Soil and Agriculture
2. Nutrient Cycles
Learning outcomes
- I can explain how nutrients move through ecosystems.
- I can describe the roles of carbon, nitrogen, and phosphorus cycles.
- I can explain the importance of nutrient recycling.
- I can identify factors that affect nutrient availability.
- I can analyze the relationship between nutrient cycles and agriculture.
Nutrient Cycles
Nutrients are chemical substances that organisms need for growth, repair, reproduction, and other life processes. Unlike energy, which enters ecosystems mainly as sunlight and eventually leaves as heat, matter is continually recycled.
Atoms of carbon, nitrogen, phosphorus, and other elements move between:
- living organisms
- soil
- water
- the atmosphere
- rocks and sediments
This movement is described by nutrient cycles, also called biogeochemical cycles.
A useful distinction is:
Energy flows through ecosystems.
Nutrients cycle through ecosystems.
Why Nutrients Must Be Recycled
Organisms continuously require materials to build cells and molecules.
Plants require nutrients to produce substances such as:
- proteins
- DNA
- chlorophyll
- cell membranes
- ATP
Animals obtain nutrients by consuming plants or other animals.
If nutrients moved through food chains but were never returned to the environment, eventually the available supply would decrease.
Decomposition solves much of this problem.
A simplified nutrient pathway is:
soil → plant → animal → waste/dead material → decomposers → soil
The same atoms can therefore be used repeatedly by different organisms.
Nutrient Reservoirs
A reservoir is a place where a nutrient is stored for some period of time.
Nutrients can be stored in:
- the atmosphere
- living organisms
- soil
- oceans
- freshwater
- rocks
- sediments
- fossil fuels
Different nutrient cycles have different major reservoirs.
For example:
- the atmosphere is an important reservoir of carbon and nitrogen
- rocks and sediments are major reservoirs of phosphorus
The location of a nutrient strongly affects how quickly it can move through an ecosystem.
The Carbon Cycle
Carbon is an essential component of living organisms.
Carbon is found in:
- carbohydrates
- lipids
- proteins
- nucleic acids
Carbon moves among the atmosphere, organisms, soil, oceans, and rocks.
Important processes in the carbon cycle include:
- photosynthesis
- feeding
- respiration
- decomposition
- combustion
- ocean-atmosphere exchange
- sedimentation
Photosynthesis and Carbon
Plants, algae, and some microorganisms remove carbon dioxide from their environment during photosynthesis.
A simplified equation is:
carbon dioxide + water → glucose + oxygen
using light energy.
Carbon from CO₂ becomes incorporated into organic molecules.
This means photosynthesis transfers carbon:
atmosphere → producers
or, in aquatic environments:
dissolved CO₂ → producers
Carbon Through Food Webs
Once carbon becomes part of plant tissue, it can move through a food web.
For example:
CO₂ → grass → rabbit → fox
The carbon atoms originally present in atmospheric carbon dioxide may eventually become part of:
- glucose
- fat
- muscle
- DNA
in different organisms.
When organisms consume food, both energy and matter are transferred.
However, they behave differently:
energy eventually leaves as heat
while
carbon can be recycled
Respiration and Carbon
Organisms release carbon dioxide during cellular respiration.
A simplified equation is:
glucose + oxygen → carbon dioxide + water + energy
Respiration therefore transfers carbon:
organic molecules → carbon dioxide
Plants, animals, fungi, and many microorganisms carry out cellular respiration.
Plants therefore participate in both:
- photosynthesis
- respiration
Decomposition and Carbon
When organisms die, their bodies contain carbon-rich organic compounds.
Decomposers such as:
- bacteria
- fungi
break down this material.
During decomposition:
- carbon can enter the soil
- carbon dioxide can be released through respiration
- some carbon can remain stored in organic matter
Without decomposition, large amounts of nutrients would remain trapped in dead organisms.
Combustion and Carbon
Combustion releases stored carbon into the atmosphere, mainly as carbon dioxide.
Materials containing carbon include:
- wood
- coal
- oil
- natural gas
For example:
fuel + oxygen → carbon dioxide + water + energy
Burning fossil fuels transfers carbon that was stored underground into the atmosphere.
Long-Term Carbon Storage
Not all carbon cycles rapidly.
