Cycles of Matter: Carbon, Oxygen & Nitrogen Cycles: Sixth Grade Science
While energy flows through the biosphere in a one-way path, the essential chemical atoms of life cycle perpetually between living organisms and Earth's abiotic reservoirs.
The Eternal Recycling of Atoms
Every breath of air you draw into your lungs contains nitrogen and carbon atoms that were once exhaled by prehistoric dinosaurs, incorporated into ancient fern fronds during the Carboniferous period, dissolved in primordial oceans, or blasted out of ancient volcanic fissures. Unlike radiant solar energy, which travels through ecosystems once before dissipating into the cosmos as heat, chemical matter on planet Earth is finite. Earth is essentially a closed thermodynamic system with respect to matter: barring the occasional meteorite, our planet has the exact same quantity of matter today that it possessed 4.5 billion years ago. To sustain life, chemical elements must cycle continuously between the living (biotic) and non-living (abiotic) realms through biogeochemical cycles (from bio meaning life, geo meaning earth, and chemical).
BIOGEOCHEMICAL CYCLE ARCHITECTURE
[ ABIOTIC RESERVOIRS ] [ BIOTIC COMMUNITY ]
* Atmosphere (Air) * Producers (Plants)
* Hydrosphere (Oceans, Lakes) <=========> * Consumers (Animals)
* Lithosphere (Rocks, Soil) * Decomposers (Microbes)
The Carbon-Oxygen Cycle: Photosynthesis and Respiration
Carbon is the universal structural backbone of all biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids. Oxygen is essential for aerobic cellular respiration and is an integral component of water and organic molecules. Because carbon and oxygen are intimately linked through photosynthesis and cellular respiration, their environmental pathways form a tightly coupled cycle.
THE GLOBAL CARBON CYCLE
Atmospheric CO2 Gas (~0.04%)
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^ |
Cellular Respiration / | Photosynthesis
& Fossil Fuel Burn / v
[ CONSUMERS ] <--- Feeding --- [ PRODUCERS ]
[ & DECOMP. ] (Trees, Phytoplankton)
| |
v (Dead Tissue Buried) v (Dead Leaves)
[ FOSSIL FUELS ] <===================='
(Coal, Oil, Natural Gas in Lithosphere)
Atmospheric Carbon Reservoir
Carbon exists in Earth's atmosphere primarily as carbon dioxide gas (CO2). Although comprising roughly 0.04 percent of the atmosphere (approximately 420 parts per million), this atmospheric pool is critical:
- Biological Uptake (Photosynthesis): Terrestrial plants and marine phytoplankton absorb atmospheric and dissolved CO2, using solar energy to chemically fix carbon into glucose (C6H12O6), simultaneously releasing oxygen gas (O2) into the air.
- Biological Release (Cellular Respiration): All living eukaryotes—plants, animals, fungi, and protists—perform cellular respiration, combining glucose with oxygen to synthesize ATP, releasing CO2 and water back into the atmosphere.
- Geologic Storage (Fossil Fuels): Hundreds of millions of years ago, vast volumes of dead plant matter (in Carboniferous swamps) and marine plankton (on ocean floors) were buried beneath thick sediments before decomposers could break them down. Under immense heat and pressure over geological time, this buried organic carbon transformed into fossil fuels: coal, crude oil, and natural gas. When humans combust fossil fuels for energy, this long-sequestered carbon is rapidly returned to the atmosphere as CO2.
- Oceanic Carbonate Reservoir: Earth's oceans hold roughly 50 times more carbon than the atmosphere. Dissolved CO2 reacts with water to form carbonic acid (H2CO3) and bicarbonate ions (HCO3-). Marine organisms (corals, mollusks, foraminifera) combine calcium ions with bicarbonate to construct calcium carbonate (CaCO3) shells. When these organisms die, their shells accumulate on the ocean floor, compressing over millions of years to form limestone rock, sequestering carbon for geological epochs.
