Biogeochemical cycles represent the continuous movement of chemical substances through Earth’s living (biotic) and non-living (abiotic) components. These pathways involve the atmosphere, oceans, land, and all organisms, facilitating the recycling of matter and energy. These cycles are essential for sustaining life on Earth, ensuring that elements remain available for biological processes.
The Water Cycle
The water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above, and below Earth’s surface. It begins with evaporation, where solar energy heats water in oceans, lakes, and soil, converting it into vapor that rises into the atmosphere. Plants also contribute through transpiration, releasing water vapor from their leaves into the air.
As this water vapor ascends, it cools and condenses into liquid water droplets or ice crystals, forming clouds. When these become heavy enough, they fall back to Earth as precipitation, which can be rain, snow, sleet, or hail. This water then either infiltrates the ground to become groundwater, flows over the land as surface runoff, or collects in bodies of water like rivers, lakes, and oceans. The cycle then repeats, making water available for all living organisms.
The Carbon Cycle
The carbon cycle details the movement of carbon atoms between Earth’s various reservoirs, including the atmosphere, oceans, land, and living things. A primary process is photosynthesis, where plants and other producers absorb carbon dioxide from the atmosphere or dissolved in water to create organic compounds. Conversely, respiration, carried out by both plants and animals, releases carbon dioxide back into the atmosphere as they break down organic matter for energy.
Decomposition also plays a role, as decomposers like bacteria and fungi break down dead organisms and waste products, returning carbon to the soil and atmosphere. Over geological timescales, some organic matter can be transformed into fossil fuels, storing carbon underground. The combustion of these fossil fuels, along with the burning of biomass, releases stored carbon dioxide back into the atmosphere, influencing the natural balance of the cycle.
The Nitrogen Cycle
The nitrogen cycle describes the processes by which nitrogen moves through the atmosphere, soil, and living organisms. Atmospheric nitrogen, which makes up about 78% of the air, is largely unusable by most organisms in its gaseous form. Nitrogen fixation is an initial step, where specialized bacteria in the soil or associated with plant roots convert atmospheric nitrogen into ammonia, a usable form.
This ammonia can then undergo nitrification, where different bacteria convert ammonia into nitrites and then into nitrates, which are absorbed by plants. Plants assimilate these nitrates to build proteins and nucleic acids, and nitrogen then moves through the food web as animals consume plants or other animals. When organisms die or excrete waste, decomposers return nitrogen to the soil as ammonia through ammonification. Finally, denitrification occurs when other types of bacteria convert nitrates back into gaseous nitrogen, releasing it into the atmosphere and completing the cycle.
The Phosphorus Cycle
The phosphorus cycle is distinct from other biogeochemical cycles because it primarily occurs within the lithosphere, involving rocks, soil, and water, with very little atmospheric involvement. The cycle begins with the weathering of rocks, which slowly releases phosphate ions into the soil and water. These dissolved phosphates are then absorbed by plants through their roots and incorporated into organic molecules.
Animals obtain phosphorus by consuming plants or other animals, integrating it into essential compounds like ATP, DNA, and the structural components of bones and teeth. When plants and animals die, decomposers break down their organic matter, returning phosphorus to the soil or water. This phosphorus can then be taken up by new plants or, over long periods, deposited in sediments and eventually form new rocks, making the phosphorus cycle one of the slowest biogeochemical cycles.