The process in which matter moves through an ecosystem is called biogeochemical cycling, also referred to as nutrient cycling. This global mechanism represents the continuous movement of chemical elements and compounds between living organisms and the non-living components of the planet. Through these cycles, elements like carbon, nitrogen, and water are constantly recycled, ensuring their availability to sustain all life forms on Earth. The stability of every ecosystem relies fundamentally on the uninterrupted flow and transformation of matter within these pathways.
Defining Biogeochemical Cycles
The term “biogeochemical” itself describes the three main components involved in this planetary recycling process. “Bio” refers to the living organisms, such as plants, animals, and microbes, that absorb and transform the elements. The “geo” component encompasses the non-living geological features, including the atmosphere, lithosphere (rocks and soil), and hydrosphere (water bodies). Finally, “chemical” signifies the elemental matter itself, such as carbon, oxygen, or nitrogen, that is being cycled.
Matter is stored in various reservoirs, or pools, which are large compartments where an element resides for a period of time. These reservoirs include the deep ocean, the atmosphere, or sedimentary rock formations. The movement of matter between these reservoirs is known as flux, and the speed at which an element moves through a specific reservoir is called its turnover rate or residence time.
The biotic and abiotic factors work together to control the movement of these elements through the ecosystem. Biotic factors, such as photosynthesis by plants, actively remove elements from the atmosphere and incorporate them into organic matter. Abiotic factors, like volcanic eruptions or the weathering of rocks, release elements from geological storage back into forms that can be utilized by organisms. This continuous exchange prevents any single element from becoming permanently locked away.
The Carbon and Water Cycles
The carbon cycle is characterized as a gaseous cycle because its primary non-living reservoir is the atmosphere, where it exists as carbon dioxide ($\text{CO}_2$). Plants, algae, and cyanobacteria act as the entry point for carbon into the biotic world through photosynthesis, converting atmospheric $\text{CO}_2$ into organic compounds like glucose. Carbon then moves through the food web and is returned to the atmosphere primarily through cellular respiration, the process by which organisms break down organic matter for energy.
The ocean represents the largest active reservoir of carbon, absorbing immense amounts of $\text{CO}_2$. This absorbed carbon is used by marine organisms to build calcium carbonate shells or cycles through marine food webs and deep ocean currents. Decomposition of dead organic matter releases carbon back into the atmosphere or into the soil, where it can be stored for long periods in the form of peat or fossil fuels.
The water cycle, or hydrologic cycle, is unique in that it physically facilitates the movement of all other elements across the globe. Driven by solar energy, water moves from the Earth’s surface to the atmosphere through evaporation from water bodies and transpiration from plant leaves. Water vapor then cools and changes back into liquid form during condensation, creating clouds.
This atmospheric water returns to the Earth’s surface as precipitation. Once on the ground, the water either infiltrates the soil to become groundwater or flows across the surface as runoff, eventually returning to rivers, lakes, and oceans. The hydrologic cycle is responsible for the physical transport of dissolved nutrients, carrying them from weathered rocks and soil into surface waters where they become available to aquatic life.
The Nitrogen and Phosphorus Cycles
Nitrogen and phosphorus are categorized as limiting nutrients because their availability frequently controls the productivity of an entire ecosystem. The nitrogen cycle is complex and highly dependent on specialized microorganisms to convert atmospheric nitrogen gas ($\text{N}_2$), which is largely inert, into biologically usable forms. Nitrogen fixation, performed by bacteria such as Rhizobium, converts $\text{N}_2$ into ammonia ($\text{NH}_3$).
The ammonia is then converted into nitrites ($\text{NO}_2^-$) and then nitrates ($\text{NO}_3^-$) by nitrifying bacteria in a process called nitrification. Plants absorb these nitrates and ammonium ions to synthesize proteins and nucleic acids. When organisms die, decomposers convert the organic nitrogen back into ammonium during ammonification, and other bacteria perform denitrification, converting nitrates back into $\text{N}_2$ gas that returns to the atmosphere.
The phosphorus cycle is a sedimentary cycle that lacks a significant atmospheric reservoir. Phosphorus is primarily stored in the lithosphere, locked within phosphate-rich rocks and sediments. The slow process of weathering and erosion releases inorganic phosphate ions ($\text{PO}_4^{3-}$) from these rocks into the soil and water.
Plants absorb these phosphate ions through their roots, and the element is then transferred through the food chain to consumers. Since phosphorus does not readily form a gas, the main path for its long-term cycling involves geological uplift, which raises ocean sediments and rock formations back above sea level, restarting the weathering process.
Human Influence on Matter Movement
Human activities have accelerated the flux rates of these biogeochemical cycles, disrupting the planetary balance. The greatest impact is on the carbon cycle, primarily through the combustion of fossil fuels, which releases carbon sequestered in the lithosphere back into the atmosphere as $\text{CO}_2$. Land-use changes, particularly deforestation, further exacerbate this by removing carbon sinks and releasing stored carbon from biomass and soil.
The nitrogen and phosphorus cycles are heavily disrupted by industrial agriculture and the widespread use of synthetic fertilizers. The Haber-Bosch process, which artificially fixes atmospheric nitrogen to create fertilizer, has more than doubled the amount of reactive nitrogen entering the global environment. This excess nitrogen and phosphorus runs off agricultural land into waterways, triggering cultural eutrophication.
Eutrophication leads to rapid, excessive growth of algae, which then die and are decomposed by bacteria. This decomposition depletes oxygen from the water and creates vast “dead zones” in coastal areas. Deforestation and urbanization also affect the water cycle by increasing surface runoff and reducing the amount of water that infiltrates the soil to recharge groundwater supplies.