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The iron cycle describes the movement of iron through Earth's systems. Iron is an essential element for many biological processes, including oxygen transport in red blood cells and enzymatic functions.

In terrestrial ecosystems, iron is primarily found in soils and rocks. Weathering of these materials releases iron into the environment, where it can be taken up by plants. Animals obtain iron by consuming plants or other animals. Decomposers, such as bacteria and fungi, break down organic matter, returning iron to the soil. Microbial activity plays a significant role in iron transformations, influencing its solubility and availability. For instance, some bacteria can oxidize or reduce iron, altering its chemical form.

In aquatic environments, iron is present in dissolved and particulate forms. Rivers and streams transport iron from land to oceans. In oceans, iron is a limiting nutrient for phytoplankton in some regions, impacting primary productivity. Iron cycles through marine sediments and the water column. Certain marine bacteria are involved in the redox reactions of iron.

Atmospheric dust can transport iron over long distances, contributing to iron deposition in remote areas, including oceans and polar regions.

The iron cycle is influenced by geological processes, biological activity, and human activities such as mining and industrial processes, which can alter the natural distribution and availability of iron. Understanding the iron cycle is crucial for comprehending nutrient cycling, ecosystem productivity, and geochemical processes on Earth.