SUMMARY

CONCEPT 22.1 Nutrients enter ecosystems through the chemical breakdown of minerals in rocks or through fixation of atmospheric gases.

22.1.1 Describe the basic roles of nutrients in organisms, and differentiate between the ways in which microorganisms, plants, and animals obtain them.

The nutrient requirements of organisms are specific to their physiology and thus differ between autotrophs and heterotrophs.

Autotrophs absorb nutrients in simple, soluble forms from their environment, while heterotrophs obtain them in more complex forms by consuming prey or detritus.

22.1.2 Summarize the steps involved in the breakdown of minerals and the subsequent release of nutrients.

The physical and chemical breakdown of minerals (weathering) releases soluble nutrients.

22.1.3 Describe the physical and chemical properties of soil that influence its fertility.

Soils are made up of mineral particles, detritus, dissolved organic matter, water containing dissolved minerals and gases, and organisms.

22.1.4 Explain the processes that fix carbon and nitrogen, converting them into usable forms for organismal function and growth.

Carbon and nitrogen enter ecosystems through fixation of atmospheric gases by autotrophs and by bacteria, respectively.

CONCEPT 22.2 Chemical and biological transformations in ecosystems alter the chemical form and supply of nutrients.

22.2.1 Describe why decomposition is a critical process in the supply of nutrients in ecosystems.

Decomposition of organic matter releases the nutrients it contains in soluble forms that can be reused by plants and microorganisms.

22.2.2 Evaluate the biological and physical factors that influence rates of decomposition.

Moisture and temperature are key controls on decomposition in terrestrial ecosystems, with the highest rates occurring in warm temperatures and intermediate moisture conditions.

Decomposition rates are influenced by the ratio of carbon to nutrient concentration, along with the chemistry of the carbon of the detritus, which determines how easily it can be degraded and used as an energy source.

22.2.3 Explain how microbial processes may alter the chemical forms of nutrients and make them either more or less available to plants.

Modification of the chemical forms of nutrients, particularly nitrogen, by microorganisms influences the nutrients’ availability to organisms or loss from the ecosystem.

Plants recycle nutrients by reabsorbing them from senescing tissues and remobilizing them when growth commences again.

CONCEPT 22.3 Nutrients cycle repeatedly through the components of ecosystems.

22.3.1 Describe the key processes involved in nutrient cycling in 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.

22.3.2 Summarize what determines the mean residence time of nutrients in ecosystems.

The mean residence time of nutrients in ecosystems is related to nutrient uptake rates, associated with biological demand, and the rate of turnover of organic matter, associated with decomposition.

22.3.3 Evaluate how the processes that determine the loss of nutrients from an ecosystem would change during succession and why the loss rates of specific nutrients may vary through time.

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.

Losses of nutrients from terrestrial ecosystems can be estimated by measuring nutrient outputs in stream water.

CONCEPT 22.4 Freshwater and marine nutrient cycles occur in a moving medium and are linked to terrestrial ecosystems.

22.4.1 Describe the concept of nutrient spiraling in moving waters and summarize the factors that control the spiral lengths.

The cycling of nutrients 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.

22.4.2 Differentiate between natural and anthropogenic causes of eutrophication.

Increases in the supply of nutrients limiting NPP, associated with both natural and anthropogenic causes, can lead to the enhancement of algal biomass in lakes and estuaries.

22.4.3 Explain why seasonal lake turnover and upwelling are important to enhancing nutrient supply in lakes and oceans.

In lakes, nutrients are cycled between the water column and the benthic sediments.

Imports of nutrients from rivers and terrestrial ecosystems support production in marine ecosystems.

REVIEW QUESTIONS

1. Describe the processes involved in the transformation of solid minerals in rock into soluble nutrients in soil. What biological factors can influence the rate of this transformation?

2. Why is nitrogen often in short supply relative to other nutrients required by plants, despite being the most abundant element in the atmosphere? How does the supply of nitrogen change during terrestrial ecosystem development?

3. Which factor is more important in controlling the mean residence times and pools of nutrients in soil organic matter in terrestrial ecosystems: the rate of input (i.e., primary productivity) or the rate of decomposition? Would you expect to find larger nutrient pools in the soils of a tropical forest than in those of a boreal forest, given that primary productivity is higher in the tropics?

4. Why might you expect nutrient input from terrestrial and stream ecosystems to be more important in tropical lakes than in temperate-zone lakes?

HONE YOUR PROBLEM-SOLVING SKILLS

How would disturbances and subsequent succession (see Chapter 17) influence nutrient cycling and nutrient losses in an ecosystem? As discussed above, nutrient losses are in part related to uptake by plants (see Figure 22.13).

1. Based on patterns of community replacement during succession discussed in Chapter 17, what pattern of nutrient loss from a catchment would you hypothesize following a disturbance in a forest? Consider how nutrient loss would change just after the disturbance, into the intermediate stages of succession, and finally into an old-growth community made up of long-lived mature trees.

2. Would the patterns of nutrient loss that you hypothesized above be the same for all nutrients? Would nutrients that limit NPP show the same patterns as nutrients that are not limiting to growth?

3. Peter Vitousek (1977) used a catchment approach to study nutrient retention by spruce-fir forests in the White Mountains of New Hampshire at different stages of secondary succession following logging. He measured nutrient loss in streams draining catchments of different successional stages. In order to evaluate the hypothesis that late successional communities would be more “leaky” than intermediate-stage communities, he examined the ratio of old-growth losses to losses during intermediate successional stages. He did this for several elements, some potentially more limiting to NPP than others. His results are shown in the table (μeq∕L, microequivalents/liter). Do the data support Vitousek's hypothesis regarding changes in nutrient losses between intermediate and late successional forest communities? Do all of the elements show the same patterns of loss for intermediate and late successional communities? How do the differences in the patterns of element losses relate to their importance to plant growth?

LIST OF KEY TERMS

aerosols alluvium atmospheric deposition biocrust biogeochemistry biological soil crust catchment cation exchange capacity chemical weathering clays

decomposition denitrification eutrophic eutrophication horizons leaching Lignin litter

loess

mean residence times mechanical weathering Mesotrophic mineralization nitrification nitrogen fixation nutrient cycling nutrients occlusion oligotrophic parent material pool sand silt

Soil

till weathering