Take a single dead leaf lying on the forest floor and ask who takes it apart. Living things called decomposers break down dead leaves, wood and other dead things, and fungi and bacteria are decomposers. They get to work when a plant or another living thing dies. Those that feed on dead things are called saprotrophs. Fungi are not plants, because they have no green chlorophyll and so cannot make their own food from sunlight. A fungus feeds another way: it grows thin threads called hyphae into wood and leaves, and together the threads are called mycelium. The fungus then makes special chemicals called enzymes, which break down the wood, and then it takes in the nutrients.

An enzyme is a molecule that speeds up a chemical reaction, and decomposers use enzymes to snip dead plants into bits they can absorb. Plant cell walls are built of three fibers: hemicellulose is softest, cellulose is sturdier and lignin is toughest. Fungi are not the only workers, because earthworms, snails, millipedes and beetles help break down dead plants too. The castings that earthworms leave behind are rich in nutrients that plants can use, and worm composting makes vermicompost, a soil booster. Termites show how much this matters. They eat wood but cannot digest cellulose alone. Bacteria, fungi and protozoa living in their guts break the cellulose down, so that the termites can absorb the nutrients.

A fallen tree is a good place to watch all of this work being done. When a tree falls, fungi that specialize in wood usually start with their hyphae, and then bacteria and beetle larvae get inside. The holes they make let in water, which helps fungi and bacteria thrive. In other words, the beetle larvae and bacteria open the wood up so that water can reach the fungi and the bacteria. Some of the mushrooms that grow on dead logs and stumps are decomposer fungi, which means they feed on the dead wood. To study how long rot takes, scientists at Harvard Forest bury blocks of wood in soil and see how long they take to rot and disappear.

Rot goes faster in some places than in others, and the differences between those places have causes that can be named. These workers do best when it is warm and wet, and in warm, wet places like tropical rainforests, dead things decay very fast. Cutting scraps into small pieces makes them rot faster, because microbes have more surface to work on, and turning a compost pile with a fork lets in air and speeds up decomposition. A compost pile also warms in stages. The middle gets warm because the living things in it are busy: microbes that like mild heat start first, and then heat-loving ones that thrive at 115 to 160 degrees F take over. A pile that gets to 130-135 degrees F kills most weed seeds and germs, but if a pile goes above about 162-172 degrees F, bacterial activity falls and breakdown slows.

People can run this process on purpose in a compost pile. It has two kinds of ingredients: carbon-rich "browns", like dry leaves, and nitrogen-rich "greens", like food scraps. Compost microbes need a balance of carbon and nitrogen. Carbon gives them energy, and nitrogen helps them grow. They work well at carbon-to-nitrogen ratios from 25:1 to 40:1, and a mix of 30 parts carbon to 1 part nitrogen is considered ideal, while finished compost has a ratio of 20 to 25:1. For a home pile, EPA advises at least two to three times as much dry leaves and other browns as food scraps by volume. Composting also needs oxygen, and decomposition with oxygen is called aerobic. A good pile feels like a wrung-out sponge, damp but not dripping, and healthy compost smells earthy, like fresh soil, not like rotten eggs.

Decomposers get their energy the way other living things do. Cells release energy from sugar with oxygen, so sugar plus oxygen makes carbon dioxide, water and energy. Plants, animals and the bacteria that eat dead plants all do this, which is why decay releases carbon dioxide. Carbon keeps moving between air, plants, soil and rocks, a movement known as the carbon cycle. Rot is one of its steps. Decomposers release carbon dioxide into the air as they break down dead plants, and plants then use that carbon dioxide to make food. When things rot, nutrients go back into the soil, and living plants use them to grow, and compost feeds plants by releasing nutrients slowly while it helps soil hold water.

Rot frees more than carbon. It also frees nitrogen, phosphorus and about two dozen other nutrients that living things need to grow. Nitrogen would stay locked inside dead plants and animals if nothing broke them down. In the nitrogen cycle, decomposing bacteria and fungi break down organic nitrogen from dead organisms and release ammonium, and other bacteria then turn ammonium into nitrite and nitrate, which plants can absorb. In other words, nitrogen that was locked in dead things goes back to plants. Composting, which lets this happen in a pile, is nature's way of recycling. Along the way, rot turns dead things back into food for new plants.

Words to know

• hyphae: fungus threads
• enzymes: reaction speeders
• saprotrophs: dead-matter feeders
• mycelium: mass of threads
• aerobic: with oxygen