The Natural Fix?

Carbon cycle

Ocean surface

a product of respiration in low oxygen environments, such as stagnant marshes and the intestines of ruminants, including cattle, sheep and goats. Methane in the atmosphere is eventu- ally oxidised to produce carbon dioxide and water. In the biosphere a significant amount of carbon is effectively ‘stored’ in living organisms (conventionally referred to as bio- mass) and their dead, undecomposed or partially decomposed remains in soil, on the sea floor or in sedimentary rock (fossil fu- els are, of course, merely the remains of long dead organisms). When the amount of atmospheric carbon fixed through pho- tosynthesis is equivalent to the amount released into the at- mosphere by respiring organisms and the burning of organic carbon, then the living or biotic part of the carbon cycle is in balance and concentrations of carbon dioxide and methane in the atmosphere should remain relatively constant (although their concentration will be affected by other parts of the carbon cycle, notably volcanic activity and dissolution and precipitation of inorganic carbon in water). Often, however, the systemmay not be balanced, at least locally. An area may be a carbon sink if carbon is accruing there faster than it is being released. Conversely, an area is a carbon source if the production of atmospheric carbon from that area exceeds the rate at which carbon is being fixed there. In terrestrial eco- systems, whether an area is a sink or a source depends very largely on the balance between the rate of photosynthesis and the combined rate of respiration and burning.

1 020

50

Dissolved organic C

40

6

700

- - -- - - - - -- - - 3

Marine biota

4

100

6

Deep ocean

38 100

0.2

150

The amount of carbon stored, the form that it is stored in and the rate of turnover – that is the rate at which carbon is organically

Sediments

11