<P> Decomposition rates vary among ecosystems . The rate of decomposition is governed by three sets of factors--the physical environment (temperature, moisture and soil properties), the quantity and quality of the dead material available to decomposers, and the nature of the microbial community itself . Temperature controls the rate of microbial respiration; the higher the temperature, the faster microbial decomposition occurs . It also affects soil moisture, which slows microbial growth and reduces leaching . Freeze - thaw cycles also affect decomposition--freezing temperatures kill soil microorganisms, which allows leaching to play a more important role in moving nutrients around . This can be especially important as the soil thaws in the Spring, creating a pulse of nutrients which become available . </P> <P> Decomposition rates are low under very wet or very dry conditions . Decomposition rates are highest in wet, moist conditions with adequate levels of oxygen . Wet soils tend to become deficient in oxygen (this is especially true in wetlands), which slows microbial growth . In dry soils, decomposition slows as well, but bacteria continue to grow (albeit at a slower rate) even after soils become too dry to support plant growth . When the rains return and soils become wet, the osmotic gradient between the bacterial cells and the soil water causes the cells to gain water quickly . Under these conditions, many bacterial cells burst, releasing a pulse of nutrients . Decomposition rates also tend to be slower in acidic soils . Soils which are rich in clay minerals tend to have lower decomposition rates, and thus, higher levels of organic matter . The smaller particles of clay result in a larger surface area that can hold water . The higher the water content of a soil, the lower the oxygen content and consequently, the lower the rate of decomposition . Clay minerals also bind particles of organic material to their surface, making them less accessibly to microbes . Soil disturbance like tilling increase decomposition by increasing the amount of oxygen in the soil and by exposing new organic matter to soil microbes . </P> <P> The quality and quantity of the material available to decomposers is another major factor that influences the rate of decomposition . Substances like sugars and amino acids decompose readily and are considered labile . Cellulose and hemicellulose, which are broken down more slowly, are "moderately labile". Compounds which are more resistant to decay, like lignin or cutin, are considered recalcitrant . Litter with a higher proportion of labile compounds decomposes much more rapidly than does litter with a higher proportion of recalcitrant material . Consequently, dead animals decompose more rapidly than dead leaves, which themselves decompose more rapidly than fallen branches . As organic material in the soil ages, its quality decreases . The more labile compounds decompose quickly, leaving an increasing proportion of recalcitrant material . Microbial cell walls also contain recalcitrant materials like chitin, and these also accumulate as the microbes die, further reducing the quality of older soil organic matter . </P> <P> Ecosystems continually exchange energy and carbon with the wider environment; mineral nutrients, on the other hand, are mostly cycled back and forth between plants, animals, microbes and the soil . Most nitrogen enters ecosystems through biological nitrogen fixation, is deposited through precipitation, dust, gases or is applied as fertilizer . Since most terrestrial ecosystems are nitrogen - limited, nitrogen cycling is an important control on ecosystem production . </P>

All ecosystems on the earth taken together are called