5 Biogeochemical Cycles
Explore how water and essential elements move between organisms, the atmosphere, water, soil, and rocks, and how human activities can change those cycles.
How Matter Cycles Through Earth
A biogeochemical cycle describes how an element or compound moves between living organisms and nonliving parts of Earth, including the atmosphere, water, soil, and rocks. A is a place where matter is stored; a is a process that moves it between reservoirs. Organisms take up matter as they grow, while waste, death, decomposition, and weathering return it to the environment.
Matter is continually recycled, unlike energy, which flows through ecosystems and is eventually released as heat. Many cycles are connected: water dissolves and transports nutrients, and organisms move elements through food webs.
Takeaway: Track both where matter is stored and the processes that move it.
Water Connects Earth’s Systems
Water is stored in oceans, lakes, glaciers, soil, groundwater, organisms, and the atmosphere. Solar energy drives evaporation from water surfaces and transpiration from plants; together, these processes are called . Water vapor cools and condenses into clouds before returning to Earth as precipitation.
After precipitation falls on land, water can follow several routes:
Run off into streams and eventually reach the ocean.
Infiltrate soil and recharge groundwater.
Be taken up by organisms.
Gravity moves surface water downhill and groundwater through aquifers. For example, rain can enter forest soil, be absorbed by a tree, and later return to the atmosphere through transpiration.
Takeaway: Water connects many reservoirs and carries dissolved materials between them.
Carbon Moves Through Living Things and Long-Term Stores
Carbon is found in the atmosphere mainly as carbon dioxide, in ocean water, in living things, in soils, and in rocks and fossil fuels. Plants and algae take in during and build carbon-containing molecules. Carbon then moves through food webs as organisms eat plants or other organisms.
and decomposition return much of this carbon to the atmosphere or water as . In oxygen-poor environments, decomposition can also produce methane. Some carbon remains stored for long periods in soils, ocean sediments, and rocks.
Burning fossil fuels and clearing forests transfer stored carbon into the atmosphere more rapidly. These activities change the speed and direction of carbon flows.
Takeaway: Carbon cycles through organisms, air, water, and long-term stores such as sediments and rocks.
Microbes Transform Nitrogen
Most nitrogen is in the atmosphere as nitrogen gas, , which most organisms cannot use directly. In , certain bacteria convert it into usable compounds; lightning and industrial processes also contribute to fixation.
Plants absorb nitrogen mainly as ammonium or nitrate. Animals obtain it by eating plants or other animals. Decomposers convert nitrogen in wastes and dead organisms into ammonium through . Other bacteria perform , converting ammonium to nitrite and then nitrate. Through , bacteria convert nitrate back into nitrogen gas, returning it to the atmosphere.
Takeaway: Bacteria drive key transformations that make nitrogen available to organisms and return it to the atmosphere.
Phosphorus Moves Mainly Through Land and Water
Phosphorus is needed to build DNA, cell membranes, and molecules involved in energy transfer. Unlike carbon and nitrogen, phosphorus has no major atmospheric gas phase. The therefore moves mainly through rocks, soil, water, and organisms.
Weathering releases phosphate from rocks into soil and water. Plants and algae take it up, and it moves through food webs. Waste and decomposition return phosphate to soil and water. Runoff carries some phosphate into rivers and oceans, where it may be taken up by organisms or settle into sediments. Over geologic time, uplift can expose these sediments as rock again.
Because this cycle depends strongly on rock weathering and sediment movement, it is generally slow.
Takeaway: Phosphorus moves through land and water reservoirs without a major atmospheric gas stage.
Nutrients, Human Impacts, and Ecosystems
Sulfur and mineral nutrients such as potassium, calcium, and magnesium also cycle among rocks, water, soil, and organisms. Weathering releases minerals, organisms take them up, and decomposition returns them. Sulfur can also move through the atmosphere, including as gases from volcanoes and from human activities such as fossil-fuel combustion.
Human activities can alter the amount and movement of nutrients. Fertilizer, manure, wastewater, and urban runoff add nitrogen and phosphorus to waterways. Excess nutrients can fuel algal blooms. When the algae die and decompose, oxygen in the water can fall, harming aquatic life. Burning fossil fuels and deforestation also change carbon and nitrogen flows.
Takeaway: Human activities can speed up or redirect cycles, with consequences for ecosystem conditions.