Matter cycling describes the continuous movement of chemical elements through Earth’s living and non-living components. This process ensures matter is transformed and reused within various systems. Elements like carbon, nitrogen, and water are constantly exchanged between the atmosphere, oceans, land, and living organisms. This continuous recycling underpins life on Earth.
The Fundamental Concept of Matter Cycling
Matter cycling involves the movement of chemical substances through distinct Earth components, called reservoirs. These reservoirs are locations where matter accumulates (e.g., atmosphere, oceans, soil, living organisms). Movement between these storage areas occurs via fluxes, which describe transfer rates.
Matter transforms into different forms (e.g., gases, liquids, solids, organic or inorganic compounds) as it cycles. For instance, carbon can exist as atmospheric carbon dioxide, dissolved in oceans, or as organic matter in living tissues. Physical, chemical, and biological forces drive these transformations. These include solar energy, gravity, and microbial metabolic activities.
Key Biogeochemical Cycles
The Water Cycle
The water cycle, or hydrologic cycle, describes water’s continuous movement on, above, and below Earth’s surface. Solar energy drives this cycle, causing evaporation from oceans and land, and sublimation of ice. Water vapor condenses to form clouds, returning to Earth as precipitation (e.g., rain, snow). On the surface, water flows as runoff, infiltrates into soil as groundwater, or is released by plants through transpiration.
The Carbon Cycle
The carbon cycle involves carbon’s movement between Earth’s atmosphere, oceans, land, and living organisms. Plants absorb atmospheric carbon dioxide through photosynthesis, converting it into organic compounds. Carbon moves through food chains as organisms consume plants and animals. Respiration and decomposition of organic matter release carbon dioxide back into the atmosphere. Geological processes (e.g., weathering, volcanism) and the formation/combustion of fossil fuels also contribute to carbon movement over vast timescales.
The Nitrogen Cycle
The nitrogen cycle describes how atmospheric nitrogen is converted into various forms as it circulates through terrestrial, aquatic, and atmospheric ecosystems. While atmospheric nitrogen gas is not directly usable by most organisms, microorganisms convert it into accessible forms. Nitrogen fixation converts atmospheric nitrogen into ammonia, which plants can assimilate. Other microbial processes, like nitrification, transform ammonia into nitrates, while ammonification decomposes organic nitrogen into ammonia. Denitrification returns nitrogen gas to the atmosphere, completing the cycle.
The Phosphorus Cycle
The phosphorus cycle involves phosphorus movement through the lithosphere, hydrosphere, and biosphere. Unlike other major cycles, it lacks a significant atmospheric gaseous phase. Phosphorus is primarily found in rocks and minerals; its release into soil and water occurs through weathering. Plants absorb dissolved phosphate from soil, transferring phosphorus through food chains. When organisms die, decomposers return phosphorus to soil and water, where it can settle as sediment, slowly re-entering the geological cycle through uplift over long periods.
The Vital Role of Matter Cycling
Matter cycling underpins the availability of elements necessary for life. These cycles ensure essential nutrients (e.g., nitrogen, phosphorus) are continuously supplied to support organism growth and ecosystem functioning. Without these processes, nutrients would become locked in reservoirs, unavailable for biological use.
The cycles also regulate Earth’s climate. For example, the carbon cycle influences atmospheric carbon dioxide levels, affecting global temperatures. The water cycle also impacts temperature regulation through evaporation and condensation, distributing heat across the planet. These cycles collectively maintain ecosystem balance and stability, supporting diverse life and contributing to planetary health.
Human Impact on Global Matter Cycles
Human activities have altered the natural balance of matter cycling, leading to widespread environmental consequences. Fossil fuel combustion and deforestation increase atmospheric carbon dioxide, contributing to climate change and ocean acidification. Deforestation reduces plant carbon dioxide absorption, further impacting the carbon balance.
Agricultural practices, particularly extensive use of synthetic nitrogen fertilizers, have profoundly affected the nitrogen cycle. Excess nitrogen from runoff can lead to eutrophication in aquatic environments, causing algal blooms and oxygen depletion that harm marine life. Industrial emissions and fossil fuel burning also release nitrogen oxides, contributing to smog and acid rain.
Human influence on the water cycle includes damming rivers, diverting water for irrigation, and increasing impervious surfaces through urbanization. These actions can disrupt natural water flow, reduce water availability, and alter water quality through pollution. Similarly, phosphorus mining, fertilizer application, and wastewater discharge overload ecosystems with phosphorus. This excess phosphorus can lead to eutrophication in freshwater bodies, negatively affecting aquatic biodiversity and water quality.