AP Environmental Science

## Learning Outcomes

- Understand how **composting** works as a controlled aerobic process
- Explain why composting usually reduces pollution compared with sending organic waste to landfills
- Identify key environmental benefits of compost use, including soil improvement and erosion control
- Describe the main drawbacks of composting, including odors, pests, pathogens, and contamination
- Distinguish between well-managed composting and poorly managed composting
- Apply this knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand composting as a waste reduction method, with a focus on the following syllabus points:

- Know that composting is the controlled **aerobic** decomposition of organic matter
- Explain how composting diverts organic waste from landfills and reduces landfill impacts
- Understand why landfill decomposition is usually **anaerobic** and associated with methane production
- Identify environmental benefits of compost, including improved soil structure, water retention, and erosion reduction
- Recognise that poorly managed compost can create odors, attract pests, and produce methane or nitrous oxide
- Understand contamination risks, including pathogens, heavy metals, and unsuitable feedstocks
- Compare composting with landfilling and, where relevant, with anaerobic digestion
- Apply composting to pollution, waste reduction, and climate-related FRQ responses

## 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 is the difference between **aerobic** decomposition and **anaerobic** decomposition?
2. Why does composting usually produce less methane than landfilling food waste?
3. Name two environmental benefits of applying compost to soil.
4. What problems can develop if a compost pile becomes too wet or oxygen-poor?
5. Why can contaminated compost become an environmental or human health problem?

## Introduction

Composting is one of the main waste reduction methods in AP Environmental Science because it does more than just “get rid of” waste. It changes organic waste into a useful product. Instead of sending food scraps, leaves, and yard trimmings to a landfill, composting returns nutrients and organic matter to soil.

This topic matters because it links several APES ideas at once: solid waste disposal, greenhouse gas emissions, soil quality, nutrient cycling, runoff, erosion, and human management of pollution. On the exam, composting is often tested by asking you to explain a benefit, compare it with landfills, or evaluate a drawback caused by poor management.

> **Key Term: composting**
> The controlled aerobic decomposition of organic matter such as food scraps, yard waste, manure, and paper into a stable soil amendment called compost.

### How composting works and why oxygen matters

Composting is a biological process driven mainly by microorganisms such as bacteria and fungi. They break down dead organic matter into simpler substances. As they feed, the pile heats up, materials shrink in volume, and the final product becomes dark, crumbly, and soil-like.

The key condition is **oxygen**. In a well-managed compost pile, oxygen-requiring decomposers dominate. They break down material mainly into carbon dioxide, water, and stable organic matter. This is very different from what happens in a landfill, where buried organic waste is packed tightly and oxygen is limited.

> **Key Term: aerobic decomposition**
> The breakdown of organic matter in the presence of oxygen.
>
> **Key Term: anaerobic decomposition**
> The breakdown of organic matter in the absence of oxygen, often producing methane.

This distinction is central to APES. Composting is not simply “rotting.” It is **controlled** rotting under aerobic conditions. Landfills, by contrast, usually create anaerobic conditions. That difference changes the gases produced.

Why does this matter so much? Because methane is a strong greenhouse gas. When food waste is buried in landfills, microbes decompose it without oxygen and release methane. By moving that same waste into an oxygen-rich compost system, methane formation is reduced.

A useful exam fact is that municipal solid waste landfills were the **third-largest source of human-related methane emissions in the United States in 2022**, and wasted food contributed about **58% of landfill methane emissions**. You do not need to memorise every statistic, but using one such figure in an FRQ can strengthen an answer.

Composting depends on balancing materials and conditions. Nitrogen-rich “greens” such as food scraps and grass clippings help microbes grow. Carbon-rich “browns” such as dry leaves, cardboard, and straw provide structure and energy. If the pile has too many greens, it may become wet, smelly, and low in oxygen. If it has too many browns, decomposition slows.

Moisture also matters. A pile should be damp, not waterlogged. Turning the pile helps bring oxygen into deeper layers. If oxygen levels drop, parts of the compost can switch from aerobic to anaerobic conditions, and then the pollution benefits are reduced.

> **Test Tip:** If an APES question asks why composting lowers greenhouse gas emissions, do not stop at “it reduces waste.” The stronger answer is: composting keeps organic waste out of anaerobic landfills, so less methane forms.

### Environmental benefits of composting

The most obvious benefit is waste diversion. Composting reduces the amount of organic matter sent to landfills and incinerators. This extends landfill life and lowers pressure to find new disposal space.

But APES usually wants more than that. Composting is important because the end product improves soil.

Finished compost adds **organic matter** and humus-like material to soil. This improves soil structure, which means soil particles form better aggregates. Better structure allows both water infiltration and water retention. These are not opposites. A healthy soil can let water soak in while also holding more moisture for later plant use.

> **Key Term: soil amendment**
> A material added to soil to improve its physical, chemical, or biological properties.

This is why compost helps in both drought and heavy rain. In dry conditions, compost-rich soils hold more water, reducing plant stress. In wet conditions, better infiltration means less surface runoff. Less runoff means less soil erosion and less transport of sediments and nutrients into streams.

That chain is a common APES reasoning pattern:

- compost increases soil organic matter
- soil holds water better and allows more infiltration
- runoff decreases
- erosion decreases
- less sediment and nutrient pollution reaches waterways

Compost also supports nutrient cycling. Instead of treating food scraps and leaves as useless waste, composting returns their nutrients to the soil. This can reduce dependence on some inorganic fertilizers. That matters because overuse of inorganic fertilizers can increase leaching and contribute to eutrophication in nearby water bodies.

