<P> When plant tissues are shed or are eaten, the nitrogen in those tissues becomes available to animals and microbes . Microbial decomposition releases nitrogen compounds from dead organic matter in the soil, where plants, fungi, and bacteria compete for it . Some soil bacteria use organic nitrogen - containing compounds as a source of carbon, and release ammonium ions into the soil . This process is known as nitrogen mineralization . Others convert ammonium to nitrite and nitrate ions, a process known as nitrification . Nitric oxide and nitrous oxide are also produced during nitrification . Under nitrogen - rich and oxygen - poor conditions, nitrates and nitrites are converted to nitrogen gas, a process known as denitrification . </P> <P> Other important nutrients include phosphorus, sulfur, calcium, potassium, magnesium and manganese . Phosphorus enters ecosystems through weathering . As ecosystems age this supply diminishes, making phosphorus - limitation more common in older landscapes (especially in the tropics). Calcium and sulfur are also produced by weathering, but acid deposition is an important source of sulfur in many ecosystems . Although magnesium and manganese are produced by weathering, exchanges between soil organic matter and living cells account for a significant portion of ecosystem fluxes . Potassium is primarily cycled between living cells and soil organic matter . </P> <P> Biodiversity plays an important role in ecosystem functioning . The reason for this is that ecosystem processes are driven by the number of species in an ecosystem, the exact nature of each individual species, and the relative abundance organisms within these species . Ecosystem processes are broad generalizations that actually take place through the actions of individual organisms . The nature of the organisms--the species, functional groups and trophic levels to which they belong--dictates the sorts of actions these individuals are capable of carrying out and the relative efficiency with which they do so . </P> <P> Ecological theory suggests that in order to coexist, species must have some level of limiting similarity--they must be different from one another in some fundamental way, otherwise one species would competitively exclude the other . Despite this, the cumulative effect of additional species in an ecosystem is not linear--additional species may enhance nitrogen retention, for example, but beyond some level of species richness, additional species may have little additive effect . </P>

What is meant by ecosystem how it works