Biogeochemistry refers to the quantity and cycling of chemicals in ecosystems. Biogeochemistry can be studied at various spatial scales, ranging from communities, landscapes (or seascapes), and over Earth as a whole. Biogeochemistry involves the study of chemicals in organisms, and also in non-living components of the environment.

An important aspect of biogeochemistry is the fact that elements can occur in various molecular forms that can be transformed among each other, often as a result of biological reactions. Such transformations are an especially important consideration for nutrients, i.e., those chemicals that are required for the healthy functioning of organisms. As a result of biogeochemical cycling, nutrients can be used repeatedly–nutrients contained in dead biomass can be recycled through inorganic forms, back into living organisms, and so on. Biogeochemistry is also relevant to the movements and transformations of potentially toxic chemicals in ecosystems, such as metals, pesticides, and...