root cells Root epidermis CortexExchange zone in 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.

Carbon Sinks

Carbon dioxide in the atmosphere causes global warming and air temperatures to rise. A carbon sink is created when carbon dioxide is removed from the atmosphere and locked up in a form away from the atmosphere and not causing a rise in air temperature. In soils a carbon sink is created when the addition of carbon is greater than the removal of carbon. This can occur in an ecosystem when the rate of photosynthesis is faster than the rate of respiration and soil organic matter increases.