Nutrients are the fundamental chemical elements that sustain life and govern the productivity of all ecosystems on Earth. These atomic building blocks, such as carbon (C), nitrogen (N), and phosphorus (P), are required by organisms to construct proteins, nucleic acids, and cell structures. The ability of an ecosystem to support life depends entirely on the availability and continuous movement of these elements, which connects the living world (biotic) with the non-living world (abiotic), including the atmosphere, water, and soil.
Cycling vs. Energy Flow
The passage of nutrients through an ecosystem operates under fundamentally different rules than the flow of energy. Energy enters the ecosystem almost entirely from the sun and moves in a linear, unidirectional path from producers to consumers. At each step in a food web, energy is converted to heat and lost, requiring a constant input of solar energy to sustain life.
Nutrients, in contrast, are finite substances that must be conserved and reused; they move in circular pathways known as biogeochemical cycles. These cycles involve the transfer of elements between four major reservoirs: the atmosphere, the hydrosphere, the lithosphere, and the biosphere. The general mechanism involves two key biological steps: assimilation, where producers take up inorganic elements, and decomposition, where bacteria and fungi break down dead organic matter to return elements to the soil or water.
The Movement of Carbon and Oxygen
The movement of carbon and oxygen is tightly linked and is largely driven by the processes of photosynthesis and respiration, involving a rapid exchange with the atmosphere and oceans. Carbon exists primarily in the atmosphere as carbon dioxide (\(\text{CO}_2\)), where it is taken up by plants, algae, and some bacteria. During photosynthesis, these organisms use solar energy to convert \(\text{CO}_2\) and water into energy-rich glucose molecules, effectively transferring atmospheric carbon into the biosphere.
The oxygen released as a byproduct of photosynthesis is then used by nearly all other organisms during cellular respiration. Respiration is the reverse process, where organisms break down organic molecules for energy and release \(\text{CO}_2\) back into the atmosphere or water. This rapid exchange constitutes the biological, or “fast,” carbon cycle, occurring over days to years. A much slower, geological carbon cycle moves carbon into long-term reservoirs, such as when marine organisms die and their carbon-rich shells sink to form sedimentary rock, eventually transforming into fossil fuels.
The Movement of Nitrogen
Nitrogen is an essential component of proteins and nucleic acids, yet its movement is the most complex of the major nutrient cycles because the vast atmospheric reservoir of dinitrogen gas (\(\text{N}_2\)) is unusable by most life. The cycle relies heavily on specialized microorganisms to convert nitrogen into usable forms like ammonium (\(\text{NH}_4^+\)) and nitrate (\(\text{NO}_3^-\)). This conversion begins with nitrogen fixation, where certain bacteria and archaea, including symbiotic Rhizobium in plant roots, use the enzyme nitrogenase to break the strong triple bond of \(\text{N}_2\) and reduce it to ammonia (\(\text{NH}_3\)).
The next major step is nitrification, a two-step aerobic process performed by different groups of bacteria in the soil. First, ammonia is oxidized into nitrite (\(\text{NO}_2^-\)) by bacteria like Nitrosomonas. Immediately afterward, other bacteria, notably Nitrobacter, convert the toxic nitrite into the more stable nitrate. Plants can readily assimilate both nitrate and ammonium from the soil to build their organic molecules.
When plants and animals excrete waste or die, a process called ammonification occurs, where decomposers convert the organic nitrogen compounds back into ammonium. The cycle is completed by denitrification, primarily carried out by anaerobic bacteria, such as Pseudomonas, in oxygen-poor environments like waterlogged soils. These bacteria use nitrate instead of oxygen for respiration, reducing it back to \(\text{N}_2\) gas, which returns the nitrogen to the atmosphere.
The Movement of Phosphorus
The phosphorus cycle is unique among the major nutrient cycles because it is a sedimentary cycle, lacking a significant gaseous phase. Because of this, the element does not circulate through the atmosphere like carbon or nitrogen, making the cycle considerably slower and mainly driven by geological and hydrological processes.
The movement of phosphorus begins with the weathering of phosphate-bearing rocks and minerals in the Earth’s crust. Physical forces like rain, wind, and chemical breakdown release the phosphorus, usually in the form of phosphate ions (\(\text{PO}_4^{3-}\)), into the soil and water. Plants absorb this dissolved inorganic phosphate through their roots in a process of assimilation.
After moving through the food web and the eventual death of organisms, decomposers return the phosphorus to the soil or water. Over long periods, phosphate runoff is carried to oceans, where it settles and precipitates to the seafloor, becoming locked into marine sediments. This burial can remove phosphorus from circulation for millennia until geological uplift and subsequent weathering expose the sedimentary rock, restarting the slow cycle.