Matter on Earth cycles through various forms and locations, a continuous process for sustaining life. These cycles move chemical elements and compounds through Earth’s atmosphere, oceans, land, and living organisms. This recycling reuses limited resources for new generations of life. Matter is conserved; elements are transformed and redistributed, not created or destroyed. Understanding these processes illustrates how interconnected Earth’s systems support diverse ecosystems.
The Water Cycle
Water continuously moves across Earth, shifting in liquid, solid, and gaseous states. This global circulation, powered by solar energy, begins with evaporation, where liquid water transforms into vapor and rises into the atmosphere from water bodies and moist surfaces. Transpiration also contributes to atmospheric water vapor, as plants release water from their leaves. As water vapor ascends, it cools and condenses into liquid droplets or ice crystals, forming clouds.
These water droplets or ice crystals grow within clouds until they become too heavy to remain suspended, falling back to Earth as precipitation such as rain, snow, sleet, or hail. On the ground, precipitation follows several paths. Some water flows over the land surface as runoff, entering streams, rivers, and oceans. Other water infiltrates the ground, replenishing groundwater reserves.
Major water reservoirs include the vast oceans, holding most of Earth’s water, as well as lakes, rivers, glaciers, and underground aquifers. The atmosphere also holds water vapor before it returns to the surface.
The Carbon Cycle
Carbon, a building block of life, moves continuously through the atmosphere, oceans, land, and living organisms. Photosynthesis is a primary process: plants absorb atmospheric carbon dioxide to create organic compounds, storing carbon in their tissues. Animals acquire carbon by consuming plants or other animals. Respiration releases carbon dioxide back into the atmosphere as organisms break down organic matter for energy.
Decomposers like bacteria and fungi break down dead organic material, returning carbon to the soil and atmosphere. Carbon also stores long-term in various reservoirs. Oceans absorb atmospheric carbon dioxide, and marine organisms use it to build shells and skeletons. Over geological timescales, accumulated organic matter can form fossil fuels such as coal, oil, and natural gas. Combustion (e.g., burning fossil fuels or biomass) releases stored carbon back into the atmosphere as carbon dioxide.
The Nitrogen Cycle
Nitrogen, crucial for proteins and nucleic acids, transforms as it cycles through Earth’s systems. Atmospheric nitrogen gas, making up about 78% of the air, is not directly usable by most organisms. Nitrogen fixation is the initial step, converting atmospheric nitrogen into ammonia or ammonium, primarily by specialized bacteria in soil or associated with plant roots. These fixed forms are then available for plant uptake.
Following fixation, nitrification occurs, where bacteria convert ammonia into nitrites and then nitrates, which plants readily absorb. Assimilation occurs as plants absorb these usable forms from the soil, incorporating them into organic molecules. Animals obtain nitrogen by consuming plants or other animals. When organisms die or excrete waste, decomposers convert organic nitrogen back into ammonia through ammonification. Denitrification completes the cycle, as bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere.
The Phosphorus Cycle
The phosphorus cycle differs from the water, carbon, and nitrogen cycles as it lacks a significant atmospheric gaseous phase. Phosphorus primarily cycles through rocks, soil, water, and living organisms. The cycle begins with rock weathering, releasing phosphate ions into soil and water. This slow geological process makes the phosphorus cycle slower than other biogeochemical cycles.
Once released, plants absorb inorganic phosphate from soil, incorporating it into organic molecules. Animals obtain phosphorus by eating plants or other animals. When plants and animals die, decomposers break down organic matter, returning phosphorus to soil and water. Over long periods, phosphorus accumulates in sediments, forming new phosphate-containing rocks that act as a long-term reservoir. This sedimentation can lock phosphorus away for millions of years before geological uplift and weathering reintroduce it.