Biogeochemical Cycles

To understand how ecosystems function on a planetary scale, we look at biogeochemical cycles, which trace the movement of specific chemical elements through biological, geological, and chemical systems.

Source: Generalized biogeochemical cycle by Muhumuza Moses, CC BY-SA 3.0.

As elements traverse the globe, they are stored in different environmental compartments. Chemical elements reside in different ecosystem reservoirs for varying amounts of time. The duration an element spends in a specific reservoir is called the residence time. For a biology teacher, clarifying residence time is essential to helping students understand why human disruption of these cycles (like digging up fossil fuels) causes such rapid environmental instability. We are taking elements with a residence time of millions of years and suddenly moving them into reservoirs where they turn over in mere days.

Let us trace the four primary cycles you must master: water, carbon, nitrogen, and phosphorus.

We often teach the water cycle as a gentle, passive loop, but it is actually a massive physical engine. The global water cycle is driven primarily by solar energy, which provides the heat required to change the state of water, and is heavily influenced by Earth's gravitational forces, which pull water back toward the core.

Reservoirs and Upward Movement (Overcoming Gravity)

The ocean is the largest reservoir of water on Earth. In fact, oceans contain approximately 97 percent of the water on Earth. To move water out of this immense sink, solar energy must break the hydrogen bonds between liquid molecules.

There are three primary avenues for water to enter the atmosphere:

• Evaporation: This is the physical process of liquid water turning into water vapor.
• Transpiration: This is the biological release of water vapor from plant surfaces into the atmosphere. As plants open their stomata to acquire carbon dioxide, they inevitably lose water.
• Sublimation: In colder climates, sublimation is the direct phase transition of solid ice or snow into water vapor, bypassing the liquid phase entirely.

Ecologists frequently use the term evapotranspiration, which is the combined loss of water to the atmosphere from both land surfaces and plant leaves, to measure the total water vapor entering the air from a terrestrial ecosystem.

Downward Movement (Yielding to Gravity)

Once aloft, water vapor cools. Condensation is the physical conversion of atmospheric water vapor into liquid water droplets, forming clouds. Eventually, gravity reclaims this mass: precipitation occurs when atmospheric water falls to the surface of Earth as rain or snow.

Upon striking land, water has two primary fates:

1. Infiltration: This is the downward movement of surface water into soil and porous rock. Deep below, groundwater is water stored in subterranean porous rock layers called aquifers.
2. Surface runoff: When precipitation outpaces infiltration, surface runoff is the flow of liquid water over land when the soil is completely saturated. Runoff plays a massive role in shaping landscapes and, as we will see, transporting nutrients into aquatic ecosystems.

Carbon is the primary structural element of all biological organic molecules. Because carbon forms the backbone of life, the carbon cycle is intimately tied to the biological processes of energy acquisition and use.

The Fast Cycle: Biology

Biological organisms actively trade carbon with the atmosphere.

• Photosynthesis removes carbon dioxide gas from the atmosphere. Through this process, photosynthetic organisms use carbon dioxide to synthesize organic carbohydrates.
• To unlock the energy stored in those carbohydrates, cellular respiration chemically breaks down organic molecules. Consequently, cellular respiration by living organisms releases carbon dioxide into the atmosphere.
• When organisms die, decomposition of organic matter by soil microbes releases carbon dioxide into the environment, completing the biological loop.

Source: Auto-and heterotrophs by Derivative by Mikael Häggström , using originals by Laghi l , BorgQueen , Benjah-bmm27 , Rkitko , Bobisbob , Jacek FH , Laghi L and Jynto, CC BY-SA 3.0.

The Slow Cycle: Oceans and Geology

While biology moves carbon in days or years, geology and oceanography move it over millennia.

The ocean is the largest active carbon reservoir on Earth. At the water's surface, a constant chemical exchange occurs: carbon dioxide diffuses constantly between the atmosphere and the surface waters of the ocean. Once dissolved, water and carbon dioxide react. Dissolved carbon dioxide in ocean water reacts to form carbonic acid, which subsequently dissociates into bicarbonate ions.

This is where the biological intersects with the geological. Marine organisms use dissolved bicarbonate ions to construct calcium carbonate shells. Over millions of years, calcium carbonate shells from dead marine organisms accumulate on the ocean floor to form limestone. Because of this slow accumulation, sedimentary rocks containing limestone represent the largest long-term carbon reservoir on Earth.

Eventually, subduction drags these rocks deep into the Earth, where heat melts them. To return this carbon to the atmosphere naturally, volcanic eruptions naturally release carbon dioxide from the mantle of Earth into the atmosphere.

The Human Impact

The modern environmental crisis is fundamentally a disruption of residence time.

1. The combustion of fossil fuels rapidly transfers carbon from geological reservoirs to the atmosphere, bypassing the millions of years it took to sequester it.
2. Simultaneously, deforestation reduces the total amount of carbon stored in the terrestrial biosphere, removing the very "sinks" that could absorb that excess atmospheric carbon.

If you ask a student what air is made of, they usually say oxygen. In reality, the atmosphere is the largest reservoir of nitrogen on Earth, and nitrogen gas makes up approximately 78 percent of the atmosphere by volume.

Source: Nitrogen Cycle by Cicle_del_nitrogen_de.svg : * Cicle_del_nitrogen_ca.svg : Johann Dréo ( User:Nojhan ), traduction de Joanjoc d'après Image:Cycle azote fr.svg . derivative work: Burkhard ( talk ) Nitrogen_Cycle.jpg : Environmental Protection Agency derivative work: Raeky ( talk ), CC BY-SA 3.0.

