What it shows
Dead leaves, wood, dead animals and droppings (together called detritus) are broken down by decomposers: bacteria, fungi and small animals called detritivores. Decomposition returns nutrients to the soil, so it keeps an ecosystem running. How fast it goes depends mostly on the climate and on what the dead matter is made of.
Ecologists measure it with litter bags: mesh bags of leaves left on the ground and weighed every few months. The mass left usually falls exponentially: M = M₀ e^(−kt), where k is the decomposition rate. This page works out k from three things:
- temperature: decomposers work about twice as fast for every 10 °C warmer (Q10 = 2), and stop when frozen;
- moisture: too dry and they cannot work; waterlogged soil is short of oxygen, so they slow down again;
- toughness: lignin (in wood) and chitin resist breakdown; leaves rich in nitrogen and sugars go fast.
It fits Class 12 Biology: ecosystem (decomposition and the factors affecting it, and nutrient cycling). Who feeds on whom, decomposers included, is in food web builder; the same exponential maths runs backwards in population growth, and the plants that will take up the released nutrients are in photosynthesis lab.
How to use it
- Pick a place (rainforest, temperate forest, grassland, desert, tundra, bog) or slide your own temperature and moisture.
- Pick what is in the bag: soft leaves, tough leaves or wood.
- Slide Years to watch the bag empty. The facts show the mass left, k, the half-life and the time until 95 % is gone; the graph compares your bag with the same bag in the other places (grey).
Worked example
Soft leaves in a temperate forest (12 °C, moisture 0.6): k = 1.2 × 2^((12 − 25)/10) × 1 × 1 ≈ 0.487 per year. After one year e^(−0.487) ≈ 61.4 % is left. The half-life is ln 2 / k ≈ 1.42 years, and 95 % is gone after ln 20 / k ≈ 6.1 years. The same leaves in a tropical rainforest (27 °C, moisture 0.7) have a half-life of about 7 months, nearly six times shorter than in the tundra. Wood in the temperate forest takes about 11.9 years to lose half.
Check yourself
Five fresh questions: a half-life from k, the effect of Q10 on the rate, grams left after some years, naming a step, and where decomposition is fastest. Nothing is saved.
FAQ
Why do bogs make peat? Waterlogged soil has very little oxygen, so decomposers work very slowly and dead plants pile up faster than they rot.
Why is rainforest soil poor, if decomposition is fast? Nutrients are released quickly and taken straight back up by plants, or washed away by heavy rain, so little stays in the soil.
Is this from my textbook? It covers the standard school topic with its own model and examples. It is an independent study aid.
Where the numbers come from
- Standard school ecology: the steps of decomposition (fragmentation, leaching, catabolism, humification, mineralisation) and the factors that control its rate (temperature, moisture and oxygen, lignin and chitin content). The exponential litter-bag model M = M₀e^(−kt) and a Q10 of 2 are standard simple models. Written for fibub; no textbook text, figures or exercises are used.
- The rate at 25 °C, the moisture curve, the toughness factors and the climates of the six places are round, made-up teaching values.
- Checked against a separate Python version (rates and half-lives) and scipy.integrate.solve_ivp for the mass left after 3 years.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.