Week 3: Matter Cycling & Decomposition
Grade 8 Science | Rosche
Driving Question: Why don't dead leaves pile up forever in forests? Where does all the matter go?
The Phenomenon: The Disappearing Leaves Mystery
Anchoring Context & Focus Question
Before We Begin: Activate Your Prior Knowledge
Think back to Week 1: The 10% rule showed you that energy flows ONE direction through trophic levels and is lost as heat at each step. Now ask yourself: If energy is "lost" at each level, what about matter? Do the atoms in dead leaves disappear, or does something else happen? Keep this question in mind as you examine the evidence below.
Scientists studying forest ecosystems discovered something puzzling:
- A forest produces about 5,000 kg of leaf litter per hectare each year
- After 100 years, that should be 500,000 kg piled up
- But the leaf layer on forest floors is only a few centimeters thick
- Where do all the leaves GO? They don't just "disappear"!
St. Louis Connection
Forest Park's 1,300 acres produce thousands of tonnes of leaf litter annually (Figure 1, top-left above), yet the forest floor stays thin. Bacteria, fungi, and invertebrates break down dead leaves, returning nutrients to soil. Without these decomposers, the park would be buried under decades of leaves! This continuous matter cycling supports 3,000+ tree species and provides ecosystem services worth millions: carbon storage, stormwater filtration, and urban cooling.
Vocabulary
Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!
| Term | Spanish | Definition |
|---|---|---|
| decomposition | descomposición | Breaking down dead organisms into simpler substances (CO₂, H₂O, nutrients) |
| decomposer | descomponedor | Organisms (bacteria, fungi) that break down dead matter |
| nutrient cycling | ciclo de nutrientes | How nutrients move from soil to organisms and back continuously |
| matter | materia | Physical substance made of atoms (conserved in reactions) |
| organic matter | materia orgánica | Dead plant and animal material in soil |
| bacteria | bacteria | Microscopic single-celled decomposers |
| fungi | hongos | Decomposers like mushrooms and mold |
Worked Example
Step-by-Step Problem Solving
Problem Scenario
A fallen log has a mass of 10 kg. After months of decomposition, only 2 kg of solid material remains. A student concludes that 8 kg of matter was destroyed. Use conservation of matter and the products of decomposition to account for the missing mass, including matter that left the log as gases or water vapor.
Step-by-Step Solution
Step 1: Identify what you know & what changed
"BEFORE: 10 kg log (solid
wood)
AFTER: 2 kg solid remains + ??? (where did 8 kg go?)
KEY LAW: Conservation of mass - atoms cannot be created or
destroyed"
Step 2: Identify decomposition products
"Decomposers (bacteria, fungi)
break down organic matter into:
- CO2 gas (carbon dioxide) - released to atmosphere
- H2O (water vapor) - evaporates or absorbed
- Mineral nutrients (N, P, K) - released to soil"
Step 3: Apply conservation of mass
"10 kg log = 2 kg solid + 8 kg
gases/vapor
The 8 kg 'missing' mass became CO2 and H2O that left as gases!
Total mass is CONSERVED: 10 kg in = 10 kg out (just in different
forms)"
Now YOU Complete Steps 4-5:
Step 4: Explain how this is DIFFERENT from energy. Does energy cycle back? (Hint: think about the 10% rule from Week 1)
Step 5: Where does the CO2 from decomposition eventually go? Can it become part of a living organism again? Explain the complete cycle.
Fading Support: Weeks 1-2 showed more steps. This week, steps 4-5 are YOUR turn. By Week 4, you'll do most steps independently! This builds your problem-solving stamina.
Station 1 – Decomposition
Investigation
20 Points | ~18 Minutes
Model how decomposers break down matter and track where atoms go.
Your Mission: Model How Decomposers Break Down Matter
Decomposition Process - What Really Happens
When organisms die, they don't just "rot away" - their matter is transformed by decomposers!
| Component | Decomposers | Products |
|---|---|---|
| Carbon (C) | Bacteria, fungi | CO2 gas → atmosphere |
| Hydrogen/Oxygen | Bacteria, fungi | H2O → evaporates |
| Nitrogen (N) | Bacteria | Nitrates → soil |
| Phosphorus (P) | Bacteria, fungi | Phosphates → soil |
Key Insight: Matter vs. Energy
This is THE key distinction in ecosystem science:
- ENERGY flows ONE direction (sun → producers → consumers → HEAT lost)
- MATTER cycles continuously (dead organisms → decomposers → soil → plants → new organisms)
- The 10% rule applies to ENERGY, not matter!
- The SAME atoms can cycle for millions of years!
Remember: Energy flows OUT. Matter cycles WITHIN.
Station 2 – Nutrient Cycle Analysis
20 Points | ~15 Minutes
Analyze how nutrients cycle through ecosystems and distinguish from
energy flow.
Your Mission: Analyze How Nutrients Cycle Through Ecosystems
Ecosystem Matter Budget (Forest Example)
| Component | Amount (kg/ha) | What Happens | Key Insight |
|---|---|---|---|
| Living biomass | 200,000 | Trees, plants, animals | Matter stored in living things |
| Dead organic matter | 150,000 | Being decomposed | Matter being transformed |
| Annual leaf fall | 5,000 | New dead matter added | INPUT to decomposition |
| Annual decomposition | ~5,000 | Dead matter broken down | OUTPUT = INPUT (balance!) |
KEY DISTINCTION: Matter vs. Energy
This table shows THE most important concept in ecosystem science:
| Property | ENERGY (Week 1) | MATTER (Week 3) |
|---|---|---|
| Movement | Flows ONE direction | CYCLES continuously |
| What happens | Lost as heat at each level | Conserved & recycled |
| % transferred | ~10% (10% rule) | ~100% (conservation of mass) |
| Can be reused? | NO (lost forever) | YES (atoms conserved) |
Enrichment & Extension
Optional deep dives into ecosystem science, scientist profiles, and
environmental justice.
Optional content if you finish early or want to go deeper.
Scientist Spotlight: Dr. Eugene Odum
Dr. Eugene Odum (1913-2002) is known as the "father of ecosystem ecology." He revolutionized how scientists understand energy flow and matter cycling in ecosystems. His groundbreaking 1953 textbook, Fundamentals of Ecology, established ecology as a rigorous science by showing that ecosystems follow thermodynamic laws: energy flows through food webs while matter cycles via decomposition. His framework—"energy flows, matter cycles"—is the foundation of modern ecosystem science and the key concept you're learning this week!
Environmental Justice: Industrial Pollution and Nutrient Cycling
Industrial pollution in North St. Louis doesn't just poison the air—it disrupts nutrient cycling in soil. Heavy metals (lead, arsenic, cadmium) from decades of manufacturing contaminate soil in predominantly Black neighborhoods. These pollutants interfere with decomposition processes by killing bacteria and fungi that break down organic matter and return nutrients to soil. Organizations like Gateway Greening and Urban Harvest STL work to remediate contaminated soil through composting and microbial restoration. They understand that environmental justice requires restoring natural nutrient cycles that industrial pollution destroyed.
Week 3 Complete!
Next Week: Food Web Complexity & Stability – Why can some ecosystems survive a drought but others collapse?