Matter cycling, or biogeochemical cycling, is the continuous movement of elements required for life through Earth’s systems. These cycles describe how non-living chemical components circulate between the atmosphere, hydrosphere, lithosphere, and the biosphere, forming a vast, interconnected global recycling system. Elements like carbon, nitrogen, and phosphorus are the building blocks of all biological molecules. Their uninterrupted flow is fundamental to sustaining life, and tracking them helps scientists understand the delicate balance governing Earth’s ecosystems.
The Fundamental Difference Between Energy and Matter
The flow of energy and the cycling of matter are distinct, yet interdependent, processes that maintain ecosystems. Energy moves in a one-way, linear direction, starting with solar radiation captured by plants. It is transferred through the food chain before being dissipated as heat, necessitating a constant input of new solar energy.
In contrast, the matter that makes up living organisms is finite and must be continuously recycled within the biosphere. This recycling involves three main biological groups. Producers, such as plants, convert inorganic compounds into organic matter using external energy. Consumers then transfer this organic matter by feeding on producers or other consumers.
Decomposers, primarily bacteria and fungi, break down dead organic material and waste products. They convert complex organic compounds back into simple, inorganic nutrient forms, such as carbon dioxide and nitrates. This action replenishes the soil and water with the elements producers need to begin the cycle anew.
The Dynamic Carbon Cycle
Carbon forms the structural skeleton for all organic molecules. Its cycle involves a rapid biological exchange alongside a much slower geological one. The fast, or biological, cycle is driven by two opposing processes: photosynthesis and cellular respiration. During photosynthesis, plants absorb atmospheric carbon dioxide ($\text{CO}_2$) and convert it into sugars, moving carbon from the atmosphere into the biosphere.
The carbon moves through the food web as organisms consume plant matter. Both plants and consumers release carbon back to the atmosphere as $\text{CO}_2$ through cellular respiration, the process that breaks down sugars to release energy. When organisms die, decomposers break down the remaining organic carbon, also releasing $\text{CO}_2$ back into the air or soil.
The slow cycle involves the long-term sequestration of carbon in massive geologic reservoirs. Carbon dissolves in ocean water, forming carbonate sediments that sink to the seabed and become incorporated into limestone rock over millions of years. Buried organic matter also transforms into fossil fuels like coal, oil, and natural gas under high pressure and temperature. These reservoirs hold carbon out of circulation, releasing it only through slow geologic processes like volcanic activity or rock weathering.
Human activity has disrupted this balance by rapidly mobilizing carbon from these deep reservoirs. Burning fossil fuels releases vast amounts of $\text{CO}_2$ into the atmosphere at a rate far exceeding natural release. Deforestation also reduces the number of plants performing photosynthesis, diminishing the biosphere’s capacity to draw carbon out of the atmosphere. This imbalance causes atmospheric $\text{CO}_2$ concentrations to rise, impacting global climate and ocean chemistry.
The Complex Nitrogen Cycle
Nitrogen is a structural component of proteins and nucleic acids like DNA, making it indispensable for life. Although atmospheric nitrogen gas ($\text{N}_2$) makes up about 78% of the air, its triple bond renders it inert and chemically inaccessible to most organisms. The nitrogen cycle relies almost entirely on specialized microorganisms to convert $\text{N}_2$ into usable forms.
The process begins with nitrogen fixation, where bacteria like Rhizobium convert $\text{N}_2$ into ammonia ($\text{NH}_3$) or ammonium ($\text{NH}_4^+$). These bacteria often live symbiotically within the root nodules of legumes, exchanging fixed nitrogen for plant sugars. The next transformation is nitrification, a two-step process carried out by different groups of nitrifying bacteria in the soil.
First, bacteria such as Nitrosomonas oxidize ammonium into nitrite ($\text{NO}_2^-$), which is toxic to plants. Bacteria like Nitrobacter then convert the nitrite into nitrate ($\text{NO}_3^-$). Nitrate is the form most readily absorbed by plants through assimilation, incorporating the nitrogen into organic molecules.
When organisms die or excrete waste, fungi and decomposition bacteria perform ammonification, converting organic nitrogen compounds back into ammonium. This ammonium can re-enter the nitrification step or be returned to the atmosphere. The final step, denitrification, is performed by anaerobic bacteria, such as Pseudomonas, which convert nitrate back into nitrogen gas ($\text{N}_2$), primarily in oxygen-poor environments. This returns the nitrogen to the atmospheric reservoir, completing the cycle.
The Sedimentary Phosphorus Cycle and Global Disruption
The phosphorus cycle is unique because it is primarily sedimentary and lacks a significant atmospheric gaseous phase. Phosphorus, as phosphate ions ($\text{PO}_4^{3-}$), is a component of energy-carrying molecules like adenosine triphosphate (ATP) and the backbone structures of DNA and RNA. The largest natural reservoir is locked up in phosphate-containing rocks and marine sediments.
The cycle begins with the slow, geological process of weathering, where wind and water erode rocks, releasing dissolved phosphate into soils and water. Plants absorb this phosphate, which is then transferred through the food web to consumers. Because natural release from rocks is slow, phosphorus is often a limiting factor for plant growth in many ecosystems.
When organisms die, decomposers return the phosphate to the soil or water, where it can be reused or settle in aquatic environments to form new sedimentary layers. Over geologic time, tectonic uplift brings these marine sediments back to the surface, restarting weathering. Human activity has drastically accelerated this cycle by mining phosphate rock to produce large quantities of agricultural fertilizers.
This intervention has tripled the global mobilization of phosphorus, causing it to accumulate in soils beyond natural levels. Runoff of these fertilizers into rivers and lakes creates nutrient overloads. This excess input causes rapid growth of algae and cyanobacteria, a process called eutrophication. When these dense algal blooms die, their decomposition consumes vast amounts of dissolved oxygen, creating oxygen-depleted zones hostile to aquatic life. The rapid movement of nutrients demonstrates how human actions have destabilized the balance of these biogeochemical cycles.