AP Environmental Science

## Learning Outcomes

- Explain how **biogeochemical cycles** show the conservation of matter in natures
- Distinguish clearly between **matter cycling** and **energy flow**
- Identify major **reservoirs**, **sources**, and transfer processes in the carbon, nitrogen, phosphorus, and water cycles
- Describe how matter moves through trophic levels and returns to abiotic parts of Earth
- Explain the role of **decomposers** in recycling nutrients
- Apply cycle knowledge to human impacts such as fertilizer runoff, combustion, and eutrophication
- Use AP-style reasoning to compare cycles and answer FRQ prompts accurately

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how biogeochemical cycles conserve matter, with a focus on the following syllabus points:

- Know that **matter is conserved** and moves through the biosphere, atmosphere, hydrosphere, and geosphere in biogeochemical cycles
- Identify **reservoirs** such as the atmosphere, oceans, soils, rocks, groundwater, sediments, and living organisms
- Explain how matter moves through the environments by **photosynthesis, respiration, feeding, waste production, decomposition, weathering, runoff, infiltration, and sedimentation**
- Understand the main features of the **carbon, nitrogen, phosphorus, and hydrologic cycles**
- Explain that **decomposers** return matter from dead organisms and waste to soil, water, and air
- Distinguish between the **nitrogen cycle**, which has a major atmospheric reservoir, and the **phosphorus cycle**, which does not
- Understand how human activity alters cycles through **fossil fuel combustion, fertilizer use, sewage discharge, and nutrient runoff**
- Explain how excess nitrogen or phosphorus can cause **eutrophication** in aquatic systems

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What does the law of conservation of matter mean in an natural system?
2. Why is the phosphorus cycle slower than the carbon or nitrogen cycles?
3. How do decomposers help recycle matter through trophic levels?
4. What is the difference between a reservoir and a source in a biogeochemical cycle?
5. How can fertilizer runoff lead to eutrophication?

## Introduction

Biogeochemical cycles are one of the most tested ideas in AP Environmental Science because they connect ecology, chemistry, Earth systems, and human impact. In this topic, the key idea is simple but easy to blur in an exam answer: **matter cycles, energy flows**. Carbon, nitrogen, phosphorus, and water all move through organisms and the physical environment, but they do not disappear when an organism dies or when a food chain ends.

This matters in trophic-level questions because students often focus only on who eats whom. That explains transfer through a food chain, but it does not fully explain what happens to atoms and nutrients afterward. To score well, you must show how matter moves into organisms, through food webs, and back into air, soil, water, or rock.

> **Key Term: biogeochemical cycle**
> A pathway by which matter moves through biological, geological, and chemical parts of Earth systems.

## Introduction to matter conservation in living systemss

The **law of conservation of matter** states that matter is not created or destroyed in ordinary environmental processes. Instead, atoms are rearranged, stored, transferred, dissolved, buried, released, or taken up again.

That means a carbon atom in atmospheric carbon dioxide can become part of a leaf by photosynthesis, then enter a rabbit that eats the leaf, then enter a fox that eats the rabbit, and later return to the atmosphere through respiration or decomposition. The atom changes location and chemical form, but it does not vanish.

> **Key Term: law of conservation of matter**
> Matter cannot be created or destroyed; it can only be transformed and transferred.

A strong APES answer usually tracks matter through both **biotic** and **abiotic** components. Biotic components include plants, animals, fungi, and bacteria. Abiotic components include air, water, rocks, soils, and sediments. If your explanation stops at “plants absorb nutrients” or “animals eat plants,” it is incomplete. You must also explain how the matter returns.

> **Key Term: reservoir**
> A storage location for a substance within a biogeochemical cycle.

A useful distinction is this:

- A **reservoir** is where matter is stored
- A **source** releases matter
- A **sink** stores matter for a relatively long time
- A **flux** is the movement between reservoirs

For example, the atmosphere is a carbon reservoir. A forest can act as a carbon sink when it stores more carbon in biomass than it releases. Burning fossil fuels is a carbon source because it adds carbon dioxide to the atmosphere.

> **Test Tip:** In FRQs, do not just name a cycle. Name a **reservoir** and a **process**. For example, write “phosphate is stored in rock and released by weathering,” not just “this is part of the phosphorus cycle.”

### Matter cycles but energy does not

This is one of the highest-yield contrasts in Unit 1. Matter is recycled through food webss. Energy is not recycled in the same way. Energy enters mostly as sunlight, moves through producers and consumers, and is eventually lost as heat.

Students often write that “energy is reused by decomposers.” That is inaccurate. Decomposers recycle **matter**, not energy. They break down dead tissue and wastes, returning nutrients and elements to abiotic reservoirs.

A quick comparison helps:

- **Matter:** reused, transformed, cycled
- **Energy:** transferred, degraded, lost as heat

This explains why the same phosphorus atoms can be used again and again, but living systemss still need a continuous energy input from the Sun.

