Biogeochemical cycles

Background and evolution

Energy flows through ecosystems in one direction and is lost as heat, but matter is recycled. The atoms in a leaf, a fish or a human body have been part of rocks, air, oceans and other living things many times. The term "biogeochemical" captures this: biological processes (photosynthesis, respiration, decomposition), geological processes (weathering, volcanism, sedimentation) and chemical changes all move elements between reservoirs.

Scientists divide the reservoirs into large, slow-moving pools, such as rocks and the deep ocean, and smaller, fast-cycling pools, such as the atmosphere, living things and soils. Over geological time the cycles have been roughly balanced, which kept atmospheric composition and climate within ranges suitable for life. Since the Industrial Revolution, human activities have moved elements faster than natural processes can absorb, with consequences such as climate change, ocean acidification, dead zones and acid rain.

Key features and provisions

• Carbon cycle: plants take in CO2 by photosynthesis; respiration, decay, fires and burning of fossil fuels return it; oceans absorb and release CO2; carbon is stored for millions of years in limestone and fossil fuels.
• Nitrogen cycle: most nitrogen is unusable N2 gas. Fixation by bacteria (such as Rhizobium in legume root nodules and cyanobacteria) converts it to ammonia; nitrification by bacteria turns it into nitrates; plants absorb nitrates; ammonification returns nitrogen from dead matter; denitrification releases N2 back to air.
• Phosphorus cycle: released by weathering of rocks, taken up by plants, passed through food chains, and returned to sediments; very slow.
• Sulphur cycle: includes volcanic emissions, burning of coal and oil (producing sulphur dioxide and acid rain), and microbial processes.
• Water cycle: evaporation, condensation, precipitation and runoff link all other cycles.