Carbon can remain stored for long periods in:
- soils
- ocean sediments
- carbonate rocks
- fossil fuels
- deep ocean water
These are sometimes described as carbon stores or carbon sinks, depending on context.
Carbon can therefore participate in both:
fast biological cycling
and
slow geological cycling
Human Effects on the Carbon Cycle
Human activities can alter the carbon cycle.
Important examples include:
- burning fossil fuels
- deforestation
- land-use change
- cement production
Burning fossil fuels increases the transfer:
geological carbon → atmospheric CO₂
Deforestation can both release stored carbon and reduce the amount of vegetation available to remove CO₂ through photosynthesis.
The Nitrogen Cycle
Nitrogen is essential for producing:
- amino acids
- proteins
- DNA
- RNA
- other nitrogen-containing molecules
The atmosphere contains a very large amount of nitrogen gas, N₂.
However, most plants cannot directly use atmospheric N₂.
It must first be converted into biologically available nitrogen compounds.
Nitrogen Fixation
Nitrogen fixation converts atmospheric nitrogen gas into forms that can eventually be used by plants.
Nitrogen-fixing microorganisms are particularly important.
Some live freely in soil.
Others live in associations with plants such as legumes.
Examples of legumes include:
- beans
- peas
- lentils
- clover
Some nitrogen-fixing bacteria live in nodules on the roots of these plants.
Why Nitrogen Fixation Matters
Atmospheric nitrogen is abundant, but N₂ is chemically stable.
Most organisms cannot simply take nitrogen gas and build proteins from it.
Nitrogen fixation makes nitrogen more biologically accessible.
A simplified pathway is:
atmospheric N₂ → nitrogen compounds in soil → plants → animals
Nitrogen fixation therefore connects the enormous atmospheric nitrogen reservoir to living ecosystems.
Nitrification
Nitrification is a process carried out by microorganisms in which ammonium is converted into nitrite and then nitrate.
Simplified:
ammonium → nitrite → nitrate
Nitrate is a form of nitrogen that many plants can absorb through their roots.
Soil microorganisms therefore play an important role in determining nutrient availability.
Plant Uptake
Plants absorb nitrogen-containing ions through their roots.
They use nitrogen to make substances such as:
- amino acids
- proteins
- nucleic acids
- chlorophyll
Nitrogen then moves through food chains when animals consume plants or other animals.
For example:
soil nitrate → wheat → human
Ammonification
Dead organisms and waste contain nitrogen.
Decomposers break down these nitrogen-containing compounds.
During ammonification, organic nitrogen is converted into ammonium compounds.
Simplified:
organic nitrogen → ammonium
This returns nitrogen from dead material and waste to the soil nutrient pool.
Denitrification
Denitrification is carried out by certain bacteria, especially under low-oxygen conditions.
These bacteria convert nitrate into nitrogen gases that return to the atmosphere.
Simplified:
nitrate → nitrogen gas
This completes an important part of the nitrogen cycle:
atmosphere → soil → organisms → soil → atmosphere
Summary of the Nitrogen Cycle
The major processes can be organized as:
N₂ in atmosphere
↓
nitrogen fixation
↓
ammonium compounds
↓
nitrification
↓
nitrates
↓
plant uptake
↓
animals
↓
waste and dead organisms
↓
ammonification
Nitrogen can also return to the atmosphere through:
nitrate → denitrification → N₂
Nitrogen and Agriculture
Nitrogen frequently limits plant growth.
Farmers may increase available nitrogen by:
- applying nitrogen fertilizers
- adding manure
- growing legumes
- using crop rotations
- maintaining organic matter
Increasing available nitrogen can increase crop growth when nitrogen is limiting.
However, excessive nitrogen can create environmental problems.
The Phosphorus Cycle
Phosphorus is another essential nutrient.
It is found in important biological molecules including:
- DNA
- RNA
- ATP
- phospholipids
Phosphorus is also important in bones and teeth.
Unlike carbon and nitrogen, the phosphorus cycle has no major atmospheric gas phase.
Much of Earth's phosphorus is stored in:
- rocks
- minerals
- sediments
Weathering and Phosphorus
Phosphorus-containing rocks gradually break down through weathering.
This can release phosphate compounds into:
- soil
- streams
- rivers
Plants can absorb phosphate ions through their roots.