## Major cycles and how matter moves through trophic levels

### The carbon cycle

Carbon moves among the atmosphere, oceans, biomass, soils, sediments, and rocks. In APES, the most important process to start with is **photosynthesis**. Producers remove carbon dioxide from the atmosphere or water and turn it into organic molecules such as glucose. That places carbon into biomass.

Animals then consume plants or other animals, so carbon moves through trophic levels by feeding. Carbon also returns to the environment through **cellular respiration**, which releases carbon dioxide. When organisms die, decomposers break down tissues and release carbon back to soil and atmosphere.

> **Key Term: photosynthesis**
> The process by which producers use light energy to convert carbon dioxide and water into sugars.

A simplified path looks like this:

1. Carbon dioxide in the atmosphere
2. Producer absorbs it by photosynthesis
3. Herbivore eats producer
4. Carnivore eats herbivore
5. Respiration and decomposition return carbon to the atmosphere or soil

Combustion is another major return pathway. When wood or fossil fuels burn, stored carbon is released rapidly as carbon dioxide. This is especially important because fossil fuels are a **long-term carbon reservoir**. Human combustion transfers that carbon into the active atmosphere much faster than natural geologic processes would.

Oceans are also major carbon reservoirs. They absorb atmospheric carbon dioxide, but this does not mean all absorbed carbon is harmlessly removed forever. Some is used by marine organisms, some remains dissolved, and some becomes part of sediments. Warmer water generally holds less dissolved gas, so ocean warming can reduce this storage role.

A common student error is to treat the atmosphere as the only carbon reservoir. It is not. Oceans, soils, biomass, and sedimentary rocks all matter.

> **Test Tip:** If asked to explain how the carbon cycle conserves matter, trace one carbon atom from **CO₂ → plant biomass → consumer tissue → decomposer or atmosphere**. That direct path is clear and exam-friendly.

### The nitrogen cycle

Nitrogen makes up about **78% of the atmosphere**, but most organisms cannot use atmospheric nitrogen gas directly. This is the key difficulty in the nitrogen cycle: the biggest reservoir exists, but it is not in a biologically useful form for most plants.

The cycle begins with **nitrogen fixation**, which converts atmospheric nitrogen into ammonia or related usable forms. After that, **nitrification** converts ammonium into nitrite and then nitrate. Plants then take up these forms through **assimilation**. Animals get nitrogen by eating plants or other animals.

When organisms excrete waste or die, decomposers convert organic nitrogen back into ammonium through **ammonification**. Finally, **denitrification** returns nitrogen to the atmosphere as nitrogen gas.

> **Key Term: nitrogen fixation**
> The conversion of atmospheric nitrogen gas into biologically available nitrogen compounds.

The sequence students should know is:

- fixation
- nitrification
- assimilation
- ammonification
- denitrification

Why does this show matter conservation? Because the nitrogen is continuously transferred among air, soil, water, and organisms. It changes form, but the nitrogen atoms remain in the system unless moved elsewhere physically.

This cycle strongly affects trophic levels. Nitrogen is needed for amino acids, proteins, and nucleic acids, so it is essential to plant growth and therefore to the entire food web. If available nitrogen is low, primary productivity is limited. That then limits herbivores and higher consumers.

Human impacts on this cycle are major. Fertilizer use, fossil fuel combustion, sewage, and livestock waste all increase reactive nitrogen in the environment. This can lead to water acidification, nutrient pollution, and eutrophication.

A frequent confusion is that plants can use nitrogen gas directly from the atmosphere. In most cases, they cannot. They usually absorb ammonium or nitrate from soil or water.

### The phosphorus cycle

The phosphorus cycle is often simpler to describe but easy to confuse with the nitrogen cycle. Its most important distinction is that **phosphorus has no major atmospheric reservoir** in the APES model. That makes it slower and more dependent on local geology.

Most phosphorus is stored in rocks, soil, sediments, water, and organisms. Weathering releases phosphate from rock into soil and water. Plants take it up through assimilation, and consumers obtain it by feeding. Decomposition returns phosphorus to soil or water. Some phosphorus is transported to aquatic systems and eventually becomes part of sediments, where it can be stored for long periods.

> **Key Term: limiting factor**
> A factor that restricts growth, productivity, or population size.

Because phosphorus is often scarce and not highly soluble, it can be a **limiting factor** for plant growth. It is especially important in ATP, DNA, RNA, cell membranes, bones, shells, and teeth.

The slow pace of the phosphorus cycle matters. Carbon and nitrogen often move through atmospheric pathways. Phosphorus mostly does not. That means weathering, runoff, sedimentation, and uplift are central.

A good comparison:

- **Nitrogen:** major atmospheric reservoir, fast biological cycling
- **Phosphorus:** rock and sediment reservoirs, no major gas phase, slower cycling

Human activity disrupts the phosphorus cycle mainly through mining phosphate rock and applying phosphorus-rich fertilizers. When phosphate runs off into lakes and streams, it can trigger eutrophication.

