Biomass is a measure of the amount of biological substance minus its water content found at a given time and place on the earth's surface. Although sometimes defined strictly as living material, in actual practice the term often refers to living organisms, or parts of living organisms, as well as waste products or non-decomposed remains. It is a distinguishing feature of ecological systems and is usually presented as biomass density in units of dry weight per unit area. The term is somewhat imprecise in that it includes autotrophic plants, referred to as phytomass, heterotrophic microbes, and animal material, or zoomass. In most settings, phytomass is by far the most important component. A square meter of the planet's land area has, on average, about 22.05 – 26.46 lb (10 – 12 kg) of phytomass, although values may vary widely depending on the type of biome. Tropical rain forests average about 45 kg/m2 while a desert biome may have a value near zero. The global average for heterotrophic biomass is approximately 0.1 kg/m2, and the average for human biomass has been estimated at 0.5 g/m2 if permanently glaciated areas are excluded.
The nature of biomass varies widely. Density of fresh material ranges from a low of 0.14 g/cm3 for floats of aquatic plants to values greater that 1 g/cm3 for very dense hardwood. The water content of fresh material may be as low as 5% in mature seeds or as high as 95% in fruits and young shoots. Water levels for living plants and animals run from 50 to 80%, depending on the species, season, and growing conditions. To insure a uniform basis for comparison, biomass samples are dried at 221°F (105°C) until they reach a constant weight.
Organic compounds typically constitute about 95% by weight of the total biomass, and nonvolatile residue, or ash, about 5%. Carbon is the principle element in biomass and usually represents about 45% of the total. An exception occurs in species that incorporate large amounts of inorganic elements such as silicon or calcium, in which case the carbon content may be much lower and nonvolatile residue several times higher. Another exception is found in tissues rich in lipids (oil or fat), where the carbon content may reach values as high as 70%.
Photosynthesis is the principle agent for biomass production. Light energy is used by chlorophyll-containing green plants to remove (or fix) carbon dioxide from the atmosphere and convert it to energy rich organic compounds or biomass. It has been estimated that on the face of the earth approximately 200 billion tons of carbon dioxide are converted to biomass each year. Carbohydrates are usually the primary constituent of biomass, and cellulose is the single most important component. Starches are also important and predominate in storage organs such as tubers and rhizomes. Sugars reach high levels in fruits and in plants such as sugar cane and sugar beet. Lignin is a very significant non-carbohydrate constituent of woody plant biomass.
Resources
Books
Lieth, H. F. H. Patterns of Primary Production in the Biosphere. Stroudsburg, PA: Dowden, Hutchinson, and Ross, distributed by Academic Press, 1978.
Smil, V. Biomass Energies: Resources, Links, Constraints. New York: Plenum Press, 1983.
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