The matter that makes up all life and non-living components of Earth is continuously recycled through a global network of transfers known as biogeochemical cycles. These cycles describe the pathways by which chemical elements move between the living components (the biosphere) and the non-living components, including the atmosphere, the hydrosphere, and the lithosphere. The balance of these material exchanges is fundamental to sustaining all biological processes and acts as the primary mechanism for regulating Earth’s planetary systems, including global climate and temperature stability.
The Movement of Water and Carbon
The hydrologic cycle, or water cycle, acts as the planet’s heat distribution and purification system, driven primarily by solar energy. Water absorbs heat during evaporation, turning liquid water from oceans, lakes, and soil into vapor, carrying latent heat into the atmosphere. This vapor travels via atmospheric circulation before cooling and undergoing condensation, forming clouds, and eventually returning to the surface as precipitation. Plants also aid this movement through transpiration, releasing water from leaf pores into the atmosphere.
Once water reaches the land, it either flows as surface runoff, which shapes geological features, or infiltrates the ground to replenish groundwater reserves. The ocean is the source for roughly 86% of global evaporation. The movement of water through its various phases—liquid, solid, and gas—is linked with Earth’s energy balance and the maintenance of habitable temperatures.
The carbon cycle describes the movement of the element that forms the structural basis of all organic molecules. Carbon dioxide (\(text{CO}_2\)) is taken from the atmosphere by plants through photosynthesis, converting the gas into complex sugars and biomass. This stored carbon then moves through the food web as organisms consume plants and each other.
Carbon returns to the atmosphere through cellular respiration, where living organisms break down organic compounds for energy, releasing \(text{CO}_2\) as a byproduct. Long-term storage occurs when carbon is sequestered in the deep ocean, forming marine sediments, or when dead organic matter is buried to form fossil fuels. The rapid exchange of carbon between the atmosphere, biosphere, and surface ocean is balanced by a slower geological cycle involving the weathering of rocks and volcanic activity.
The Cycles Essential for Life: Nitrogen and Phosphorus
The nitrogen cycle is complex because atmospheric nitrogen (\(text{N}_2\)), which makes up about 78% of the air, is chemically inert and unusable by most life forms. Nitrogen is required for synthesizing proteins and DNA, but it must first be “fixed,” or converted into reactive compounds like ammonia (\(text{NH}_3\)). This conversion, known as nitrogen fixation, is carried out primarily by specialized bacteria, often living symbiotically in the root nodules of legumes or freely in the soil.
Once fixed, nitrogen enters the terrestrial cycle, where microbial groups perform sequential transformations. Nitrifying bacteria convert ammonia first into nitrites (\(text{NO}_2^-\)) and then into nitrates (\(text{NO}_3^-\)), which plants absorb through assimilation. Denitrifying bacteria complete the cycle by converting nitrates in waterlogged soils back into \(text{N}_2\) gas, returning the element to the atmosphere. This microbial-driven sequence regulates the planet’s supply of biologically available nitrogen.
The phosphorus cycle is unique because it is a purely sedimentary cycle, lacking a gaseous atmospheric phase. Phosphorus, a component of ATP and cell membranes, is primarily stored in phosphate-bearing rock formations within the Earth’s crust. The cycle begins when weathering and erosion slowly release phosphate ions into the soil and water systems.
Plants absorb these dissolved phosphate ions, and animals obtain the element by eating these plants. The element is returned to the soil or water through waste and decomposition. Because new phosphorus enters the active cycle only through the slow geological uplift and weathering of rock, it is considered the slowest of the major biogeochemical cycles, making it a common limiting nutrient for plant growth in many ecosystems.
Human Impact on Earth’s Natural Rhythms
For centuries, biogeochemical cycles maintained a steady state, but human activities have rapidly accelerated and disrupted the natural flow of these materials. The most prominent disruption is to the carbon cycle, primarily through the combustion of fossil fuels. This releases carbon that had been locked away in the lithosphere, creating a rapid flux into the atmosphere, mainly as \(text{CO}_2\). This flux exceeds the capacity of the oceans and biosphere to absorb it, leading to a rise in global atmospheric concentrations.
The nitrogen cycle has been altered through the industrial production of fertilizers via the energy-intensive Haber-Bosch process. This synthetic fixation of atmospheric nitrogen now rivals the total amount fixed naturally by global ecosystems. Excess nitrogen from agricultural runoff and sewage enters waterways, leading to eutrophication. Dense algal blooms consume vast amounts of oxygen upon decomposition, creating anoxic “dead zones” in coastal areas.
The phosphorus cycle is also destabilized by agricultural practices, specifically the mining of phosphate rock for fertilizer application. Since phosphorus lacks an atmospheric reservoir, the element is extracted from a slow geological sink and rapidly introduced to the land, where much is lost to runoff. This runoff pollutes aquatic environments and accelerates eutrophication. Furthermore, large-scale deforestation and the construction of dams alter the water cycle by reducing local evapotranspiration and changing the natural flow and storage of water.