### The hydrologic cycle

Water is also matter, so it follows the same conservation principle. Water moves among oceans, atmosphere, ice, groundwater, rivers, lakes, soil, and organisms. It changes state and location, but total matter is conserved.

Major processes include:

- evaporation
- condensation
- precipitation
- runoff
- infiltration
- percolation
- groundwater flow
- transpiration

A useful APES distinction is this:

- **infiltration** = water entering soil
- **percolation** = water moving downward through soil and rock
- **runoff** = water flowing across land surface

The hydrologic cycle links directly to trophic levels because organisms contain a lot of water and rely on water for metabolism, photosynthesis, and transport of dissolved nutrients. Plants use water in photosynthesis and release it through transpiration. Animals return water vapor through respiration.

Water is not “used up” globally, but local availability absolutely can change. Deforestation, urbanization, and groundwater withdrawal alter the movement of water, increasing runoff and reducing infiltration.

## Decomposition, human disruption, and FRQ application

### Why decomposers are central to matter conservation

Decomposers are the missing link in many student answers. Without them, matter would stay locked in dead tissues and wastes much longer, reducing nutrient availability for producers.

When bacteria and fungi decompose dead organisms:

- carbon returns as carbon dioxide or methane
- nitrogen returns as ammonium
- phosphorus returns as phosphate to soil or water

That is why decomposers are essential to nutrient cycling. They connect trophic levels back to abiotic reservoirs.

A strong cause-and-effect chain is:

1. Organisms die or produce waste
2. Decomposers break down organic matter
3. Nutrients return to soil, water, or air
4. Producers absorb those nutrients again
5. Matter re-enters the food web

This is the clearest proof that matter is conserved and recycled.

### Human disruption and eutrophication

Human activity often speeds up movement from one reservoir to another. That is a common APES theme.

Examples:

- Burning fossil fuels transfers long-stored carbon to the atmosphere
- Fertilizer use increases reactive nitrogen and phosphorus in soils
- Runoff carries those nutrients to aquatic systems
- Nutrient enrichment leads to algal growth
- Decomposition of excess algae uses dissolved oxygen
- Low oxygen harms fish and other aquatic life
  > **Key Term: eutrophication**
  > Nutrient enrichment of a water body that leads to excessive algal or plant growth and often oxygen depletion.

Nitrogen and phosphorus are the main nutrients involved in eutrophication. Carbon is not usually the main focus in this process. Students often miss that.

A simple freshwater example:

- Farmer applies fertilizer
- Rain causes runoff
- Nitrate and phosphate enter a pond
- Algae bloom
- Algae die
- Decomposers break them down
- Dissolved oxygen drops
- Fish kills occur

This is a good example of matter conservation because the nutrients are still present, but their redistribution creates environmental harm.

### How to answer APES questions on this topic

When a question says **describe**, give the process accurately.

When it says **explain**, show why or how, using a cause-and-effect chain.

When it says **compare**, state a direct contrast, such as atmospheric vs non-atmospheric reservoir.

For an FRQ on conservation of matter, a good structure is:

1. State that matter is not created or destroyed
2. Name the cycle
3. Identify one or more reservoirs
4. Explain transfers between biotic and abiotic parts
5. Include decomposers or return pathways
6. If relevant, add a human impact

Worked mini-example:

> A phosphorus atom can be released from rock by weathering, absorbed by a plant as phosphate, transferred to an herbivore by feeding, returned to soil by decomposition, and later carried by runoff to sediments. This shows conservation of matter because the phosphorus changes location and form, but is not destroyed.
>
> **Test Tip:** On FRQs, avoid vague lines like “nutrients move around the environment.” Examiners want named processes such as **weathering, assimilation, ammonification, runoff,** or **denitrification**.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Biogeochemical cycles show that **matter is conserved** in natural systemss
- Matter cycles through the **biosphere, atmosphere, hydrosphere, and geosphere**
- A **reservoir** is a storage location for a substance within a cycle
- Matter is recycled, but **energy flows one way** and is eventually lost as heat
- In the carbon cycle, **photosynthesis** stores carbon in biomass and **respiration, decomposition, and combustion** return it
- In the nitrogen cycle, most organisms cannot use atmospheric **N₂** directly and rely on fixation and other transformations
- The phosphorus cycle is slower because phosphorus has **no major atmospheric reservoir**
- The water cycle includes **evaporation, precipitation, runoff, infiltration, and transpiration**
- **Decomposers** are essential because they return nutrients from dead matter and waste to abiotic reservoirs
- Human activities such as **fossil fuel combustion** and **fertilizer runoff** change the rate and direction of matter transfer
- Excess nitrogen and phosphorus in water can cause **eutrophication**
- APES answers should name both **reservoirs** and **processes**, not just list cycle names

## Key Terms and Concepts

- biogeochemical cycle
- law of conservation of matter
- reservoir
- photosynthesis
- nitrogen fixation
- limiting factor
- eutrophication


Trophic Levels - Biogeochemical cycles conserve matter