The Cycle of Matter in Ecosystems

Here's what's puzzling: the matter — all the wood, bark, and leaves that made up that tree — didn't just vanish into thin air. The tree weighed hundreds of pounds. Where did all that material go? And here's an even bigger mystery: the new plants growing in that same spot are building their stems, leaves, and roots out of something. Where are they getting the matter they need to grow?

• Where did all the matter that made up the fallen tree go as it decomposed?
• How do the new plants growing in the same spot get the matter they need to build their stems and leaves?
• If matter can't be created or destroyed, how does it move between living things and the environment around them?

To explain what happened to the disappearing log, we need to understand a powerful idea in science: matter cycles between living organisms and the nonliving environment. Matter isn't used up — it moves from place to place, changing form along the way. The atoms that make up a tree today might have been part of the air, the soil, or even another living thing in the past.

The Investigation: Imagine your class builds a sealed terrarium using a large glass jar with a tight lid. Inside, you place moist soil, a small plant, a few dead leaves, and some tiny pill bugs (decomposers). You seal the jar shut so nothing can get in or out. Then you observe it for four weeks.

Materials: Large glass jar with lid, potting soil, small fern or moss, dried leaves, 4–5 pill bugs, spray bottle with water, ruler, observation journal.

Key question: If no new matter can enter or leave the sealed jar, how do the organisms inside continue to survive and grow?

What students would observe:

• The plant continues to grow, producing new leaves
• Water droplets form on the inside of the glass (water cycle!)
• The dead leaves slowly shrink and crumble as pill bugs eat them
• The soil stays moist and dark
• After several weeks, the dead leaves are mostly gone, but the plant has grown larger

How Matter Cycles in a Sealed Terrarium

This investigation models the Science and Engineering Practice of developing and using models. The sealed terrarium is a model of how matter cycles in a real ecosystem. By observing what happens when no matter can enter or leave, students can gather evidence that matter is being recycled — not created or destroyed — within the system.

The terrarium investigation reveals something remarkable. Even though the jar was sealed — no new matter could get in — the plant kept growing, the pill bugs stayed alive, and the dead leaves slowly disappeared. How is this possible? Because matter was cycling inside the system.

Let's trace the path of a single carbon atom to see how this works. Imagine a carbon atom that starts in a molecule of carbon dioxide (CO₂) floating in the air inside the jar. During photosynthesis, the plant absorbs that CO₂ through its leaves and uses energy from sunlight to combine the carbon with hydrogen and oxygen from water. The result is a sugar molecule — and our carbon atom is now part of the plant's leaf.

Eventually, that leaf dies and falls to the soil. A pill bug eats the dead leaf. Inside the pill bug's body, the sugar is broken down through a process called cellular respiration — the pill bug combines the sugar with oxygen to release energy for moving, growing, and living. This process produces CO₂ and water as waste products. Our carbon atom is now back in the air as part of a CO₂ molecule — right where it started! Bacteria in the soil do the same thing with bits of leaf matter the pill bugs don't eat.

This is the key insight: the same matter keeps moving in a loop. Carbon dioxide from the air becomes part of a plant, the plant matter gets eaten or decomposes, and the carbon returns to the air as CO₂ or to the soil as nutrients. Water follows a similar cycle — absorbed by roots, released by leaves, condensing on the jar, and dripping back to the soil.

Terrarium Observation Data (Sample Results)

The data shows a clear pattern: as the dead leaves decreased, the plant increased in size. The total amount of matter inside the sealed jar stayed the same, but it moved from one form (dead leaves) to another (plant growth, CO₂ in air, nutrients in soil). This is evidence that matter is conserved — it isn't created or destroyed, it just changes form and location.

The Cycling of Matter in a Forest Ecosystem

The cycling of matter in ecosystems is an example of one of the most important patterns in all of science: matter is conserved within a system — it is transferred in and out of organisms, but it never disappears. Scientists call this a Crosscutting Concept because the same pattern shows up in many different areas of science, not just ecosystems.

Let's look at how the idea of Energy and Matter: Flows, Cycles, and Conservation appears across different science topics:

Do you see the pattern? In every case, matter doesn't appear out of nowhere and doesn't vanish. It changes form — solid to gas, living to nonliving, one compound to another — but the total amount stays the same. Scientists can actually measure this: if you weigh all the matter before and after a change, the weight is the same. This is the law of conservation of matter, and it's one of the most fundamental principles in science.

This pattern also helps us understand our anchoring phenomenon. The fallen log didn't "disappear" — its matter was transformed. Some became CO₂ gas in the air. Some became nutrients dissolved in the soil. And some of that matter was taken up by the new plants growing nearby. The log's matter is still in the ecosystem, just in different forms and different places.

Understanding how matter cycles isn't just an abstract science idea — it directly affects our lives and the decisions we make about the environment. Here are some real-world examples of how people use this knowledge:

• Matter — Anything that has weight and takes up space. All living and nonliving things are made of matter, including air, water, soil, plants, and animals.
• Photosynthesis — The process plants use to take in carbon dioxide from the air and water from the soil and, using energy from sunlight, turn them into sugars and oxygen. This is how plants build new matter for growth.
• Decomposer — An organism (like bacteria, fungi, or certain insects) that breaks down dead plants and animals into simpler substances, returning nutrients to the soil and carbon dioxide to the air.
• Conservation of Matter — The scientific principle that matter cannot be created or destroyed. In any process, the total amount of matter stays the same — it just changes form or moves to a different place.
• Cycling of Matter — The continuous movement of matter (like carbon and water) through living organisms and the nonliving environment. The same atoms get used again and again.
• Carbon Dioxide (CO₂) — A gas in the air made of one carbon atom bonded to two oxygen atoms. Plants take it in for photosynthesis; animals and decomposers release it during respiration.
• Cellular Respiration — The process by which living organisms break down sugars using oxygen to release energy. It produces carbon dioxide and water as waste products. This is how matter gets returned to the environment from living things.
• Nutrients — Substances in soil (like nitrogen, phosphorus, and potassium) that plants absorb through their roots and use to grow. Decomposers help release these nutrients from dead organisms back into the soil.