2.2 – Nutrient Cycles in Ecosystems
_________________ are chemicals that are required for plant and animal growth and other life processes.
Nutrients accumulate in Earth’s atmosphere, oceans, and land. These accumulations are called
______________. Nutrients continually flow into and out of these stores. This process is called
_______________________.
___________________________ such as land clearing, farming, building cities, mining, industry and
transportation can affect nutrient cycles. Without human interference, nutrient cycles are usually
________________, with the same amount of nutrients entering the stores as leaving them.
There are ______ chemical elements that limit the amount and types of life in an ecosystem:
 Carbon
 Hydrogen
 Oxygen
 Nitrogen
 Phosphorus
The Carbon Cycle
All living things contain __________ in their cells.
Carbon stores are found in many places on Earth:
 Inside all living things
 Inside decaying organic matter in the soil
 Dissolved in the oceans
 Fossil fuel (oil and gas) deposits under the ground
___________________is the process by which layers of sediment build up on the ground. Over time, the
layers are turned into rock.
___________________is a chemical that is formed when carbon dioxide from the atmosphere becomes
dissolved in ocean water.
There are several processes that move ______________ through ecosystems.
1. Photosynthesis
 Plants gather sunlight and water, and using carbon dioxide from the air, are able to make
carbon-containing carbohydrates (sugars) for food.
 Photosynthesis also occurs in some types of microorganisms such as bacteria.
 When herbivores (primary consumers) eat plants, they take the carbon in the plants into
their bodies.
2. Cellular respiration
 Plants and animals release carbon dioxide gas into the air as a waste product of their cells.
Cells produce CO2 as they grow, repair, and reproduce.
3. Decomposition
 When plants and animals die and decompose, carbon dioxide is released into the
atmosphere.
4. Other processes
 The cycling of ocean currents releases CO2 that is dissolved in the oceans into the
atmosphere.
 Sometimes volcanoes release CO2 into the atmosphere when they erupt.
 Forest fires release carbon dioxide into the atmosphere as they burn.
Human activities are increasing the amount of ____________________ in the air. This is
due mostly to the burning of ___________________________. However, clearing land
for _________________ and ________________________ also decreases the amount of plants that are
present to remove CO2 from the air. The combination of putting more CO2 into the air and removing
places where the CO2 can be stored upsets the natural balance of the carbon cycle.
The Nitrogen Cycle
The largest store of nitrogen is in the _______________________, where it exists as N2, nitrogen gas.
Some nitrogen is also stored in lakes and oceans, soil, and living organisms. Most organisms can not use
the nitrogen as it exists in the air, so the nitrogen needs to be altered to a usable form.
_______________________________is the process by which nitrogen gas from the atmosphere is
changed into compounds that contain __________________ (NO3-) or __________________________
(NH4+). Both of these compounds are usable by _______________.
A small amount of nitrogen fixation occurs when __________________ strikes. Most nitrogen fixation
occurs in the ___________ by _________________________________ during the decomposition
process. Certain species of cyanobacteria also fix nitrogen in aquatic ecosystems.
Not all __________ live closely with nitrogen-fixing bacteria, so other bacteria, called
__________________ bacteria, use a two-step process called _____________________ to ensure the
nitrogen in the soil is usable for the plants.
Nitrogen is returned to the atmosphere in a process called ________________________.
This process involves nitrogen becoming N2 again due to _________________________ bacteria.
Excess nitrate and ammonium that are not used by plants are washed by _____________ into streams and
ground water. The unused nitrogen may settle into sediments on the bottom. Eventually, the sediments
will form into rock and the nitrogen will no longer be available.
Some human activities that alter the amount of nitrogen in the environment are:

Burning _______________ such as gasoline.

Clearing ______________ and _________________.

Using chemical _________________ that contain nitrogen.
When there is excess nitrogen in the soil (usually due to chemical fertilizers), a process called
_______________ takes place, where water washes unused nitrogen into nearby water sources.
Too much nitrogen in an aquatic ecosystem can result in _______________________, the process by
which water plants grow too much and kill off other species in the ecosystem by depriving them of
sunlight and nutrients.
The Phosphorus Cycle
___________________ is an essential element in the molecule that carries energy to plant and animal
cells. In plants, it contributes to roots, stem strength and seed production. In animals, it is an important
mineral in bone tissues.
Phosphorus is not stored in the __________________ like carbon and nitrogen. It is only stored in
phosphate-containing rocks and sediments on ocean floors.
______________________ is the process of breaking rocks up into smaller pieces over a long period of
time (due to rain, weather, and plant growth). When a rock is weathered, phosphate is released into the
environment.The phosphate is usually washed into lakes
and oceans where it sits on the bottom as ________________. Over time, rocks are lifted
out of the water in a process called ________________________. When the rocks are re-exposed, the
weathering process can again take place, and the phosphorus can be re-released into the environment.
Humans contribute to the amount of phosphorus in the environment when they:

_____________ phosphate rocks to make fertilizers and detergents.

_____________. Animal (and human) wastes are a large source of phosphates.

Slash and burn _______________.
Too much phosphorus in the environment can cause the ___________ of certain organisms, especially
those in aquatic ecosystems.