This presents a biological paradox. Nitrogen is a critical elemental component of amino acids and nucleic acids. Life needs nitrogen constantly to build proteins and DNA. Yet, despite swimming in an ocean of nitrogen gas ($N_2$), most biological organisms cannot directly utilize atmospheric nitrogen gas because its atoms are locked together by an incredibly stable triple covalent bond.

Fixing the Unusable

To make nitrogen biologically available, the triple bond must be broken.

• Nitrogen fixation is the chemical conversion of atmospheric nitrogen gas into ammonia ($NH_3$).
• This intense chemical feat requires massive energy. In nature, lightning strikes cause abiotic nitrogen fixation in the atmosphere.
• However, the vast majority is handled biologically. Biological nitrogen fixation is performed exclusively by specific bacteria and cyanobacteria. Some of these microbes are free-living in the soil, while some nitrogen-fixing bacteria live in symbiotic relationships within the root nodules of leguminous plants (like beans, peas, and clover).

Source: Root nodules on fava bean plant by Jengod, CC BY-SA 4.0.

The Microbial Assembly Line: Nitrification

Once converted to ammonia, the nitrogen enters a bacterial assembly line. Nitrification is the biological oxidation of ammonia into nitrites and subsequently into nitrates.

Teacher's Note for Exam Prep: You must know the specific actors here.

1. Nitrosomonas bacteria oxidize ammonia into nitrites during the nitrogen cycle.
2. Passing the baton, Nitrobacter bacteria oxidize nitrites into nitrates during the nitrogen cycle.

Uptake, Use, and Return

Once in the form of nitrates or ammonia, plants can finally use it. Assimilation is the process by which plants incorporate nitrates or ammonia into plant tissues. Because animals cannot pull nitrogen from the soil or air, animals acquire necessary nitrogen strictly by consuming plants or other animals.

When plants and animals produce waste or die, the nitrogen must be recycled. Ammonification is the biological conversion of organic nitrogen from dead tissues or animal waste into ammonia.

Finally, to close the loop, nitrogen must be returned to the atmosphere. Denitrification is the biological reduction of nitrates into atmospheric nitrogen gas. This process is handled by a different suite of microbes: denitrifying bacteria typically thrive in anaerobic soil environments (like waterlogged bogs or deep muds), where they use nitrates instead of oxygen for their own respiration.

Human Intervention: The Haber-Bosch Process

In the early 20th century, humans learned how to break the triple bond of nitrogen gas industrially. Industrial nitrogen fixation through the Haber-Bosch process produces artificial agricultural fertilizers. While this allowed the global human population to explode, it came at a cost. Excessive application of agricultural nitrogen fertilizers leads to harmful nutrient runoff into waterways, disrupting local ecosystems.

The phosphorus cycle is the outlier of the major biogeochemical cycles. It is the slow, steady grind of geology.

Source: Phosphorus cycle by Bonniemf Incorporates work by NASA Earth Science Enterprise, CC BY-SA 3.0.

Phosphorus is an essential component of nucleic acids, cell membranes, and ATP. Without it, life cannot store genetic information, define cellular boundaries, or transfer microscopic energy. Yet, unlike carbon, nitrogen, and water, the phosphorus cycle lacks a significant atmospheric gaseous phase. Dust particles may blow in the wind, but there is no phosphorus-based gas equivalent to $CO_2$ or $N_2$.

Because it is strictly bound to land and water, the phosphorus cycle operates at a much slower rate than the carbon and nitrogen cycles.

The Geologic Conveyor Belt

The largest reservoir of phosphorus on Earth is sedimentary rock. To introduce phosphorus into the biosphere, we rely on the slow destruction of stone.

1. Weathering of rocks gradually releases phosphate ions into soils and bodies of water.
2. Once dissolved in soil moisture, plants absorb dissolved inorganic phosphate directly from the soil.
3. Similar to nitrogen, animals obtain phosphorus solely by consuming plants or other animals.
4. When organisms die, decomposers break down dead organisms to return phosphate compounds to the soil.

What happens to the phosphate that washes into the sea? Dissolved phosphate in aquatic systems frequently settles to the ocean floor. In the dark, crushing depths, settled ocean phosphate slowly forms new sedimentary rock over long geological time scales. It will remain locked there for millions of years until geologic uplift eventually exposes marine sedimentary rocks to the surface of Earth, and the weathering process begins anew.

Note: There is one notable shortcut in the biological phosphorus cycle. Guano deposits from seabirds serve as a highly concentrated natural source of phosphorus. Seabirds eat marine fish (loaded with oceanic phosphorus) and deposit their waste on coastal islands, effectively transporting massive amounts of phosphorus from the ocean back to land.

The Consequence of Acceleration: Eutrophication

Just as we disrupt the nitrogen cycle, we dramatically alter the phosphorus cycle. We mine phosphate rock to create artificial fertilizers. When it rains, excess phosphorus runoff from agricultural fertilizers frequently causes rapid algal blooms in freshwater lakes.

This creates a deadly chain reaction. The algae block sunlight from aquatic plants below, then rapidly die off. As decomposers feast on the dead algae, their cellular respiration consumes all the dissolved oxygen in the water. This phenomenon—eutrophication—is the severe depletion of oxygen in water bodies caused by the decomposition of massive algal blooms. Eutrophication turns vibrant, living lakes into dead zones, a stark reminder of what happens when a naturally slow biogeochemical cycle is flooded by rapid, human-driven inputs.