Biogeochemical cycles describe the movement of elements between the living (biotic) and nonliving (abiotic) parts of the world. These cycles are the fundamental recycling mechanisms that sustain ecosystems. Biotic components include all organisms, forming the biosphere. Abiotic components are the atmosphere, the hydrosphere (water), and the lithosphere (Earth’s crust and rocks). Constant cycling prevents essential nutrients from becoming permanently trapped in unusable forms.
Carbon: The Foundation of Biological Cycling
Carbon is the backbone of all organic molecules and cycles rapidly between the atmosphere and the biosphere. Through photosynthesis, plants draw carbon dioxide from the atmosphere to create sugars, transferring carbon into the biotic realm. This carbon moves through the food web as organisms consume plants and each other. Carbon returns to the atmosphere via cellular respiration, where organisms release carbon dioxide as a byproduct.
A much slower, geological cycle involves the long-term storage of carbon in deep-sea sediments, limestone rock, and fossil fuels. Over millions of years, organic matter that escapes decomposition can be buried and transformed into oil, coal, and natural gas. Geological processes, such as the weathering of rocks or volcanic activity, are the only natural mechanisms that return this stored carbon back to the atmosphere.
Nitrogen: The Essential Atmospheric Nutrient
Nitrogen is a fundamental component of proteins and nucleic acids, but its largest reservoir is the atmosphere, where it exists as an inert gas (\(\text{N}_{2}\)) unusable by plants and animals. Specialized soil bacteria perform nitrogen fixation, converting atmospheric \(\text{N}_{2}\) into ammonia or ammonium, making it biologically available. Plants then use assimilation to incorporate this fixed nitrogen into living tissue.
The cycle continues with nitrification, a two-step process where bacteria convert ammonium into nitrites, and then quickly oxidize nitrites into nitrates. Nitrates are the form most readily absorbed by plant roots. Finally, denitrification, performed by bacteria in low-oxygen conditions, converts nitrates back into \(\text{N}_{2}\) gas, returning the element to the atmosphere. The continuous action of these soil microbes acts as the critical bridge linking the vast atmospheric pool of nitrogen with terrestrial and aquatic ecosystems.
Phosphorus: The Sedimentary Limiting Factor
The phosphorus cycle is distinct because it lacks a significant atmospheric gaseous phase; it does not cycle through the air. The primary nonliving reservoir for phosphorus is sedimentary rock in the Earth’s crust. The initial step is the slow geological process of weathering, which breaks down phosphate-rich rocks. This releases inorganic phosphate ions into the soil and water.
Plants absorb these phosphate ions through their roots, and the element moves through the food web. Phosphorus is often a limiting nutrient in aquatic systems, controlling the growth of producers like algae. Over long periods, phosphate can become locked up in sediments at the bottom of oceans and lakes. Only through immense geological uplift and mountain-building events, occurring over millions of years, can this sedimentary phosphorus be brought back to the surface.
The Universal Role of Decomposers
The universal action of decomposers, primarily bacteria and fungi, links all biogeochemical cycles. These organisms return elements from the biotic world back to the abiotic environment after death or waste excretion. When organisms die, the elements within their complex organic molecules are temporarily unavailable for new life.
Decomposers break down this dead organic matter through mineralization, converting complex organic compounds into simple, inorganic forms. This essential recycling function prevents nutrients from remaining sequestered in dead biomass, which would halt elemental cycles. Mineralization ensures a continuous supply of inorganic nutrients that producers can absorb to support the next generation of life.