Biosphere Contents

1. Organic Matter

The conversion of solar energy into organic matter by photosynthesis is termed primary productivity

Primary productivity is the productivity of green plants and other autotrophic organisms. Autotrophs produce the organic compounds they require from simple inorganic materials, most commonly using energy from the sun through photosynthesis.

Gross primary productivity is the total amount of energy produced by primary producers. Net primary productivity is gross primary production minus the energy used in respiration. It is net primary production that contributes to growth and long-term changes in plant biomass.

Total net primary productivity on Earth at present is about 105 Pg of carbon each year (1 Pg = 1015 grams). This is comprised of 56.4 Pg C yr-1 in terrestrial ecosystems and 48.3 Pg C yr-1 produced in aquatic ecosystems (Geider 2001). The total amount of biomass on the Earth at any one time is 565-570 Pg C, of which 560 Pg C is in terrestrial ecosystems (Groombridge and Jenkins 2002). In general, the carbon content of most plant and animal tissues is 45-50% of its dry mass. Since most organisms contain large amounts of water, the total plant mass is actually 7-10 times larger than plant dry mass.

Example rates of gross primary production and net primary production in two ecosystems

The time that any one unit of carbon is stored as biomass varies for different parts of the biosphere. This explains the huge difference in biomass, on a global scale, between terrestrial and aquatic ecosystems, even though the productivity of these ecosystems is similar. Phytoplankton usually live for only a few days or weeks. Deciduous leaves on trees last only one season, but the trunks of trees may last many tens of years and even centuries. The longest living plant is thought to be a swathe of seagrass in the Mediterranean which has been estimated to be between 80,000 and 200,000 years old (Arnaud-Haond et al. 2012). Some carbon is stored for millions of years in organic material which is deposited in sediment and converted to fossil fuels - coal, oil or gas.

What does primary productivity refer to?