Chapter 21 Summary

CONCEPT 21.1 Nutrients enter ecosystems through the chemical breakdown of minerals in rocks or through fixation of gases in the atmosphere.

• The nutrient requirements of organisms are specific to their physiological demands and thus differ between autotrophs and heterotrophs.
• Autotrophs absorb nutrients in soluble forms from their environment, while heterotrophs obtain them by consuming prey or detritus.
• The physical and chemical breakdown of rock minerals (weathering) releases soluble nutrients.
• Soils are made up of mineral particles, detritus, dissolved organic matter, water containing dissolved minerals and gases, and organisms.
• Carbon and nitrogen enter ecosystems through fixation of atmospheric gases by autotrophs and by prokaryotes, respectively.

CONCEPT 21.2 Chemical and biological transformations in ecosystems alter the chemical form and supply of nutrients.

• Decomposition of organic matter releases the nutrients it contains in soluble forms that can be reused by plants and microorganisms.
• Modification of the chemical forms of nutrients, particularly nitrogen, by microorganisms influences their availability as nutrients or loss from the ecosystem.
• Plants recycle nutrients by reabsorbing them from senescing tissues and remobilizing them when growth commences again.

CONCEPT 21.3 Nutrients cycle repeatedly through the components of ecosystems.

• Nutrient cycling rates are controlled primarily by the rate of decomposition, which in turn is controlled by climate and the chemistry of plant litter.
• Losses of nutrients from terrestrial ecosystems can be estimated by measuring nutrient outputs in stream water.
• Changes in the relative amounts of nutrients supplied by weathering and decomposition determine the specific nutrients that limit primary production at different stages of ecosystem development.

CONCEPT 21.4 Freshwater and marine ecosystems receive nutrient inputs from terrestrial ecosystems.

• Nutrient cycling in streams and rivers can be thought of as a spiral of repeated biological uptake and incorporation into organic forms followed by release in inorganic forms.
• In lakes, nutrients are cycled between the water column and the bottom sediments.
• Imports of nutrients from rivers and terrestrial ecosystems support production in marine ecosystems.