An ecosystem functions as a dynamic system where living organisms interact with their non-living surroundings. Matter, which comprises the elements and compounds forming both living and non-living components, is not created or destroyed within these systems. Instead, it undergoes continuous reuse and recycling. Understanding these cycles is fundamental.
Foundations of Matter Cycling
Matter cycling differs from energy flow within an ecosystem; energy moves through in a unidirectional path, while matter is continuously recycled. Organisms play distinct roles in this cycling. Producers, such as plants, convert inorganic matter into organic compounds, primarily through photosynthesis. These organisms form the base of the food web, making matter available to other life forms.
Consumers obtain matter by ingesting other organisms. Primary consumers feed on producers, secondary consumers consume primary consumers, and tertiary consumers feed on secondary consumers. As matter moves through these trophic levels, it is incorporated into the bodies of the organisms. Ultimately, decomposers, including bacteria and fungi, break down dead organic matter and waste products. This process returns essential elements back to the environment in inorganic forms, making them available for producers to reuse. Matter can also be stored for varying periods in “reservoirs” or “sinks,” such as the atmosphere, oceans, rocks, and living organisms.
The Carbon Cycle
Carbon continuously moves through Earth’s systems, traveling between the atmosphere, oceans, living organisms, and the Earth’s crust. In the atmosphere, carbon exists primarily as carbon dioxide (CO2). Photosynthesis is a key process where producers, like plants, absorb CO2 from the atmosphere or water to create organic molecules. This process removes carbon from the atmosphere.
Conversely, living organisms, including plants, animals, and microbes, release carbon dioxide back into the atmosphere through respiration. Decomposers also contribute to this release when they break down dead organic matter and waste products. Carbon is stored in the oceans as dissolved CO2, which can be absorbed or released in exchange with the atmosphere. Over long geological timescales, carbon can be stored in rocks and fossil fuels. Natural processes like wildfires or volcanic activity can also release stored carbon.
The Nitrogen Cycle
Nitrogen is an essential element for life, forming components of proteins and nucleic acids. Although atmospheric nitrogen (N2) constitutes about 78% of the air, most organisms cannot directly use it in this gaseous form. The nitrogen cycle involves microbial transformations to convert nitrogen into usable forms. Nitrogen fixation, primarily carried out by certain bacteria and cyanobacteria, converts atmospheric N2 into ammonia (NH3) or ammonium (NH4+), which are usable by plants. These nitrogen-fixing bacteria can be free-living in the soil or symbiotic, living in the root nodules of plants like legumes.
Following nitrogen fixation, nitrification occurs, a two-step process where specific bacteria convert ammonia into nitrites (NO2-) and then into nitrates (NO3-). Plants absorb these nitrates and ammonium ions through assimilation. When plants and animals die, or through animal waste, decomposers convert organic nitrogen back into ammonia or ammonium in a process called ammonification. Finally, denitrification, carried out by other bacteria, converts nitrates back into gaseous nitrogen (N2), returning it to the atmosphere.
The Water Cycle
The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the Earth’s surface. This cycle is driven by solar energy, which facilitates the various phase changes of water. Evaporation is the process where liquid water from oceans, lakes, and rivers transforms into water vapor and rises into the atmosphere. Plants also contribute water vapor to the atmosphere through transpiration, where water evaporates from their leaves.
As water vapor rises and cools, it undergoes condensation, changing back into liquid water droplets to form clouds. When these droplets accumulate and become too heavy, precipitation occurs, returning water to the Earth’s surface as rain, snow, or other forms. Upon reaching the ground, water can flow over the land as runoff, eventually entering rivers and oceans. Alternatively, it can seep into the ground through infiltration, becoming soil moisture or groundwater.
The Phosphorus Cycle
The phosphorus cycle involves the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike other major cycles, the atmosphere plays a limited role because phosphorus compounds do not readily enter a gaseous phase. Phosphorus is an essential component of DNA, ATP (the energy currency of cells), and bones. The cycle begins with the weathering of rocks, which releases phosphate ions into the soil and water. This process is slow, making the phosphorus cycle one of the slowest biogeochemical cycles.
Plants absorb inorganic phosphate from the soil or water through their roots. This phosphorus then moves through the food web as consumers obtain it by eating plants or other animals. When organisms die or excrete waste, decomposers break down organic matter, returning phosphorus to the soil and water. Over geological time, phosphorus in water bodies can settle as sediments on the ocean floor. These sediments can eventually form new rocks, and through geological uplift, the phosphorus can once again be exposed to weathering, continuing the cycle.