The phosphorus then becomes incorporated into biological molecules.
A simplified pathway is:
rock → weathering → phosphate in soil → plant
Phosphorus Through Food Webs
Animals obtain phosphorus by consuming:
- plants
- other animals
For example:
soil phosphate → grass → cow
Phosphorus becomes part of the animal's:
- cells
- ATP
- DNA
- bones
When organisms produce waste or die, decomposers help return phosphorus compounds to the environment.
Phosphorus in Water
Phosphorus can be transported into aquatic systems by:
- erosion
- runoff
- wastewater
- decomposition
Some phosphorus eventually settles into sediments.
Over very long periods, geological processes can form new phosphorus-containing rocks.
The phosphorus cycle is therefore generally slower than many parts of the carbon cycle.
Comparing the Three Major Cycles
| Feature | Carbon | Nitrogen | Phosphorus |
|---|---|---|---|
| Important atmospheric reservoir | Yes | Yes | No major one |
| Important rock reservoir | Yes | Limited role | Yes |
| Major biological importance | Organic molecules | Proteins and nucleic acids | ATP, DNA, membranes |
| Important microorganisms | Decomposers | Nitrogen-cycle bacteria | Decomposers |
| Agricultural importance | Organic matter | Major plant nutrient | Major plant nutrient |
| Major human influence | Fossil fuels, deforestation | Fertilizers, farming | Fertilizers, mining |
The cycles interact rather than operating independently.
Decomposition Connects Nutrient Cycles
Decomposition is central to nutrient recycling.
When organisms die, nutrients do not disappear.
Decomposers break down organic material and return nutrients to forms that can re-enter ecosystems.
For example:
dead plant
↓
decomposition
↓
nutrients released
↓
plant roots absorb nutrients
↓
new plant growth
Without decomposers, nutrients would increasingly become locked inside dead biological material.
Soil as a Nutrient Store
Soil is an important temporary reservoir for nutrients.
It may contain:
- nitrate
- ammonium
- phosphate
- organic carbon
- decomposing organic material
Plants depend heavily on the nutrients available in soil.
However, nutrient concentration alone does not determine availability.
Other soil properties also matter.
Factors Affecting Nutrient Availability
Nutrient availability can be influenced by:
- soil pH
- soil moisture
- temperature
- oxygen availability
- organic matter
- microbial activity
- soil texture
- erosion
- leaching
A nutrient can be present in soil but still be difficult for plants to absorb.
Soil pH and Nutrients
Soil pH influences the chemical form and availability of many nutrients.
If soil becomes too acidic or too alkaline:
- some nutrients may become less available
- some elements may become excessively available
- microbial activity may change
This can reduce plant growth even when the total amount of nutrients in the soil appears sufficient.
Organic Matter and Nutrient Cycling
Organic matter provides a reservoir of nutrients.
As decomposers break it down:
organic nutrients → mineral nutrients
These nutrients can then become available to plants.
Adding materials such as:
- compost
- crop residues
- manure
can therefore support nutrient cycling as well as improve soil structure.
Temperature and Nutrient Cycling
Temperature affects the activity of soil organisms.
When conditions are suitable, warmer temperatures can increase:
- microbial activity
- decomposition
- nutrient release
Very cold conditions can slow these processes considerably.
This is one reason nutrient cycling rates differ among ecosystems and seasons.
Soil Moisture
Microorganisms require water, so extremely dry soil can slow decomposition.
However, saturated soil may contain little oxygen.
Low oxygen can alter microbial processes, including parts of the nitrogen cycle.
Soil moisture therefore influences both:
- nutrient release
- nutrient transformations
Leaching
Leaching occurs when water carries dissolved nutrients downward through soil.
For example:
nitrate dissolved in soil water → moves downward with rainfall
If nutrients move below plant roots, they become less available to crops.
Leaching can also transport nutrients into:
- groundwater
- streams
- rivers
This represents both a loss of soil fertility and a potential pollution problem.
Erosion and Nutrient Loss
Nutrients can also be lost when soil is eroded.
Topsoil often contains substantial amounts of:
- organic matter
- nitrogen
- phosphorus
When topsoil is carried away by wind or water, nutrients move with it.