Compost use may also support beneficial soil organisms. Soil microbes, fungi, and invertebrates help decompose matter, recycle nutrients, and maintain soil quality. So composting benefits are not only chemical; they are biological as well.

The main benefits you should be ready to explain are:

- reduced landfill waste
- reduced methane risk compared with landfilling
- creation of nutrient-rich soil additives
- improved soil water-holding capacity
- improved soil structure
- reduced runoff
- reduced erosion
- support for beneficial soil organisms
- no major toxic issues when compost is properly managed

Notice the last phrase: **when properly managed**. That qualification matters because many drawbacks of composting are management problems rather than unavoidable features.

> **Key Term: nutrient cycling**
> The movement and reuse of nutrients through organisms, soil, water, and the environment.
>
> **Test Tip:** In FRQs, build a full chain. For example: “Adding compost increases water retention, which reduces runoff, which decreases erosion and lowers sediment pollution in streams.” This scores better than listing isolated benefits.

### Drawbacks and limitations of composting

Composting is beneficial, but it is not impact-free. APES questions often reward students who can explain both sides.

One common drawback is odor. A properly aerated compost pile should have an earthy smell, but poorly managed compost can smell strongly unpleasant. This usually happens when the pile becomes compacted, too wet, or overloaded with nitrogen-rich waste. Under those conditions, oxygen falls and anaerobic decomposition begins.

When oxygen is low, undesirable gases may form, including methane and hydrogen sulfide. So composting does not automatically eliminate greenhouse gas emissions. It only lowers them **if the process stays aerobic**.

A second drawback is pests, sometimes called **vectors**. Food-rich compost piles can attract rats, flies, and other animals, especially if meat, dairy, grease, or exposed food waste are included in systems not designed to handle them.

> **Key Term: vector**
> An organism, such as a rat or fly, that can spread disease or contamination.

This is why many school-level or backyard compost systems limit what materials can be added. Municipal or industrial compost systems may accept wider feedstocks, but only if they have the equipment and management needed to control odors, temperature, and oxygen.

A third drawback is contamination. If the incoming waste stream contains plastics, glass, metals, pesticides, or persistent chemicals, the final compost may be lower quality or unsafe for use. In APES, this matters because compost that looks “natural” can still become a pollution source if the feedstock was contaminated.

Heavy metals are a special concern because they do not break down biologically. If they enter compost, they can remain in the finished product and later accumulate in soils.

Pathogens are another important issue, especially when animal manure is included. If compost does not heat sufficiently, harmful microorganisms may survive. USDA guidance identifies pathogens of concern such as **E. coli**, **Salmonella**, and **Listeria** in manure-based materials.

> **Key Term: pathogen**
> A disease-causing microorganism such as a bacterium, virus, or fungus.

Proper composting reduces pathogen risk because hot compost kills many harmful microbes. For compost containing animal materials, a commonly cited standard is **131°F** under controlled conditions. In contained or in-vessel systems, this can mean at least 3 days. In windrow systems, it typically means a longer period plus repeated turning.

This leads to another drawback: management and infrastructure demands. Composting needs collection systems, sorting, labor, monitoring, and space. If the incoming organics are mixed with trash, contamination increases. If the site is badly chosen, neighbors may complain about smell or traffic. If materials must be transported long distances, some environmental gains are reduced.

So composting is a waste reduction method, but not a zero-cost one. It needs active management.

### Why composting is a waste reduction method, not just disposal

Students sometimes describe composting as if it were just another disposal method like landfilling. That misses the main point.

Landfilling stores waste. Composting converts waste into a useful product. That is why composting fits under **waste reduction**.

It reduces waste in three linked ways:

1. It diverts organic material from the waste stream before landfill disposal.
2. It reduces the volume of material needing final disposal.
3. It turns waste into a resource that improves soil and can reduce demand for some other inputs.

This is very similar to the broader environmental principle of circular use of materials. Organic waste is not simply discarded. It re-enters the system.

A useful comparison is with **anaerobic digestion**. Both composting and anaerobic digestion divert organic waste from landfills. But composting is aerobic and mainly aims to produce compost, while anaerobic digestion is oxygen-free and mainly aims to capture methane-rich biogas for energy. Both can reduce landfill methane, but they are different processes and should not be confused.

Composting is often the better answer when the exam asks about making a soil amendment, improving water retention, or reducing erosion. Anaerobic digestion is often the better answer when the focus is renewable energy production from waste.

The best evaluative judgement is this: composting is usually environmentally beneficial, especially compared with landfilling organic waste, **but only when feedstocks are sorted and conditions are well controlled**.

That final phrase is often where stronger APES answers separate themselves. Composting is not automatically harmless. Its success depends on oxygen, moisture, temperature, and contamination control.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Composting is the controlled aerobic decomposition of organic matter
- Aerobic composting differs from landfill decomposition, which is usually anaerobic
- Anaerobic landfill conditions promote methane production
- Composting reduces landfill waste and usually lowers methane emissions compared with landfilling
- Municipal solid waste landfills were the third-largest source of human-related methane emissions in the United States in 2022
- Wasted food contributed about 58% of landfill methane emissions
- Compost improves soil structure, water retention, and nutrient content
- Compost application can reduce runoff and slow soil erosion
- Poorly managed compost can create odors, attract pests, and produce methane or nitrous oxide
- Compost can become contaminated by plastics, heavy metals, pathogens, or other unsuitable materials
- Composting is a waste reduction method because it turns waste into a useful soil amendment
- Successful composting depends on sorting, oxygen, moisture, and process control

## Key Terms and Concepts

- composting
- aerobic decomposition
- anaerobic decomposition
- soil amendment
- nutrient cycling
- vector
- pathogen


Waste Reduction Methods - Composting benefits and drawbacks