In biology, the nutrient cycle is a concept that describes how nutrients move from the physical environment into living organisms, and subsequently are recycled back to the physical environment. This movement of nutrients, essential for life, from the environment into plants and animals and back again, is a vital function of the ecology of any region. In any particular environment, the nutrient cycle must be balanced and stable if the organisms that live in that environment are to flourish and be maintained in a constant population.

Nutrient cycling is typically studied in terms of specific nutrients, with each nutrient in an environment having its own particular pattern of cycling. Among the most important nutrient cycles are the carbon nutrient cycle and the nitrogen nutrient cycle. Both of these cycles make up an essential part of the overall soil nutrient cycle. There are many other nutrient cycles that are important in ecology, including a large number of trace mineral nutrient cycles.

The carbon nutrient cycle is perhaps the most basic and essential cycle for life on Earth, as life on our planet is carbon-based. Carbon is present in large quantities in the air in the form of carbon dioxide. When plants photosynthesize, they take carbon dioxide from the air, and use it to create molecules of sugar, starch, protein, and other foods. This is the first part of the carbon cycle. The carbon that is present in plants is now available to herbivores, or plant-eating animals.

Herbivores, such as deer, consume the plants and use the carbon to build, repair, and multiply cells of their own bodies. This moves the carbon from plant life to animal life. A carnivore such as a wolf may then kill and eat the deer, using the carbon from the meat of the deer to grow and reproduce. Eventually both plants and animals die, and the carbon in their bodies is recycled into the soil. Carbon based matter in the soil is broken down by fungi and bacteria, releasing individual carbon atoms into the soil and also back into the air as carbon dioxide.

The nitrogen nutrient cycle is somewhat similar to the carbon cycle. Instead of atmospheric nitrogen being trapped by photosynthesis, however, this occurs by a process called fixation. Nitrogen fixation is typically performed by certain types of bacteria. One such family of bacteria, called Rhizobium, lives in the roots of peas and beans. For this reason, crops of peas or beans are often grown by farmers when the soil needs to be enriched with nitrogen.