The Nitrogen Cycle: From Air to Protein
Nitrogen is an essential biological element required to build amino acids (the monomers of proteins) and nucleotides (the building blocks of DNA, RNA, and ATP).
Although nitrogen gas (N2) comprises approximately 78 percent of Earth's atmosphere, plants and animals cannot use atmospheric nitrogen gas directly! The two nitrogen atoms in an N2 molecule are held together by an extraordinarily strong, unreactive triple covalent bond (N#N). Animals and plants lack the biochemical enzymes required to break this triple bond. To enter living systems, nitrogen must be converted into biologically usable forms (ammonia, nitrates) through the nitrogen cycle, driven by specialized soil and aquatic bacteria.
THE NITROGEN CYCLE
Atmospheric Nitrogen Gas (N2: 78%)
==================================
^ |
/ Denitrification | Nitrogen Fixation
/ (Pseudomonas) | (Rhizobium in legumes)
/ v
[ NITRATES: NO3- ] [ AMMONIA: NH3 / NH4+ ]
^ |
| Nitrification | Nitrification
| (Nitrobacter) | (Nitrosomonas)
+--- [ NITRITES: NO2- ] <'
The Five Steps of the Nitrogen Cycle
SUMMARY OF NITROGEN CYCLE PROCESSES
Process Starting Molecule Ending Molecule Key Organisms
---------------------------------------------------------------------------------
Nitrogen Fixation Nitrogen gas (N2) Ammonia (NH3 / NH4+) Rhizobium, Cyanobacteria
Nitrification Ammonia (NH3) Nitrites (NO2-) then Nitrosomonas &
Nitrates (NO3-) Nitrobacter bacteria
Assimilation Nitrates (NO3-) Plant Proteins & DNA Plant root absorption
Ammonification Dead organic tissue Ammonia (NH3) Decomposers (Fungi/Bact)
Denitrification Nitrates (NO3-) Nitrogen gas (N2) Denitrifying bacteria
Step 1: Nitrogen Fixation
The conversion of inert atmospheric nitrogen gas (N2) into biologically usable ammonia (NH3) or ammonium (NH4+):
- Biological Fixation: Driven by specialized nitrogen-fixing bacteria. Many live symbiotically inside specialized root nodules of legume plants (beans, peas, clover, alfalfa), such as Rhizobium. The plant supplies the bacteria with photosynthesized sugars, while the bacteria provide the plant with fixed ammonia. Free-living soil bacteria (Azotobacter) and aquatic cyanobacteria (Anabaena) also fix nitrogen.
- Atmospheric Fixation: Lightning strikes possess enough energy to break N2 triple bonds, allowing nitrogen to react with atmospheric oxygen to form nitrates that fall in precipitation (accounting for about 10 percent of natural fixation).
- Industrial Fixation: The human-engineered Haber-Bosch process uses high temperature, pressure, and natural gas to manufacture chemical synthetic nitrogen fertilizers.
Step 2: Nitrification
A two-step aerobic process performed by specialized nitrifying soil bacteria:
- Soil bacteria (Nitrosomonas) oxidize ammonia (NH3) into nitrite ions (NO2-).
- A second group of bacteria (Nitrobacter) oxidizes toxic nitrites into nitrate ions (NO3-), the primary chemical form of nitrogen preferred by plant roots.
Step 3: Assimilation
Plant roots absorb dissolved nitrates (NO3-) and ammonium (NH4+) from soil water via active transport. Inside plant cells, the nitrogen is incorporated (assimilated) into organic molecules: amino acids, structural proteins, and nucleic acids. Animals obtain organic nitrogen by consuming plants or other animals.
Step 4: Ammonification (Decomposition)
When plants and animals die, or produce nitrogenous metabolic waste (urea, uric acid, feces), decomposers (fungi and heterotrophic bacteria) break down the organic nitrogen compounds, releasing inorganic ammonia (NH3) back into the soil.