Therefore:
soil erosion → nutrient loss → reduced soil fertility
Eroded material can also carry nutrients into waterways.
Nutrient Cycles and Agriculture
Agriculture changes natural nutrient cycles because crops are harvested.
Consider a natural ecosystem:
plant grows → plant dies → decomposes → nutrients return to soil
Now consider agriculture:
crop grows → crop harvested → nutrients removed from field
Each harvest exports nutrients from the agricultural system.
If these nutrients are not replaced, soil fertility may decline.
Nutrient Removal by Harvesting
Imagine wheat absorbs nitrogen, phosphorus, and other nutrients from soil.
When the grain is harvested and transported away, some of those nutrients leave the field.
Repeated harvesting can therefore create:
soil nutrients → crop → harvest → nutrients removed
Farmers must manage this nutrient loss to maintain long-term productivity.
Fertilizers
Fertilizers supply nutrients needed by plants.
Common fertilizers contain combinations of:
- nitrogen
- phosphorus
- potassium
These are often represented as:
N-P-K
Fertilizers can improve plant growth when particular nutrients are limiting.
However, applying more fertilizer than crops can use is inefficient and potentially harmful.
Fertilizer Runoff
If fertilizer is applied in excess, nutrients may be transported by runoff.
A possible pathway is:
fertilizer → soil → rainfall → runoff → river → lake
Nitrogen and phosphorus entering aquatic ecosystems can stimulate excessive growth of algae and aquatic plants.
This can contribute to eutrophication.
Eutrophication
Eutrophication occurs when excessive nutrients stimulate unusually high biological productivity in a water body.
A simplified sequence is:
excess nitrogen/phosphorus
↓
rapid algal growth
↓
large quantities of algae die
↓
decomposition increases
↓
microorganisms consume more oxygen
↓
dissolved oxygen decreases
↓
fish and other organisms may experience stress or die
This demonstrates how agricultural nutrient management can affect ecosystems far from the farm itself.
Manure and Nutrient Cycling
Animal manure contains nutrients including nitrogen and phosphorus.
Properly managed manure can:
- return nutrients to soil
- add organic matter
- reduce dependence on synthetic fertilizers
However, excessive application can cause:
- nutrient runoff
- nitrate leaching
- water pollution
The amount and timing of application are therefore important.
Crop Rotation
Crop rotation means growing different crops in a planned sequence.
Rotations can improve nutrient management because different crops:
- require different nutrients
- have different root systems
- leave different residues
- interact differently with soil organisms
Rotations that include legumes can be particularly useful for nitrogen management.
Legumes and Nitrogen
Legumes such as beans, peas, and clover can form associations with nitrogen-fixing bacteria.
The bacteria live in root nodules and convert atmospheric nitrogen into biologically useful nitrogen compounds.
This can increase nitrogen entering the agricultural system.
A simplified relationship is:
atmospheric N₂ → bacteria → biologically available nitrogen → plant
Legumes are therefore often included in crop rotations.
Cover Crops
Cover crops are grown partly to protect and improve soil.
They can help nutrient cycling by:
- taking up nutrients that might otherwise leach away
- adding organic matter
- reducing erosion
- supporting soil organisms
When residues decompose, some nutrients can return to the soil.
Composting and Nutrient Recycling
Food scraps, plant material, and agricultural residues contain nutrients.
Instead of treating these materials only as waste, they can sometimes be composted.
A simplified cycle becomes:
plants → food/waste → compost → soil → plants
Composting therefore returns some nutrients to productive use.
It also adds organic matter to soil.
Precision Nutrient Management
Modern agriculture can use measurements to determine:
- which nutrients are needed
- where they are needed
- how much should be applied
- when they should be applied
Tools may include:
- soil testing
- plant tissue testing
- yield data
- soil sensors
- mapping technologies
The goal is to better match nutrient supply with crop demand.
A useful principle is:
right nutrient + right amount + right place + right time
Example: Nitrogen in a Farm System
Consider a field growing corn.
Nitrogen may enter through:
- fertilizer
- manure
- decomposition
- biological nitrogen fixation elsewhere in the rotation
Nitrogen may leave through:
- crop harvesting
- leaching
- runoff
- denitrification
- erosion
The farmer's challenge is to provide enough nitrogen for crop growth while minimizing unnecessary losses.
