Creekcare

Urban Soils

Urban Biocarbon Sinks

Keywords: Soil carbon, stormwater harvesting, carbon emissions, climate change, global warming
Ted Floyd
June 2013

When plants grow they absorb Carbon Dioxide from the atmosphere and generate bio-carbon sinks. In cities, stormwater can be harvested and the captured water used to irrigate plants increasing the size of biocarbon sinks.

Ecosystems gain Carbon by photosynthesis and lose Carbon Dioxide by respiration. In natural systems there is a balance between photosynthesis and respiration and there is no loss or gain of Carbon. Biocarbon sinks are generated when photosynthesis is greater than respiration.

In many farming systems, respiration is greater than photosynthesis and Carbon Dioxide is lost to the atmosphere. With careful nurturing of plants and soils, Carbon Dioxide can be absorbed from atmosphere and a biocarbon sink generated.

Spoil Carbon is essential to prevent soil degradation and many farmers are now working towards increasing soil Carbon so as to improve soil health and sustainability. Effort should also be made to increase soil Carbon in urban areas so as to improve soil health and generate biocarbon sinks.

In many densely populated cities, land is scarce and expensive. There are many competing demands for available land. Schemes for the introduction of nature into cities should be multifunctional. Harvesting stormwater reduces floods and if the captured water is used to irrigate plants, biocarbon sinks will work towards reduced global warming.

Trees, shrubs, grasses, vegetables and ornamental flowers will all produce carbon compounds. Perennial plants tend to generate bigger sinks than annuals. Natural coastal estuaries store a large amount of carbon. Wetlands, mangroves and seagrasses are all good biocarbon sinks. With many plants, what you see growing above ground level is only half the Carbon. Underground in the soil is a large store of Carbon.

Lignin and cellulose decompose slowly and form long lasting soil humus. Sugars and carbohydrates decompose quickly and disappear from soil. Sugars are mainly decomposed by bacteria and lignin decomposed by fungi.

Photograph looking up trunk at canopy of tree

Big Tree, Botanic Gardens, Orange

Renewable energy reduces the amount of Carbon Dioxide added to the atmosphere. Biocarbon sinks absorbs current Carbon Dioxide from the atmosphere. A comprehensive global warming strategy should consist of renewable energy working in tandem with biocarbon sinks.

Photosynthesis and respiration

Biocarbon sinks are generated by growing plants. Under sunlight, Carbon Dioxide from the atmosphere is converted into plant material by photosynthesis, in green leaves. Organic material is transported to all parts of the plant and Carbon is incorporated into the different plant organs. Stems and roots are usually the major Carbon sinks in growing plants.

When parts of a plant die it becomes incorporated in the leaf litter on the soil surface. Animals and microbes eat the leaf litter and organic material becomes incorporated in the soil. Soil organic matter is decomposed into smaller Carbon compounds and Carbon Dioxide. Finally, dark coloured humus is formed containing hard to decompose Carbon compounds

Junee Urban Wetlands

Root exudates are excreted into the soil near the growing tip of roots. Up to 20% of organic material synthesised by photosynthesis becomes root exudates and they are an important source of Carbon in soils. Surrounding roots is the rhizosphere containing a high number of bacteria and other soil microbes. These microbes feed on root exudates and the rhizosphere is a highly active zone important in plant nutrition and health.

Soil carbon tends to be higher in wet, cold environments and lower in dry, hot environments.

Respiration occurs in all plants, animals and microbes. All living organisms gain energy from Carbon compounds and emit Carbon Dioxide during respiration.

Formation of biocarbon sinks

Composting, manures, organic fertilizers and mulching add Carbon to garden soils. Water infiltration and water holding capacity are improved when Carbon is added to soils. Organic carbon in soils is food for microbes.

Microbes and small soil animals improve soil health, increase plant growth and increase Carbon stored in soils. Digging breaks up fungi mycelium and reduces microbe growth. Pesticides, insecticides and fertilizers are no good for microbes.

Contour gardens improve water absorption and can increase plant growth. Swales, rain gardens and leaky wells improve water absorption into soils.

Leaky Weir, Broken Hill

Deep roots encourage soil carbon. Perennial plants often have deep roots. Annual lawn grasses have shallow roots.

All plant material collected while gardening should be returned to the garden. Garbage trucks should not be laden with dead garden waste heading to a rubbish dump. It should be remembered organic waste buried in landfill will partially decompose into methane a greenhouse gas more powerful than Carbon Dioxide.

Water runoff from house rooves should be collected in rainwater tanks and used to irrigate gardens. Gardeners should aim to prevent any water flowing into council gutters. Council parks and gardens should be irrigated by stormwater harvesting.

In cities biocarbon sinks are increased when the area of permeable soils is increased. Permeable soil, increases water infiltration and encourages plant growth. Area covered by roads, footpaths and paving should be reduced so as to increase plant growth.

Tar and concrete are no good for plant growth. Urban expansion and the spread of roads in growing outer suburbs produce impermeable surfaces reducing plant growth and destroying biocarbon sinks. Compact cities have many advantages including reduction of urban spread and reduction of the alienation of land suitable for plant growth and generation of biocarbon sinks.

Vegetated Creek Floodplain, Orange

Some scientists claim peak soil is a bigger problem than peak oil. Will food production decrease because of soil degradation? Increasing soil health is essential to ensure food security.

The generation of biocarbon sinks in farmlands and in cities is a powerful weapon in the control of climate change.

The common denominator in food security and Carbon Dioxide emissions is soil organic matter (soil carbon). The march of human progress has destroyed soil organic matter causing land degradation and the release of Carbon Dioxide into the atmosphere. We need to reverse this trend worldwide.

Further information

Soil carbon is essential to prevent soil degradation, desertification and to maintain food security.

Soil carbon is also an important biocarbon sink and important in maintaining climate stability.

Soil carbon is important for soil health and builds up microbe population. Water infiltration, water storage and structure in soils are improved by soil carbon. Erosion is reduced by soil Carbon.

Very Narrow Street with Pot plants inner Sydney

Soil Carbon Key To Sustainability http://www.tedthefloyd.wordpress.com

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