Chapter 3, section 3.4 Chapter Review

Describe the carbon, nitrogen, and phosphorus cycles, and explain how human activities are affecting each cycle.

Carbon cycle: The series of processes by which carbon compounds are interconverted in the environment, involving the incorporation of carbon dioxide into living tissue by photosynthesis and its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels. Ex: Within a few hundred years, we have extracted and burned huge quantities of fossil fuels that took millions of years to form. This has added large quantities of CO2 to the atmosphere (see the red arrows in Figure 3.16) faster than the carbon cycle can recycle it. There is considerable scientific evidence that this disruption of the carbon cycle is helping to warm the atmosphere and change the earth's climate. The oceans remove some of this CO2 but as a result, the acidity of ocean waters is rising. This is bad news for organisms that are adapted to less-acidic ocean waters. Nitrogen cycle: The series of processes by which nitrogen and its compounds are interconverted in the environment and in living organisms, including nitrogen fixation and decomposition. Ex: People also alter the nitrogen cycle in aquatic ecosystems by adding excess nitrates (NO −). These nitrates contaminate bodies of water through agricultural runoff of fertilizers, animal manure, and discharges from municipal sewage treatment systems. This plant nutrient can cause excessive growths of algae that can disrupt aquatic systems. Phosphorus cycle: The cyclic movement of phosphorus (P) through water, the earth's crust, and living organisms is called the phosphorus cycle. Ex: Human activities, including the removal of large amounts of phosphate from the earth to make fertilizer, disrupt the phosphorus cycle. By clearing tropical forests we expose the topsoil to increased erosion, which reduces phosphate levels in the tropical soils.

Explain how human activities are affecting the water cycle.

First, people sometimes withdraw freshwater from rivers, lakes, and aquifers, at rates faster than natural processes can replace it. As a result, some aquifers are being depleted and some rivers no longer flow to the ocean. Second, people clear vegetation from land for agriculture, mining, road building, and other activities, and cover much of the land with buildings, concrete, and asphalt. This increases water runoff and reduces infiltration that would normally recharge groundwater supplies. Third, people drain and fill wetlands for farming and urban development. Left undisturbed, wetlands provide the ecosystem service of flood control. Wetlands act like sponges to absorb and hold overflows of water from drenching rains and rapidly melting snow.

What is an aquifer?

Ground- water collects in aquifers, which are underground layers of sand, and water-bearing rock.

What happens to matter in an ecosystem?

Matter, in the form of nutrients, cycles within and among ecosystems and the biosphere, and human activities are altering these chemical cycles.

What is the key concept for this section?

Matter, in the form of nutrients, cycles within and among ecosystems and the biosphere, and human activities are altering these chemical cycles.

What percentage of the earth's water supply is available to humans and other species as liquid freshwater?

Only about 0.024% of the earth's vast water supply is available to humans and other species as liquid freshwater in accessible groundwater deposits and in lakes, rivers, and streams. The rest of the planet's water is too salty, is too deep underground to extract at affordable prices, or is stored as ice in glaciers.

What is a nutrient cycle?

The elements and compounds that make up nutrients move continually through air, water, soil, rock, and living organisms within ecosystems, in cycles called nutrient cycles, or biogeochemical cycles.

Describe the hydrologic cycle or water cycle.

The hydrologic cycle, or water cycle, collects, purifies, and distributes the earth's fixed supply of water.

What is surface runoff?

This water flows over land surfaces into streams, rivers, lakes, wetlands, and the ocean from which some of the water evaporates. Some of the water seeps into the upper layers of soils and is used by plants, and some evaporates from the soils back into the atmosphere.

Summarize the unique properties of water.

Water exists as a liquid over a wide temperature range because of forces of attraction between its molecules. Liquid water changes temperature slowly because it can store a large amount of heat without a large change in its own temperature. It takes a large amount of energy to evaporate water because of the attractive forces between its molecules. Liquid water can dissolve more com- pounds than other liquids. Water filters out wavelengths of the sun's ultraviolet radiation that would harm some aquatic organisms. Unlike most liquids, water expands when it freezes.

Define groundwater.

Water that seeps deeper into the soil is known as groundwater.

Explain how nutrient cycles connect past, present, and future life.

nutrient cycles connect past, present, and future forms of life. Some of the carbon atoms in your skin may once have been part of an oak leaf, a dinosaur's skin, or a layer of limestone rock. your grandmother, George Washington, or a hunter- gatherer who lived 25,000 years ago may have inhaled some of the nitrogen (n2) molecules you just inhaled.