This can be thought of as a nutrient budget:
nutrient inputs − nutrient outputs = change in nutrient storage
Example Nutrient Budget
Suppose a simplified field receives:
120 kg nitrogen/ha
during a growing season.
The crop removes:
90 kg nitrogen/ha
through harvesting.
Other processes remove:
20 kg nitrogen/ha
through leaching and other losses.
Total outputs:
90 + 20 = 110 kg/ha
Change in nitrogen storage:
120 − 110 = +10 kg/ha
This simplified calculation suggests that 10 kg/ha remains within the system.
Real agricultural nutrient budgets can be considerably more complex.
Too Few Nutrients
If nutrients are removed faster than they are replaced:
nutrient availability decreases
↓
plant growth may decrease
↓
crop yield may decrease
This is sometimes called nutrient depletion.
Long-term nutrient depletion can reduce soil fertility.
Too Many Nutrients
Excess nutrients can also cause problems.
Possible consequences include:
- fertilizer waste
- nitrate leaching
- groundwater contamination
- nutrient runoff
- eutrophication
- changes in soil chemistry
Therefore, the goal is not to maximize nutrient concentrations.
The goal is to maintain an appropriate nutrient balance.
Nutrient Cycles Are Connected
The carbon, nitrogen, and phosphorus cycles interact.
For example, plant growth requires:
- carbon from CO₂
- nitrogen from soil
- phosphorus from soil
If phosphorus is unavailable, adding more nitrogen may not greatly increase growth.
This introduces the idea of a limiting nutrient.
Limiting Nutrients
A limiting nutrient is a nutrient whose low availability restricts biological growth.
Imagine a plant has plenty of:
- water
- nitrogen
- carbon dioxide
- light
but very little phosphorus.
Phosphorus may limit its growth.
Adding more nitrogen would not solve the main problem.
Agricultural management therefore requires identifying which factor is actually limiting productivity.
Nutrient Cycles and Ecosystem Health
Healthy nutrient cycling helps ecosystems maintain productivity.
Nutrients move through:
abiotic environment → producers → consumers → decomposers → abiotic environment
Disturbing one part of the cycle can affect many others.
For example:
deforestation
can reduce plant nutrient uptake and increase erosion.
excess fertilizer
can increase nutrient movement into waterways.
loss of decomposers
can reduce nutrient recycling.
Ecosystems depend on the entire cycle functioning together.
Natural Ecosystems vs Agricultural Systems
In many natural ecosystems, nutrients are efficiently recycled locally.
For example:
tree → fallen leaves → decomposition → soil nutrients → tree
Agricultural systems regularly remove biomass through harvesting:
soil → crop → harvest → food transported away
This makes agricultural systems more dependent on deliberate nutrient management.
Sustainable agriculture tries to return nutrients where practical while minimizing losses.
Evaluating Agricultural Nutrient Management
When evaluating a farm's nutrient management, useful questions include:
- Are crops receiving enough nutrients?
- Are nutrients being applied in excess?
- Is soil being tested?
- Is organic matter being maintained?
- Is erosion occurring?
- Are nutrients entering waterways?
- Are crops being rotated?
- Are cover crops being used?
- Is manure being applied appropriately?
- Are fertilizer applications timed with crop demand?
Good nutrient management balances:
CROP PRODUCTIVITY + SOIL FERTILITY + WATER QUALITY + LONG-TERM SUSTAINABILITY
Common Mistakes
Thinking Nutrients Are Used Up Permanently
Atoms are not destroyed when organisms use nutrients.
They can be transformed and recycled.
Confusing Energy Flow with Nutrient Cycling
Energy and matter behave differently.
Energy flows through ecosystems.
Matter cycles through ecosystems.
Thinking Plants Obtain Carbon from Soil
Most of the carbon incorporated into plant organic molecules comes from carbon dioxide, not directly from soil.
Thinking Plants Can Directly Use Atmospheric Nitrogen
Most plants cannot use N₂ directly.
Nitrogen must first be converted into biologically available forms.
Thinking Nitrogen Fixation Is the Same as Nitrification
They are different processes.
Nitrogen fixation: atmospheric N₂ is converted into biologically available nitrogen compounds.
Nitrification: ammonium is converted to nitrite and then nitrate.
