Water Cycle and Nutrient Cycles Study Guide

The water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above, and below the surface of the Earth.

Carbon: Photosynthesis converts carbon dioxide ( $CO_2$ ) in the atmosphere into organic compounds through the process that occurs primarily in plants and algae.

Water: During photosynthesis, plants absorb water ( $H2O$ ) through their roots, which is then used in the process to produce oxygen ( $O2$ ) and glucose ( $C6H{12}O_6$ ).

Explanation: Eutrophication is the excessive growth of algae in water bodies due to increased nutrient levels, particularly nitrogen (N) and phosphorus (P), which can lead to oxygen depletion and negatively affect aquatic life.

Definition: Transpiration is the process by which water evaporates from the aerial parts of plants, particularly from the leaves, into the atmosphere.

Infiltration: The process by which water on the ground surface enters the soil.
Percolation: The downward movement of water through the soil layers due to gravity and capillary action.

Definition: Precipitation is any form of water, liquid or solid, that falls from the atmosphere and reaches the ground, including rain, snow, sleet, and hail.

Definition: Evaporation is the process through which water changes from a liquid to a gas or vapor, typically occurring from oceans, rivers, lakes, and surface water bodies.

Relative Size: The Earth's fresh water reservoirs include:
- Atmosphere
- Estuaries
- Lakes
- Rivers and Streams
- Ice Caps and Glaciers

Largest Reservoir: The atmosphere, consisting primarily of nitrogen gas ( $N_2$ ), serves as the largest reservoir of nitrogen.

Nitrogen is added back to the atmosphere through processes such as denitrification, where bacteria convert nitrates ( $NO3^-$ ) back into nitrogen gas ( $N2$ ).

Nitrate Addition: Nitrate can be added to soil through:
- Fertilization
- Decomposition of organic matter
- Atmospheric deposition

Denitrification: The process by which nitrogenous compounds are converted back into atmospheric nitrogen.
Nitrification: The conversion of ammonia ( $NH3$ ) to nitrites ( $NO2^-$ ) and then to nitrates ( $NO_3^-$ ) by bacteria.
Assimilation: The process by which plants and other organisms take up nitrates and incorporate the nitrogen into organic molecules, such as amino acids and proteins.

No Gaseous Form: Unlike nitrogen, phosphorus does not have a gaseous form in its cycle and primarily exists in solid forms which are generally insoluble in water.

Guano, a fertilizer obtained from the excrement of fish-eating birds, is harvested for its high nitrogen (N) and phosphorus (P) content.

Nitrogen (N) and Phosphorus (P) are considered limiting nutrients, meaning they regulate the rate of primary production (i.e., plant growth) in ecosystems.

Hydrosphere: Oceans and freshwater bodies act as significant carbon sinks by absorbing carbon dioxide from the atmosphere.
Atmosphere: The atmosphere contains carbon dioxide ( $CO_2$ ) as a greenhouse gas contributing to the carbon cycle.
Soil (Geosphere): Carbon is stored in soils, including decaying organic matter, where it contributes to soil health and fertility.
Lithosphere: Carbon is stored in rocks and minerals such as limestone ( $CaCO_3$ ) and fossil fuels (coal, oil, natural gas).

Carbon returns to the atmosphere through:
- Respiration by organisms, releasing $CO_2$
- Decomposition of organic matter
- Combustion of fossil fuels

Roles of Elements: All elements discussed (Carbon, Nitrogen, Phosphorus, Sulfur, Sodium) play critical roles in biological processes, including:
- Structural components of cells
- Functionality in metabolic pathways
- Nutrient cycling crucial for ecosystem health and sustainability.