09 Biogeochemical Cycles
Learn how water, carbon, nitrogen, phosphorus, and sulfur move through organisms and Earth's physical reservoirs, and how human activities can alter these cycles.
How Matter Cycles Through Ecosystems
A describes how matter moves among living organisms and Earth's nonliving reservoirs, such as the atmosphere, water, soil, and rocks. Organisms take up elements and return them through feeding, respiration, waste, and decomposition.
Matter is continually recycled, while energy flows through ecosystems and is eventually lost as heat. Water and other materials can also connect cycles: for example, water transports dissolved carbon and nutrients.
The Water Cycle
Water moves among the ocean, atmosphere, land, and living things. Sunlight drives evaporation from water surfaces, while plants release water vapor through transpiration. Together, these processes are called . Water vapor cools and condenses into clouds, then returns to Earth as precipitation.
After precipitation, water may flow over land as runoff, soak into soil and replenish groundwater, or be taken up by organisms. Groundwater can later discharge into streams, lakes, or the ocean.
Example: Rain falling on a forest may be absorbed by tree roots and released from leaves as vapor. Some rain instead flows into a stream or travels underground toward an aquifer.
Takeaway: Water changes location and pathway as it cycles through the atmosphere, land, surface water, groundwater, and organisms.
The Carbon Cycle
Carbon is found in the atmosphere mainly as carbon dioxide, , and in organisms as part of organic molecules. It is also dissolved in the ocean or stored in soils, sediments, rocks, and fossil fuels.
Photosynthesis moves carbon from the atmosphere or water into plants and algae. Carbon then passes through food webs when organisms eat one another. Respiration and decomposition return carbon to the atmosphere or water as . Some carbon remains stored for long periods in ocean sediments and rocks.
Combustion of coal, oil, and natural gas transfers long-stored carbon to the atmosphere. Deforestation can also release carbon and reduce the amount removed from the air through photosynthesis. The ocean takes up some atmospheric , but the rates of ocean exchange and movement into the deep ocean differ.
Takeaway: Carbon cycles between organisms and physical reservoirs, with some processes moving it quickly and geological storage holding it for long periods.
The Nitrogen Cycle
Although nitrogen gas, , makes up much of the atmosphere, most organisms cannot use it directly. converts it into available forms, including ammonia or ammonium. Certain bacteria, including bacteria associated with legume roots, carry out fixation; lightning and industrial fertilizer production also contribute. Plants take up usable nitrogen from soil, and animals obtain it by eating plants or other animals.
When organisms produce waste or die, decomposers return organic nitrogen to soil as ammonium through . Soil bacteria perform , converting ammonium to nitrite and then nitrate. Through , other bacteria convert nitrate back into nitrogen gas, returning it to the atmosphere.
Takeaway: Bacterial transformations move nitrogen between forms that organisms can use and atmospheric nitrogen that most organisms cannot use directly.
Phosphorus, Sulfur, and Different Timescales
Phosphorus is an essential component of DNA, RNA, and cell membranes. Weathering releases phosphate from rocks into soil and water. Plants and algae take it up, and phosphorus moves through food webs. Waste and decomposition return phosphate to the environment. Some phosphate is carried by runoff into waterways and settles into sediments; over geologic time, sediment can form rock that may eventually be exposed again.
Unlike the water, carbon, and nitrogen cycles, the has no major gaseous stage, so its movement through ecosystems is often relatively slow. Sulfur also moves among organisms, soil, water, rocks, and the atmosphere through processes that include weathering, decomposition, volcanic activity, and fossil-fuel burning.
Takeaway: Elements share the general pattern of movement through organisms and the environment, but their reservoirs and timescales differ.
Human Impacts and Connections Between Cycles
Cycles interact: water transports dissolved carbon and nutrients, while organisms move elements through food webs. Human activities can alter how quickly materials move or where they are stored.
Fertilizers, manure, wastewater, and urban runoff can add excess nitrogen and phosphorus to waterways. This may encourage algal blooms. When excess organic matter decomposes, decomposers use oxygen, potentially lowering oxygen levels enough to harm fish and other aquatic life.
Example: After heavy rain, fertilizer can wash from a field into a stream and then a lake. The added nutrients may cause rapid algal growth. As algae die and decompose, decomposers use oxygen, potentially leaving too little for fish and other aquatic organisms.
Takeaway: Adding nutrients to aquatic systems can set off a sequence from algal growth to decomposition and oxygen loss.