Ted Floyd Creekcare

Urban Soils

Carbon Storage in Parks and Gardens

Revision September 2011
by Ted Floyd

Carbon Gardens, Carbon Farming, Soil Carbon Sinks, Soil Organic Matter

Is it time to encourage the creation of soil carbon sinks?

Carbon Farming

Adding organic matter to farm soils and creating carbon sinks will improve soil fertility, reduce erosion and establishes drought tolerance. These soil qualities improve farm productivity, reduce costs and increases farm income.

Effect of Climate on Soil Organic Matter

Soil organic matter is low in tropical soils and high in colder climates.

Organic matter decomposes quicker at higher temperatures.

In very wet environments organic matter decomposes very slowly.

For these reasons peats form in alpine soils and coastal swamps.

Organic matter of alpine soils and peats may reach nearly 100%

Wheat production using minimum tillage or establishing deep rooting perennial grasses for sheep and cattle production are two well known ways of increasing soil organic matter.

The Australian Government introduced legislation for a carbon tax in 2011 and included a Carbon Farming Initiative designed to provide incentives for Carbon Farmers to create soil carbon sinks. (1)

Carbon Gardens

Soil carbon sinks can be established in the suburbs of our towns and cities. Native bush with large trees and an extensive understory will store carbon above and below ground level. Very little original bush remains in old Sydney suburbs and now bush regeneration enthusiasts and councils are planting native vegetation in parks and gardens.

A very dense vigorous forest can store 261 tonnes of carbon per hectare. (2) A wetland may store 300 or more tonnes of Carbon per hectare.

vegetable alotments with brick building and corrugated iron water tank

Community Gardens White's Creek Photo EJ Floyd

young green trees

Newly planted native vegetation Photo Aurora Sice

large tree

White's Creek Parkland Photo Aurora Sice

pink flowering plants in wetland

White's Creek Wetland Photo Aurora Sice

Irrigation of tree-lined valleys

Irrigation will increase plant growth and carbon storage. In urban areas excess water runoff during heavy rain can be harvested for irrigating planted bush areas. Trees planted on the lower slopes of valleys, beside creeks, can be irrigated with stormwater runoff from drains flowing into the creek.

In Whites Creek Valley local residents and Leichhardt Council are planting native vegetation to form a green corridor along the valley. There is the potential in suitable areas in this valley to harvest stormwater and irrigate native trees.

Irrigation in a tree-lined valley will increase carbon storage and reduce flooding and pollution from stormwater runoff. Biodiversity will increase when there is vigorous plant growth. Soils beside creeks on the valley floor is often fertile and supports vigorous plant growth. Soils further up the slopes of a valley are usually shallower and less fertile. Soils on floodplains are often very deep and fertile. In the Sydney region when deep valleys occur with steep slopes a gully rainforest will grow.

At least 2 to 3 times more carbon is stored in wetlands than in trees. Whites Creek Wetland is a new, artificial wetland. Plant growth in this wetland is vigorous and continual maintenance is required to remove excess vegetation.

Growing a carbon sink

Australian Forest Carbon Sinks
(Tonnes of Carbon per Hectare)

TotalAbove GroundSoilSoil Carbon

Closed forests26114112046%
Open forests 127428567%
Woodland and scrub93237075%

Carbon can be stored in home gardens. Native species, especially native trees will help create carbon sinks in the back yard. Drought tolerant species should be chosen. Carbon gardens can be created in flower and vegetable gardens. (3)

Composts, mulches, manures and suitable organic fertilizers increase soil organic matter. Digging soils and leaving soil surfaces bare will reduce organic matter.

Deep roots encourage the growth of carbon sinks. Native gum trees often have very deep roots, up to 40 meters, while fruit trees only 0.6 to 1.3 meters. Lawn grasses have very shallow roots, Poa annua only 0.15 m and Kikuyu deeper up to 2.4 m.

Generally carbon sinks do not form under impermeable surfaces. Plants seldom grow under houses and paving. Large trees sometimes have roots growing under houses and footpaths. Kikuyu will grow under footpaths. To form carbon sinks it is best to reduce the area of paving and to encourage plant growth over most of the backyard.

All plant material collected while gardening should be returned to the soil. We do not need council garbage trucks laden with dead garden waste heading to the rubbish dump emitting tonnes of carbon dioxide.

Water runoff collected from the roves of houses should be directed onto garden soils. After rainwater tanks are filled up and overflowing, excess water can be directed onto the top of raised garden terraces where water is stored during rain periods. Over the following days, water will gradually seep from the top terraces down through the soil to lower levels in the garden. Gardeners should aim to prevent any water flowing into council gutters.

Addition of organic matter to soils improves infiltration and water storage. This improves drought tolerance in gardens.