Thinking Decomposers Only Remove Dead Material
Decomposers also play a major role in returning nutrients to ecosystems.
Thinking Phosphorus Has a Large Atmospheric Stage
Unlike carbon and nitrogen, phosphorus does not have a major atmospheric gas reservoir.
Assuming More Fertilizer Always Produces More Growth
Growth may be limited by another nutrient or environmental factor.
Excess fertilizer can also cause pollution.
Assuming Fertilizer Stays Where It Is Applied
Nutrients can leave fields through:
- runoff
- leaching
- erosion
Confusing Fertility with Nutrient Quantity Alone
Soil fertility also depends on:
- pH
- organic matter
- water
- aeration
- microbial activity
Key Terms
Nutrient — A chemical substance required by organisms for growth and life processes.
Nutrient cycle — Movement and recycling of nutrients through living organisms and the physical environment.
Biogeochemical cycle — Movement of chemical elements through biological, geological, and chemical parts of Earth systems.
Reservoir — A location where a nutrient is stored.
Carbon cycle — Movement of carbon through organisms, the atmosphere, oceans, soil, rocks, and sediments.
Nitrogen cycle — Movement and transformation of nitrogen through the atmosphere, organisms, soil, and water.
Phosphorus cycle — Movement of phosphorus through rocks, soil, water, sediments, and organisms.
Photosynthesis — Process that uses light energy to produce organic molecules from carbon dioxide and water.
Cellular respiration — Process through which cells release usable energy from organic molecules.
Decomposition — Breakdown of dead organic material by decomposers.
Nitrogen fixation — Conversion of atmospheric nitrogen into biologically available nitrogen compounds.
Nitrification — Microbial conversion of ammonium to nitrite and then nitrate.
Ammonification — Conversion of organic nitrogen into ammonium during decomposition.
Denitrification — Microbial conversion of nitrate into nitrogen gases.
Weathering — Breakdown of rocks that can release minerals such as phosphate.
Leaching — Movement of dissolved substances downward through soil with water.
Eutrophication — Nutrient enrichment of water that can cause excessive biological growth and subsequent oxygen depletion.
Limiting nutrient — Nutrient whose low availability restricts biological growth.
Nutrient budget — Comparison of nutrient inputs, outputs, and changes in storage within a system.
Crop rotation — Planned sequence of crops grown on the same land.
Cover crop — Crop grown partly to protect soil and improve soil conditions.
Nutrient depletion — Reduction in available soil nutrients when losses exceed replacement.
Key Takeaways
- Nutrients cycle repeatedly between organisms and the environment.
- Energy flows through ecosystems, while matter is recycled.
- Carbon, nitrogen, and phosphorus are important elements in living organisms.
- Photosynthesis transfers carbon from carbon dioxide into organic molecules.
- Respiration and decomposition return carbon to the environment.
- Burning fossil fuels transfers long-stored carbon into the atmosphere.
- Atmospheric nitrogen must be converted into biologically available forms before most plants can use it.
- Nitrogen fixation, nitrification, ammonification, and denitrification are important processes in the nitrogen cycle.
- Phosphorus is stored mainly in rocks, soils, sediments, water, and organisms rather than in the atmosphere.
- Weathering releases phosphate from rocks.
- Decomposers are essential for returning nutrients from dead organisms to ecosystems.
- Soil acts as an important nutrient reservoir.
- Nutrient availability is affected by pH, water, temperature, microorganisms, erosion, and leaching.
- Agriculture removes nutrients when crops are harvested.
- Fertilizers and manure can replace nutrients but must be managed carefully.
- Excess nitrogen and phosphorus can contribute to eutrophication.
- Crop rotation, legumes, cover crops, composting, and soil testing can improve nutrient management.
- A limiting nutrient can restrict plant growth even when other nutrients are abundant.
- Sustainable agriculture aims to maintain crop productivity without disrupting nutrient cycles or damaging surrounding ecosystems.
A useful overall model is:
ENVIRONMENT → PRODUCERS → CONSUMERS → DECOMPOSERS → ENVIRONMENT
And in sustainable agriculture:
NUTRIENT INPUTS → CROPS → HARVEST + RECYCLING → MANAGE LOSSES → MAINTAIN SOIL FERTILITY.