Biogeochemical Cycles

Core Concepts

The Water (Hydrologic) Cycle

Water cycles between the oceans, atmosphere, land, and organisms. Solar energy drives evaporation (primarily from oceans) and transpiration (from plants). Water vapor condenses and falls as precipitation. On land, water flows as surface runoff and groundwater, eventually returning to oceans. Organisms take up, use, and release water. Major human impacts: groundwater depletion, wetland drainage, altered runoff patterns through urbanization and deforestation.

The Carbon Cycle

Carbon moves between four major reservoirs: the atmosphere (CO₂), the biosphere (organic carbon in living and dead organisms), the oceans (dissolved CO₂, bicarbonate, carbonate, marine organisms), and the lithosphere (fossil fuels, limestone, sedimentary rocks). Key fluxes:

• Photosynthesis removes CO₂ from the atmosphere and fixes it into organic carbon.

• Cellular respiration returns CO₂ to the atmosphere.

• Decomposition releases CO₂ as decomposers break down dead organic matter.

• Ocean-atmosphere exchange: CO₂ dissolves in and is released from ocean surface waters.

• Sedimentation: Organic matter and calcium carbonate shells sink to the ocean floor, sequestering carbon in sediments and eventually rocks (long-term geological cycle).

• Combustion of fossil fuels releases carbon that was stored for millions of years into the atmosphere over decades. This is the primary driver of anthropogenic climate change.

• Deforestation reduces carbon uptake and releases stored carbon from biomass and soils.

The Nitrogen Cycle

Nitrogen is essential for proteins and nucleic acids. The atmosphere is 78% N₂, but N₂ is biologically unavailable to most organisms (the triple bond is extremely strong). The nitrogen cycle makes nitrogen available through several microbially mediated transformations:

• Nitrogen fixation: Conversion of N₂ to ammonia (NH₃) or ammonium (NH₄⁺). Performed by: free-living soil bacteria (e.g., Azotobacter), symbiotic bacteria in root nodules of legumes (Rhizobium), cyanobacteria in aquatic and soil environments, and lightning (a small contribution). Nitrogen fixation requires substantial energy (ATP) and the enzyme nitrogenase, which is inhibited by oxygen (hence the anaerobic environment of root nodules).

• Nitrification: Conversion of ammonium (NH₄⁺) to nitrite (NO₂⁻) and then to nitrate (NO₃⁻) by nitrifying bacteria (Nitrosomonas, Nitrobacter). Nitrate is the form most readily taken up by plants.

• Assimilation: Uptake of ammonium or nitrate by plants and incorporation into amino acids, proteins, and nucleic acids. Animals obtain nitrogen by consuming plants or other animals.

• Ammonification: Decomposers (bacteria and fungi) break down organic nitrogen (proteins, nucleic acids) from dead organisms and wastes, releasing ammonium (NH₄⁺) back into the soil.

• Denitrification: Conversion of nitrate (NO₃⁻) back to N₂ gas by denitrifying bacteria under anaerobic conditions. This returns nitrogen to the atmosphere, closing the cycle.

Human impacts on the nitrogen cycle

• Haber-Bosch process: Industrial nitrogen fixation for fertilizer production has doubled the global rate of nitrogen fixation, dramatically increasing the reactive nitrogen in the biosphere.

• Fertilizer runoff causes eutrophication of aquatic ecosystems.

• Fossil-fuel combustion releases nitrogen oxides (NOₓ), contributing to acid rain and photochemical smog.

The Phosphorus Cycle

Phosphorus is essential for nucleic acids, ATP, and phospholipids. Unlike carbon and nitrogen, phosphorus has no gaseous phase in its cycle under normal Earth-surface conditions — it cycles primarily through rocks, soil, water, and organisms.

• Weathering of phosphate-containing rocks releases phosphate (PO₄³⁻) into soil and water.

• Plants absorb phosphate from soil; animals obtain it from food.

• Decomposition returns phosphate to soil and water.

• Phosphate in aquatic systems eventually sediments and forms new phosphate rocks over geological time — a very slow process.

• Phosphorus is often the limiting nutrient for primary production in aquatic ecosystems and many terrestrial soils.

Human impacts: Phosphate mining for fertilizer, fertilizer runoff (eutrophication), and detergent phosphates (now largely regulated) increase phosphate in waterways, causing algal blooms and oxygen depletion.

ELI-10

The atoms in your body have been around for billions of years — they were once in rocks, in the air, in dinosaurs, in ancient bacteria. They get reused over and over. That is what a biogeochemical cycle is: the path an atom takes as it moves through the living and nonliving parts of the Earth.

The water cycle is the easiest to see: sun evaporates water, water vapor forms clouds, rain falls, rivers carry it back to the ocean. Repeat.

The carbon cycle: plants pull CO₂ out of the air (photosynthesis) and turn it into sugar. Animals eat the plants and breathe the carbon back out (respiration). When things die, decomposers release the carbon. Over millions of years, some dead organisms got buried and turned into coal and oil — fossil fuels. We dug them up and burned them, releasing millions of years of stored carbon in a few decades. That is climate change in a nutshell.

The nitrogen cycle: the air is 78% nitrogen, but it is locked up as N₂ — triple-bonded and unusable. Only certain bacteria can break that bond (nitrogen fixation) and turn it into forms plants can use. It is one of the most important biochemical tricks on Earth. Without it, life as we know it would starve for lack of usable nitrogen. We now make nitrogen fertilizer industrially, doubling nature’s nitrogen fixation — but the excess runs off into rivers and oceans, causing algal blooms and dead zones.

