Volume 22 • Issue 04 • 2018
Carbon : cycling or stored
There ’ s a finite amount of carbon in the earth ’ s biosphere , which is either cycling or stored .
As a basic building block of all life on earth , carbon is continuously cycled ( and recycled ) in many guises . It may be cycling as a gas ( CO2 or CH4 ) in the atmosphere and absorbed into oceans , or as a liquid or solid when part of living things or recently living things , such as plants , animals , microbes , soil humus and wood . Carbon moves easily and constantly between its gaseous , liquid and solid forms as it is cycled by plants and animals through respiration , rumination , reproduction and all other processes of life . This is naturally cycling carbon .
On the other hand , the majority of stored carbon was removed from the atmosphere and laid down during the carboniferous period 300-360 million years ago , a time when atmospheric CO2 levels were much higher than now ( 2 ). Over the millennia , this has formed into the fossil fuels oil , coal and natural gas . Whilst undisturbed , this carbon is locked away : stored and not part of the moving carbon cycles .
Natural carbon cycles all start with photosynthesis
There are many different natural carbon cycles , each of different complexity and duration , but they all start with photosynthesis – the capturing of sunlight energy by green growing plants . Some carbon recycles from the air through a plant and back to the air in as little as 24 hours as plants photosynthesise during the day and respire at night . Some cycles take weeks or months , such as when a plant grows and is eaten by an animal that metabolises its carbon and then breathes it out as CO2 once more . Other cycles last many , sometimes hundreds of years , for example when a tree grows to maturity , then is burned in a forest fire , which releases carbon back into the atmosphere .
There ’ s an annual cycle as CO2 levels in the atmosphere ebb and flow each year with the seasonal ‘ breathing ’ of the world ’ s vast boreal forests in North America , Alaska and Russia : levels are lowest in late autumn after a summer of rapid photosynthesis when CO2 is removed from the air - a long , slow global inhale ; then , as leaves fall and photosynthesis declines , atmospheric CO2 levels gradually rise over the winter peaking again in May - a long , slow exhale .
Carbon sequestration
A less well known carbon cycle , but one that is fundamentally important for all life on earth , is the capacity of soils to absorb carbon . The soil is alive with micro-organisms and when plants take carbon from the air , as described above , as well as for their own growth , they use it in a symbiotic relationship to feed the soil microbes in exchange for nutrients and other ecological functions . Plants do this by releasing exudates via their roots in the form of many different carbon compounds ; in effect they are exuding liquid carbon .
The energy provided by liquid carbon in plant exudates drives a multitude of interactions within soil , including the reproduction and life-cycles of many different soil microbes . Organic matter from the biomass of plant roots as they grow and die , plant matter trodden into the soil by grazing cows , along with the fertiliser they leave behind , all adds carbon ( and other nutrients ), which the microbes ultimately convert into complex and very stable carbon compounds called humus . Humus is the building block of soil and building new soil , by storing carbon in this stable form , is called carbon sequestration . Under healthy aerobic soil conditions , up to 40 % of the carbon captured from the air by plants can be sequestered in the soil in this way ( 3 ).
Sequestered carbon can stay locked in the soil for many , hundreds and potentially thousands of years , as in old growth forest soils and the last of the unploughed American prairies . And although there is a limit to the amount of carbon a soil can hold , there is no limit to the volume of soil that can be built , with some prairie soils reaching over six feet deep ( 4 ). Managing pastures in a way that replicates how the prairie soils were built plays a vital role in this process of carbon sequestration .
Grasslands ’ unique ability
Fossil records show us that 55 million years ago grasslands and ruminants co-evolved ( 5 ) ( 2 ) and grasses have evolved to be grazed . When combined with appropriate grazing , grasslands are the key to carbon sequestration on a rapid scale . Unlike bushes and trees that store the majority of their biomass above ground and do not respond to repeated browsing with rapid regrowth , evolution has bestowed grasslands with the unique ability to not only withstand repeated grazing but to positively benefit from it . Grasses also benefit from disturbance and fertilisation by ruminants as well as their role in preventing the plains and prairies progressing into forest .
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