Creekcare

diagram with numbers of a tree with roots below ground with kangaroo

Carbon Cycle in Soils

Photosynthesis in green leaves CO₂ in air synthesised into sugars
Sugars transported in stems to all parts of plant
Exudates from roots deposited in soil and becomes food for microorganisms
Plants eaten by animals and respiration produces energy and CO₂ returned to atmosphere
Leaf litter forms from dead leaves, bark and stems
Soil organic matter
Microrganisms
CO₂ is released during respiration by insects and microorganisms and
Inorganic plant nutrients are released into the soil in a soluble form available to plants.

WATER and CARBON in GARDEN SOILS

Vigorous plant growth, irrigated by water harvesting will establish carbon sinks in urban gardens.

Water cycle

Water harvesting: Water collected from the roof of a house and used to irrigate the home garden. Water can be stored in a water tank and used in dry times or water from the roof spread directly on the garden during rain periods.

When raindrops come to earth they are absorbed by the soil or if there is too many of them they will flow down hill over the soil surface.

Transpiration is the movement of water from soils up the roots and stems of plants and into atmosphere. Water movement up roots and stems is essential to carry plant nutrients from soils to the growing parts of the plant.

Excess rainfall is not absorbed by soils and flows downhill over the land surface into drains and creeks. Flooding is reduced when more water is absorbed by soils and less water allowed to enter drains.

Water harvesting reduces downstream flooding and reduces severity of droughts in gardens. Organic matter improves water penetration into soils and increases water storage in soils.

Irrigation increases plant growth. The use of town water may not be available in the future and to ensure good plant growth in urban gardens water harvesting is necessary. Water tanks are very useful and storage of water in soils will help plants to grow in dry times.

Water collected on the roof of a house and spread over a garden area of equal size to the house, doubles the total rainfall. For this to work the soil has to be well prepared. The ground surface needs to be very permeable to allow water to soak down into the earth.

IMPROVING WATER INFILTRATION INTO GARDEN SOILS

Increase plant cover to protect soil surface
Encourage plants with deep roots
Improve soil structure
Add mulches, green manures and compost
Encourage earth worms
Add soil improvers such as gypsum
Maintain rough soil surface
Protect the soil from raindrop impact
Reduce soil compaction
Improve subsurface drainage
Use organic fertilizers and animal manures

Growing plants encourages water to penetrate deep into soils. A variety of plants with different root systems will allow water stored in the soil at different depths to be utilized.

Carbon sinks

Carbon dioxide in the atmosphere causes greenhouse effect and air temperatres to rise. A carbon sink is created when carbon is removed from the atmosphere and locked up in a form away from the atmosphere and not causing a rise in air temperature.

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 producing food and energy by plants. Animals eat plants and each other and there food and energy is originally synthesised by photosynthesis.

Respiration:Plants and animals use oxygen from atmosphere and living compounds to produce energy and exhale carbon dioxide CO₂ to atmosphere. All living organisms respire including plants, animals and microorganisms.

In a mature native forest the rate of photosynthesis is nearly equal to the rate of respiration. There is a balance and no CO₂ is added or subtracted from the atmosphere. If the forest is cut down photosynthesis in trees stops and respiration by soil microorganisms continues and extra CO₂ is added to the atmosphere increasing the greenhouse effect. If trees are planted and nurtured in cleared land and photosynthesis is faster than respiration a carbon sink will form.

Carbon sinks can be formed from any plants growing at a faster rate than total respiration. In most farming land respiration is faster than photosynthesis so CO₂ is added to the atmosphere.

Carbon sinks in urban gardens are created by living plants growing above and below the ground surface. Green plants absorb carbon dioxide from the air and manufacture plant material by photosynthesis in leaves.

Below ground level, organic matter in soils is a very important carbon sink. Leaves and stems fall onto the soil surface, forming a leaf litter. Many small animals and microorganisms break down large dead plant remains into small pieces, incorporating broken down plants into soil organic matter.

Benefits of soil organic matter

Food for microorganisms and soil animals
Nutrient cycling
Store plant nutrients in soils
Reduce harmful changes in soil acidity and alkalinity
Improve soil structure
Improve soil aeration
Reduce erosion
Increase water infiltration
Improve water holding capacity
Create healthy soils and encourage vigorous plant growth
Are a very good carbon sink and reduce greenhouse gases

Plants are made from sugars, starches, cellulose, fats, oils, proteins and lignin. Sugars are very quickly eaten by microorganisms, cellulose is slowly converted to sugars and lignin is very resistant to decay and may survive in soils for hundreds of years. Organic matter is mainly dead, decomposing plant material. Only a small proportion originates from animals. Animal bones, CaCO₃ is a inorganic compound containing carbon.

The amount of carbon is often equal above and below ground level. Carbon in soils increases in cold or wet conditions. Woody plants, containing lignin, forms humus resistant to microbial breakdown and will persist in soils for hundreds of years.

Carbon sinks will be bigger when there is extra plant growth in soils with desirable physical properties.

The build up of soil organic matter by returning garden cuttings, compost and mulches to soils will add nutrients to the soil. The addition of organic materials to soils will improve soil physical properties. Structure is stable, improving soil aeration and water movement. Water holding capacity is increased, improving drought tolerance. Damage from erosion and the beating action of raindrops is decreased.

Cultivation and digging soils can reduce soil organic matter. Digging soils increases aeration and this can increase the decay of organic matter. Cultivation kills plants and this can reduce the build up of organic matter.

Hardwood trees and shrubs and especially gum trees form long lasting humus in soils.

Well managed garden soils will contain up to 10% organic matter. A small gum tree can store up to 1 tonne of carbon with ½ half above ground and ½ half in the soil. It is possible to store 4 tonnes of carbon in 100 square meters of garden containing several trees.