1. Biogeochemical Cycles
Chapter 2

2. Ecosystems provide vital services
All life on Earth (including humans) depends on healthy, functioning ecosystems
Ecosystem services: essential services provided by healthy, normally functioning ecosystems
When human activities damage ecosystems, we must devote resources to supply these services ourselves
Example: if we kill off insect predators, farmers must use synthetic pesticides that harm people and wildlife
One of the most important ecosystem services:
Nutrients cycle through the environment in intricate ways

3. Ecological processes provide services

4. Nutrients circulate through ecosystems
Nutrients move through the environment in complex ways
Matter is continually circulated in an ecosystem
Nutrient (biogeochemical) cycle: the movement of nutrients through ecosystems
Pool (reservoir): a location where nutrients remain for varying amounts of time (residence time)
Source: a reservoir releases more materials than it accepts
Sink: a reservoir that are accepts more than it releases
Flux: the rate at which materials move between reservoirs
Can change over time
Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 4

5. Humans affect nutrient cycling
Human activities affect nutrient cycling
Altering fluxes, residence times, and amounts of nutrients in reservoirs

6. The water cycle affects all other cycles
Water is essential for biochemical reactions and is involved in nearly every environmental system and cycle
Hydrologic cycle: the flow of liquid, gaseous, and solid water through the environment
Less than 1% is available as fresh water
Evaporation: conversion of liquid to gaseous water
Transpiration: release of water vapor by plants
Precipitation: rain or snow returns water to Earth’s surface
Runoff: water flows into streams, lakes, rivers, oceans

7. Transpiration
Animation: Transpiration
Right-click / Select “Play”

8. Water is also stored underground
Infiltration: water soaks down through rock and soil to recharge aquifers
Aquifers: underground reservoirs of spongelike regions of rock and soil that hold …
Groundwater: water found underground beneath layers of soil
Water table: the uppermost level of groundwater held in an aquifer
Water in aquifers may be ancient (thousands of years old)

9. The hydrologic cycle

10. Human impacts on the hydrologic cycle
Humans have affected almost every flux, reservoir, and residence time in the water cycle
Damming rivers slows water movement and increases evaporation
Removal of vegetation increases runoff and erosion while decreasing infiltration and transpiration
Overdrawing surface and groundwater for agriculture, industry, and domestic uses lowers water tables
Emitting air pollutants that dissolve in water changes the nature of precipitation and decreases cleansing
Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 10

11. The Carbon Cycle
Carbon cycle: describes carbon’s route in the environment
Carbon forms essential biological molecules
Through photosynthesis, producers move carbon from the air and water to organisms
Respiration returns carbon to the air and water
Oceans are the second largest reservoir of carbon
Absorb carbon from the air, land, and organisms
Decomposition returns carbon to the sediment, the largest reservoir of carbon
Ultimately, it may be converted into fossil fuels
Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 11

12. The carbon cycle

13. Humans affect the carbon cycle
Burning fossil fuels moves carbon from the ground to the air
Since mid-1700s, people have added over 275 billion tons of carbon dioxide to the atmosphere
Cutting forests and burning fields moves carbon from organisms to the air
Less carbon dioxide is removed by photosynthesis
Today’s atmospheric carbon dioxide reservoir is the largest in the past 800,000 years
The driving force behind climate change

14. The Nitrogen Cycle Nitrogen makes up 78% of the atmosphere
It is contained in proteins, DNA, and RNA
It is also essential for plant growth
Nitrogen cycle: describes the routes of nitrogen through the environment
Nitrogen gas is inert and cannot be used by organisms
It needs lightning, bacteria, or human intervention to become biologically active and available to organisms
Then it is a potent fertilizer

15. Nitrogen must become biologically available
Nitrogen fixation: nitrogen-fixing soil bacteria or lightning “fixes” nitrogen gas into ammonium
Nitrogen-fixing bacteria live in legumes (e.g., soybeans)
Nitrification: bacteria then convert ammonium ions first into nitrite ions then into nitrate ions
Plants can take up these ions
Nitrite and nitrate also come from the air or fertilizers
Animals obtain nitrogen by eating plants or other animals
Denitrifying bacteria: convert nitrates in soil or water to gaseous nitrogen, releasing it back into the atmosphere

16. The nitrogen cycle

17. Humans greatly affect the nitrogen cycle
Historically, nitrogen fixation was a bottleneck: limited the flux of nitrogen from air into water-soluble forms
Industrial fixation fixes nitrogen on a massive scale
Overwhelming nature’s denitrification abilities
Excess nitrogen leads to hypoxia in coastal areas
Nitrogen-based fertilizers strip the soil of other nutrients
Reducing soil fertility
Burning forests and fossil fuels leads to acid precipitation, adds greenhouse gases, and creates photochemical smog

18. The phosphorus cycle
Phosphorus cycle: describes the routes that phosphorus takes through the environment
No significant atmospheric component
Most phosphorus is in rocks
With naturally low environmental concentrations, phosphorus is a limiting factor for plant growth
Weathering releases phosphorus into water
Allowing it to be taken up by plants
Phosphorus is a key component of cell membranes, DNA, RNA, and other biochemical compounds

19. The phosphorus cycle

20. Humans affect the phosphorus cycle
Fertilizer from lawns and farmlands
Increases phosphorus in soil
Its runoff into water increases phytoplankton blooms and hypoxia
Wastewater containing detergents releases phosphorus to waterways

21. Controlling nutrient pollution in waterways
Reduce fertilizer use in farms and lawns
Change timing of fertilizer applications to minimize runoff
Manage livestock manure applications to farmland
Plant vegetation “buffers” around streams to trap runoff
Restore wetlands and create artificial ones to filter runoff
Improve sewage-treatment technologies
Restore frequently flooded lands
Reduce fossil fuel combustion

22. Conclusion
Life interacts with its abiotic environment in ecosystems through which energy flows and materials are recycled
Understanding biogeochemical cycles is crucial
Humans are changing the ways those cycles function
Understanding energy, energy flow, and chemistry increases our understanding of organisms
How environmental systems function
Thinking in terms of systems can teach us how to:
Avoid disrupting Earth’s processes and to mitigate any disruptions we cause
Copyright C Pearson Education, Inc., publishing as Benjamin Cummings 22