The phosphorus cycle: phosphorus comes from rocks, not the air. It washes into soil and water, plants absorb it, animals eat the plants, decomposition returns it. It is slow, and there is no atmospheric backup supply. That is why phosphorus is often the limiting nutrient — the one that runs out first. Adding too much (fertilizer runoff) causes the same overgrowth problems as excess nitrogen.

The golden rule: energy FLOWS (sun → heat). Matter CYCLES (atoms get reused). These are two different rules, and keeping them straight is essential.

ELI Example

Picture Earth as a giant recycling center. The water cycle is the plumbing — evaporation, rain, runoff. The carbon cycle is the energy economy — carbon atoms build everything from sugar to trees to oil, constantly moving between the air, living things, and buried storage. The nitrogen cycle is the fertilizer factory — bacteria are the only workers who can pull nitrogen out of the air and make it usable. The phosphorus cycle is the slow rock-to-soil-to-life pathway — no atmospheric shortcut. Humans have turbocharged the nitrogen and carbon cycles (fertilizer factories, fossil fuel burning), and the recycling center is struggling to keep up.

Do Not Confuse

• Nitrogen Fixation vs. Nitrification vs. Denitrification: Fixation = N₂ → NH₃/NH₄⁺ (make usable). Nitrification = NH₄⁺ → NO₃⁻ (convert form). Denitrification = NO₃⁻ → N₂ (return to atmosphere).

• Ammonification vs. Denitrification: Ammonification = organic N → NH₄⁺ (decomposers releasing ammonium). Denitrification = NO₃⁻ → N₂ (anaerobic bacteria returning to atmosphere). Different processes, different organisms, different products.

High-Yield Memory Anchors

• Water: evaporation → precipitation → runoff. Solar-powered.

• Carbon: photosynthesis (CO₂ → organic) ⇄ respiration (organic → CO₂). Fossil-fuel combustion adds ancient carbon to atmosphere.

• Nitrogen: Fixation (N₂ → NH₄⁺), Nitrification (NH₄⁺ → NO₃⁻), Assimilation, Ammonification, Denitrification (NO₃⁻ → N₂). Bacteria do the heavy lifting.

• Phosphorus: rocks → soil → organisms → sediments. No atmospheric component. Often limiting.

• Energy flows. Matter cycles.

Quick Check

Q1: Which process converts atmospheric nitrogen (N₂) into a form usable by plants?

A) Nitrification

B) Denitrification

C) Nitrogen fixation

D) Ammonification

Q2: A farmer applies ammonium nitrate fertilizer to a field just before a heavy rain. Explain what happens to the nitrogen in the fertilizer as it enters nearby aquatic ecosystems, and what ecological consequences result.

Q3: Compare the reservoirs of the carbon and phosphorus cycles. Why does carbon have a significant atmospheric component while phosphorus does not? How does this difference affect the speed at which each cycle can respond to perturbations?

Quick Check Answers

A1: C. Nitrogen fixation. Nitrogen fixation converts N₂ to ammonia/ammonium. Nitrification converts ammonium to nitrate. Denitrification converts nitrate back to N₂. Ammonification releases ammonium from organic matter.

A2: The fertilizer contains ammonium (NH₄⁺) and nitrate (NO₃⁻). Rain washes these into nearby streams, ponds, or lakes. The sudden influx of nitrogen (often together with phosphorus) causes eutrophication: (1) Algal and cyanobacterial populations explode (algal bloom). (2) The bloom blocks light, killing submerged aquatic plants. (3) When the algae die, decomposition by aerobic bacteria consumes dissolved oxygen. (4) Oxygen depletion (hypoxia) kills fish and other aerobic organisms, creating a “dead zone.” (5) Under anoxic conditions, denitrification may return some nitrogen to the atmosphere, but the ecosystem damage can persist.

A3: Carbon has a large atmospheric reservoir (CO₂) and exchanges rapidly between atmosphere, biosphere, and ocean surface. This fast cycling means atmospheric CO₂ can respond relatively quickly to changes in emissions or uptake. Phosphorus has no gaseous phase at Earth-surface conditions. Its major reservoir is rocks, and it enters ecosystems through the slow process of weathering. Once in aquatic sediments, phosphorus can be sequestered for geological timescales. Because phosphorus lacks a rapid atmospheric pathway, its cycle responds much more slowly to perturbations — but once loaded into an ecosystem (e.g., from fertilizer runoff), phosphorus can persist and recycle internally for long periods, sustaining eutrophic conditions long after inputs are reduced.

Chapter Summary

The water, carbon, nitrogen, and phosphorus cycles describe the movement of essential elements through biotic and abiotic compartments. Carbon cycles rapidly between atmosphere and biosphere through photosynthesis and respiration; fossil-fuel combustion releases ancient carbon. The nitrogen cycle depends on bacterial transformations: fixation, nitrification, ammonification, and denitrification. The phosphorus cycle is primarily geological, lacks an atmospheric component, and is often the limiting nutrient. Human activities have dramatically altered these cycles at global scales.

Common Mistakes

• “Plants get nitrogen directly from the air.” Plants cannot use N₂. They absorb nitrogen as nitrate (NO₃⁻) or ammonium (NH₄⁺) from the soil, which must first be produced by nitrogen-fixing and nitrifying bacteria.

• “Phosphorus cycles through the atmosphere like carbon and nitrogen.” Phosphorus lacks a gaseous phase and cycles primarily through rocks, soil, water, and organisms. There is no significant atmospheric reservoir.