Nutrient cycles, or biogeochemical cycles, are processes that move essential chemical elements through the planet’s living and non-living components. This continuous recycling ensures that the building blocks of life are consistently available to sustain organisms and entire ecosystems.
Why Nutrient Cycles Are Essential for Life
The cycling of nutrients is fundamental to all ecosystems, making a limited supply of elements perpetually available. These cycles bridge the biotic (living) and abiotic (non-living) components like soil, water, and air. This connection ensures that elements in one organism are returned to the environment to be used by another, preventing resource depletion.
These cycles involve two components: reservoirs and fluxes. Reservoirs are where nutrients are stored, such as oceans, the atmosphere, or living organisms. Fluxes are the processes that move elements between reservoirs. For example, when a tree dies and decomposes, nutrients move from the tree back to the soil, illustrating a flux.
Different organisms have distinct roles in these cycles. Producers, like plants, absorb inorganic nutrients and convert them into organic compounds. Consumers obtain these nutrients by eating plants or other animals. Decomposers, including bacteria and fungi, break down dead organic matter, releasing inorganic nutrients back into the environment for producers to use again.
Exploring Major Nutrient Cycles: Carbon and Nitrogen
The carbon cycle moves carbon between its primary reservoirs: the atmosphere, oceans, land, and living organisms. Photosynthesis is a primary driver, where plants and marine algae pull carbon dioxide from the atmosphere or water to build organic molecules. This process transfers carbon from the abiotic to the biotic world.
Carbon returns to the atmosphere through several pathways. Respiration by plants, animals, and decomposers releases carbon dioxide. The oceans also absorb and store vast quantities of atmospheric carbon dioxide. Over geological time, buried organic matter can form fossil fuels, locking carbon away from the active cycle.
The nitrogen cycle moves nitrogen through its largest reservoir, the atmosphere, which is about 78% nitrogen gas (N2). Most organisms cannot use N2 directly, so the cycle depends on nitrogen fixation. This process involves specialized bacteria in soil, water, or plant roots converting atmospheric nitrogen into usable forms like ammonia.
Once fixed, nitrogen moves through several other processes:
- Nitrification: Microbes convert ammonia into nitrates and nitrites, which plants can easily absorb.
- Assimilation: Plants absorb these nitrogen compounds into their tissues to create proteins and DNA.
- Ammonification: Decomposers convert the nitrogen in dead organic matter back into ammonia for reuse.
- Denitrification: Other bacteria convert nitrates back into N2 gas, returning it to the atmosphere and completing the cycle.
How Humans Are Changing Nutrient Cycles
Human activities have altered the balance of nutrient cycles, particularly for carbon and nitrogen. The burning of fossil fuels for energy and transportation has released immense quantities of stored carbon dioxide (CO2) into the atmosphere. Deforestation also contributes by removing trees that absorb atmospheric CO2.
Land-use changes, like converting landscapes for agriculture, alter the carbon stored in soil and vegetation. Industrial processes, such as cement manufacturing, are another source of CO2 emissions. While the oceans have absorbed much of this excess CO2, their capacity is finite.
The nitrogen cycle has been modified by agricultural practices. The Haber-Bosch process, invented in the early 20th century, enabled the industrial production of synthetic ammonia-based fertilizers. This process fixes atmospheric nitrogen on a massive scale, doubling the amount of nitrogen entering the terrestrial cycle compared to natural fixation.
The cultivation of nitrogen-fixing crops like soybeans has also increased the amount of biologically fixed nitrogen. Additionally, high-temperature combustion from vehicles and industry converts atmospheric nitrogen into reactive nitrogen oxides that enter ecosystems.
Environmental Impacts of Altered Nutrient Cycles
The increased carbon dioxide in the atmosphere is a primary driver of global climate change, as CO2 is a greenhouse gas that traps heat. This leads to rising global temperatures and more extreme weather. The absorption of excess CO2 by the oceans causes ocean acidification. This process lowers the water’s pH, making it difficult for marine organisms like corals to build their skeletons.
Nitrogen overload in ecosystems also causes environmental damage. Excess nitrogen from fertilizer runoff leads to eutrophication, which stimulates explosive algae growth in water bodies. When these algal blooms die and decompose, the process consumes dissolved oxygen, creating hypoxic “dead zones” where aquatic life cannot survive.
Surplus nitrogen has other negative effects beyond aquatic systems:
- It can contaminate drinking water sources with high levels of nitrates, posing human health risks.
- Atmospheric nitrogen oxides contribute to acid rain, which damages forests and acidifies soils and lakes.
- These compounds also help form ground-level ozone and smog, which are harmful to respiratory health.
- The overall disruption can impact soil fertility and reduce overall biodiversity.