Earth as a System: Energy, Matter, and Life · Lesson 9 of 13
Biogeochemical Cycles
“Water never really leaves the group chat; it just changes states and locations.”
• Define biogeochemical cycles. • Explain why matter must be recycled. • Describe the major processes of the water cycle. • Trace water through several Earth spheres. • Distinguish runoff from infiltration. • Explain how warming can alter the water cycle.
The water in a glass may once have been ocean spray, cloud droplets, glacier ice, groundwater or part of a leaf. Matter moves repeatedly between living and non-living parts of the Earth, often changing state or chemical form along the way.
A biogeochemical cycle is the repeated movement of matter between living organisms and the atmosphere, water, rocks and soil.
The word combines biological processes involving life, geological stores in rocks and soil, and chemical changes in matter. Cycling is essential because the supply of atoms is limited. Organisms need water and elements repeatedly, so decomposition and other transformations return materials to forms that can be reused.
Water Cycle
| Process | What happens |
|---|---|
| Evaporation | Liquid water gains energy and becomes water vapour |
| Transpiration | Plants release water vapour from their leaves |
| Condensation | Water vapour cools and forms liquid droplets |
| Precipitation | Water falls from clouds as rain, snow or other forms |
| Runoff | Water flows over land into streams, lakes or seas |
| Infiltration | Water enters soil and may recharge groundwater |
Problem
Trace one possible route through the system.
- 1.Ice melts and becomes liquid water.
- 2.Meltwater flows into a stream or soaks into soil.
- 3.A plant root absorbs some of the water.
- 4.The water travels through the plant and leaves by transpiration.
- 5.Water vapour may condense and later return as precipitation.
Problem
Why can intense rain reduce groundwater recharge?
- 1.Rain arrives faster than exposed soil can absorb it.
- 2.More water flows across the surface as runoff.
- 3.Fast runoff removes soil particles and nutrients.
- 4.Less water remains long enough to infiltrate.
- 5.A heavy rainfall event can therefore occur together with erosion and poor recharge.
Warmer conditions increase evaporation from many surfaces and allow air to hold more water vapour. Rainfall may become more intense in some places, while changed circulation can prolong dry periods elsewhere. Melting glaciers initially increase some flows but reduce the long-term frozen store. Thermal expansion of seawater and addition of meltwater contribute to sea-level rise.
Vegetation slows runoff, improves infiltration through roots and soil organisms, and returns water to the atmosphere through transpiration. Removing vegetation therefore changes several pathways at once. The water cycle is a clear example of atmosphere, hydrosphere, cryosphere, geosphere and biosphere operating together.
Water can remain stored for different lengths of time in oceans, ice, groundwater and organisms. Many branching pathways operate simultaneously.
Quiz
Which description best matches Biogeochemical Cycle?
Which term matches this description: A biogeochemical cycle is the repeated movement of matter between living organisms and the atmosphere, water, rocks and soil.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Distinguish evaporation and transpiration.
- Trace water through at least four spheres.
- Explain how vegetation can increase infiltration.
- Describe two ways atmospheric warming can alter the water cycle.
Key Takeaways
• Biogeochemical cycles recycle limited matter. • Water changes state as it moves among reservoirs. • Runoff and infiltration are competing pathways after precipitation. • Climate and land cover can change the speed and direction of water movement.