Hungry little microbes eat fallen leaf litter allowing plant nutrients to become available to growing plants. Small soil animals distribute nutrients by eating, moving around and depositing body wastes deep in the soil. Healthy soils are teaming with bacteria, fungi, actinomycetes and tiny critters, all eating fallen leaf litter and eating each other.

A small gum tree in a backyard garden may store 1 tonne of Carbon with 1/2 above ground and 1/2 in the soil. (4)

With careful management of a garden, containing several trees, it should be possible to store 4 tonnes of Carbon in a 4m x 25m garden with equal amounts of Carbon above ground and in the soil. The average home emits about 2 tonnes of carbon a year.

Greenhouse Facts-Gases

Greenhouse GasGas'Contribution in AustraliaGlobal Warming Potential
Carbon Dioxide CO274%1
Methane CH420% 21
Nitrous Oxide N2O4% 310
(Hydroflurocarbons, perflurocarbons)
1% greater 1,000
Molecular Weight of CO2

C (Carbon) = 12
O (Oxygen) = 16
Molecular Weight of CO2 = 12 + (16x2) = 44

Carbon content of CO2 = 27.3%

To Calculate C from Carbon Dioxide x by 0.273

To Calculate CO2 from Carbon x by 3.66

In a typical house two years emissions from the house can be stored in the garden. These calculations indicate it would be difficult for a home garden to store all emissions from a house in the backyard garden. A carbon neutral house could be formed by an enthusiastic greenthumb living in a solar house.

Many people enjoy pottering in the garden, it is often a time of reflection and helps to cope with the stress of modern living. While in the backyard garden people are not burning up fossil fuels watching TV and the exercise will help to prevent nasty health problems.

Modern global economies encourages growth and an increase in standard of living. Would it be better to aim for a better quality of life?

The storage of carbon in soils should be encouraged. Large quantities of carbon can be stored in urban soils and research and education programs should aim to encourage councils and home gardeners to increase the organic matter of soils in parks and gardens.


Encouraging the growth of plants in urban soils has many advantages. Flooding is reduced Transpiration Benefits For Urban Catchment Management, and Urban Catchments Enhanced By Green Corridors. Air Temperatures are reduced in urban areas by plants Cool Trees and urban creeks. Rehabilitation Of Urban Creeks will create cool spaces in the suburbs.

Many soil scientists warn we are rapidly losing good agricultural land Peak Soil. Growing food in the suburbs can help to secure food supplies in cities.

Natural carbon cycle

Carbon dioxide in the atmosphere is converted into plant material during photosynthesis. Organic carbon compounds from plants are added to the soil and eaten by microbes and animals. During respiration animals and microbes breath out carbon dioxide into the atmosphere. Carbon Cycle in Soils

The natural carbon cycle is essentially a closed loop with no carbon added or subtracted from ecosystems. Small changes do occasionally occur.

Burning fossil fuels adds carbon dioxide to the atmosphere.

Clearing native forests and the decomposition of plant material adds carbon dioxide to the atmosphere.

A carbon sink can be created in soils by adding organic matter to soils. Growing plants can help create a carbon sink. Composts and mulches can help create a carbon sink. A carbon sink is only created when organic matter is added faster than the rate of decomposition of organic matter by animals and microbes. Soil Organic Matter

Lignin and cellulose decompose slowly and form long lasting soil humus. Sugars and carbohydrates decompose quickly and disappear from soils.

In the creation of carbon sinks it is often claimed long lasting compounds like humus must be formed. Humus is beneficial. Easily decomposed compounds similar to sugars and carbohydrates should also be able to form carbon sinks when the rate of adding these compounds to the soil is faster than the rate of decomposition.

When long lasting humus is formed it is considered the carbon sink is secure for hundreds of years. For rapidly decomposing compounds the carbon sink is only temporally and will disappear if soil management changes or the plants are removed.

Biochar is a carbon compound similar to charcoal. It is made by a process called pyrolysis when organic matter is heated to high temperatures in the absence of air. The primary aim of pyrolysis is to manufacture` an alternate energy source and the solid waste is biochar. A lot of research is now carried out on biochar throughout the world and small plants are operating. (5)

Biochar is resistant to breakdown in soils and it is expected it will form long lasting soil carbon sinks. There is many questions concerning large scale manufacture of biochar. Often biochar proposals will use plant material which should be left on the ground. Cutting down forests for a pyrolysis plant is not the best way to go. Small farm size plants may have advantages. Remember biochar is a waste product looking for a use.


(1) Australian Government, Department of Climate Change and Energy Efficiency, Carbon Farming Initiative, 2011.

(2) Australian Government, National Greenhouse Gas Inventory, Land Use, Land Use Change and Forestry.

(3) Permaculture

(4) Carbon and Climate Change (Ecologically Sustainable Sydney)

(5) Biochar, CSIRO.