Step 5: Denitrification
To prevent all nitrogen from remaining trapped in soil and oceans, the nitrogen cycle must close the loop back to the atmosphere. Under anaerobic (oxygen-poor) conditions—such as waterlogged soils, deep swamps, and ocean sediments—specialized denitrifying bacteria (such as Pseudomonas) convert soil nitrates (NO3-) back into nitrogen gas (N2) and nitrous oxide (N2O), releasing nitrogen back into the atmosphere.
The Phosphorus Cycle: The Sedimentary Cycle
Unlike carbon, oxygen, and nitrogen, the phosphorus cycle does not include an atmospheric gas phase! Phosphorus is stored almost exclusively in the lithosphere (rocks and mineral sediments) and hydrosphere.
Phosphate ions (PO4^3-) are released slowly over thousands of years through the chemical and mechanical weathering of phosphate-bearing rocks. Rainwater dissolves phosphates, carrying them into soil where plant roots absorb them for constructing DNA, RNA, ATP, and cell membrane phospholipids. Animals consume plants to acquire phosphorus, and decomposers return phosphates to the soil.
Because geological weathering is sluggish, phosphorus is often the primary limiting nutrient in freshwater and terrestrial ecosystems. When humans mine phosphate rock to manufacture commercial agricultural fertilizers, agricultural runoff flushes excess phosphorus and nitrogen into lakes and rivers, triggering a destructive ecological cascade known as eutrophication:
THE EUTROPHICATION CASCADE
Excess Fertilizer Runoff (N & P) flushes into Lake
|
v
MASSIVE ALGAL BLOOM (Green scum blankets surface!)
|
v
Algae die and sink to bottom; DECOMPOSERS MULTIPLY!
|
v
Decomposing bacteria consume all DISSOLVED OXYGEN via Respiration!
|
v
HYPOXIA (Dead Zone): Fish and aquatic animals suffocate and die!
Exercises and Solutions
Exercise 1: Describe the complementary relationship between photosynthesis and cellular respiration in the global carbon-oxygen cycle, identifying the primary carbon reservoir in the atmosphere.
Solution 1: Photosynthesis and cellular respiration are complementary biochemical processes. Photosynthetic producers absorb carbon dioxide gas (CO2) from the atmosphere and combine it with water using sunlight to synthesize glucose, releasing oxygen gas (O2) as a byproduct. Aerobic organisms consume glucose and breathe in oxygen to perform cellular respiration, releasing carbon dioxide and water back into the atmosphere. The primary carbon reservoir in the atmosphere is carbon dioxide gas (CO2), currently present at approximately 0.04 percent (420 parts per million).
Exercise 2: Why are plants and animals unable to absorb and use atmospheric nitrogen gas (N2) directly, and which specific group of organisms makes nitrogen available to biological life?
Solution 2: Plants and animals cannot use atmospheric nitrogen gas directly because the two nitrogen atoms in an N2 molecule are bound by an exceptionally strong triple covalent bond that plants and animals lack the enzymes to break. Specialized nitrogen-fixing bacteria (such as Rhizobium living in legume root nodules and free-living cyanobacteria) possess the enzyme nitrogenase, which breaks the triple bond and converts N2 gas into ammonia (NH3) and nitrates (NO3-), forms that plant roots can absorb and assimilate into proteins and DNA.
Exercise 3: Explain why the phosphorus cycle is described as a sedimentary cycle, and describe the ecological process of cultural eutrophication caused by excess fertilizer runoff.
Solution 3: The phosphorus cycle is classified as a sedimentary cycle because phosphorus has no significant gaseous atmospheric phase; its primary environmental reservoir is rocks and mineral sediments in the lithosphere, released slowly through geological weathering. Cultural eutrophication occurs when excess agricultural fertilizers containing nitrogen and phosphorus wash into lakes or rivers. This nutrient surge triggers explosive algal blooms that block sunlight; when the algae die, decomposing bacteria multiply and consume nearly all dissolved oxygen in the water through respiration, creating a hypoxic dead zone where fish and aquatic life suffocate.