24.3 Earth Cycles: Water, Rock, Carbon & Nitrogen

Overview & Exam Relevance

Competency 016 of the TExES Core Subjects EC-6 Science exam (Subject Exam 904) assesses your pedagogical and conceptual understanding of Earth's major cycles: the hydrologic (water) cycle, the rock cycle, and biogeochemical cycles including the carbon cycle and nitrogen cycle. Earth operates as a closed thermodynamic system with respect to matter—atoms are neither created nor destroyed, but continuously cycled among the geosphere (lithosphere), hydrosphere, atmosphere, and biosphere—driven by solar radiation and geothermal heat.

The elementary TEKS build from observing water in different forms and weather patterns in the early grades, to describing and illustrating the water cycle and the Sun's role as its main energy source (Grade 4), to exploring weather, climate, and the management of natural resources in Grade 5.

On the TExES 391 examination, test questions routinely target subtle thermodynamic and ecological concepts: distinguishing between evaporation and transpiration, calculating groundwater flow dynamics across permeable aquifers (such as the Edwards Aquifer in Texas), tracing carbon fluxes across living and geological reservoirs, and addressing persistent student misconceptions regarding phase changes and conservation of matter.

---

The Hydrologic (Water) Cycle

The hydrologic cycle is the continuous, solar- and gravity-driven circulation of water ($H_2O$) among Earth's oceans, atmosphere, land surfaces, and subterranean aquifers.

THE GLOBAL HYDROLOGIC CYCLE

                       ┌─────────────────────────┐
                       │   SOLAR RADIATION       │
                       │   (Primary Thermal Pump)│
                       └────────────┬────────────┘
                                    │
                                    ▼
                  ┌───────────────────────────────────┐
                  │     CONDENSATION (Clouds/Fog)     │ ◄─────── Cloud Condensation
                  │     * Releases Latent Heat *      │          Nuclei (Aerosols)
                  └─────────────────┬─────────────────┘
                                    │ Precipitation
                                    │ (Gravity-Driven)
                                    ▼
       Evaporation ┌──────────────────────────────────┐ Surface Runoff
      ┌────────────┤ LAND SURFACE & VEGETATION        ├────────────┐
      │            │ (Transpiration via Stomata)      │            │
      │            └────────────────┬─────────────────┘            │
      │                             │ Infiltration &               │
      │                             │ Deep Percolation             ▼
      │                             ▼                    ┌──────────────────┐
      │            ┌──────────────────────────────────┐  │ OCEANS & SEAS    │
      │            │ GROUNDWATER AQUIFERS             │  │ (97.2% of Earth's│
      │            │ (Water Table, Karst Conduits)    ├──► Water Volume)   │
      │            └──────────────────────────────────┘  └────────┬─────────┘
      │                                                           │ Evaporation
      └───────────────────────────────────────────────────────────┘ (Latent Heat Absorbed)

1. Primary Physical Mechanisms

• Evaporation: The phase change of liquid water into gaseous water vapor occurring at the surfaces of oceans, lakes, streams, and damp soils below the boiling point. Solar insolation supplies the necessary latent heat of vaporization (approximately $2,260\text{ J/g}$), breaking intermolecular hydrogen bonds. Oceans account for approximately 86% of global evaporation.
• Transpiration: The biological evaporation of water from the internal tissues of plants into the atmosphere, predominantly through microscopic leaf pores called stomata. Vascular plants absorb liquid water and dissolved mineral nutrients through root hairs, transport it upward via xylem vessels through capillary action and negative transpirational pull, and release excess moisture as vapor. In heavily vegetated biomes, combined evapotranspiration contributes significant moisture to the regional atmosphere.
• Condensation: The phase change of gaseous water vapor into liquid water droplets or solid ice crystals. As warm, moist air expands and cools adiabatically upon rising, its relative humidity climbs until reaching saturation (100% relative humidity) at the dew point temperature. Gaseous water molecules coalesce around microscopic suspended aerosol particles known as cloud condensation nuclei (CCN)—such as sea salt crystals, volcanic dust, or smoke particles—forming visible clouds or fog. Crucially, condensation releases stored latent heat into the surrounding atmosphere, fueling convection in thunderstorms and hurricanes.
• Precipitation: Water in liquid or solid form that falls from clouds under the downward force of gravity when coalescing droplets or ice crystals become too massive to remain suspended by atmospheric updrafts. Precipitation manifests as rain, snow, sleet (rain freezing into ice pellets while falling through a deep sub-freezing boundary layer), hail (concentric layers of ice accreted in severe cumulonimbus updrafts), or freezing rain (supercooled liquid droplets that freeze instantaneously upon striking subfreezing surfaces).
• Infiltration and Percolation: When precipitation reaches land surfaces, a portion soaks into the upper soil horizon through infiltration. Driven by gravity, this infiltrated water continues migrating downward through permeable sediment pores and fractured bedrock via percolation, replenishing subterranean moisture reserves.
• Groundwater & Aquifer Systems:
  • Zone of Aeration (Vadose Zone): Upper subsurface zone where pore spaces are filled with both air and water.
  • Water Table: The dynamic upper boundary of the zone of saturation. During periods of heavy precipitation, the water table rises; during extended droughts or excessive agricultural pumping, it drops.
  • Zone of Saturation: Subterranean rock and sediment layers where all pore spaces and fractures are completely filled with liquid water.
  • Porosity vs. Permeability: Porosity represents the percentage of total void space within a rock or sediment volume available to hold water. Permeability represents the interconnectedness of these void spaces, governing the speed and ease with which fluids transmit through the rock. Clay exhibits high porosity but extremely low permeability (trapping water), whereas gravelly sand exhibits both high porosity and high permeability.
  • Texas Water Focus: The Edwards Aquifer: A world-renowned karst aquifer in Central Texas developed within porous, faulted Cretaceous limestone. The Edwards Aquifer serves as the primary municipal drinking water source for over two million people (including San Antonio) and supports endangered aquatic species at major discharge points (such as Comal Springs and San Marcos Springs). Because of widespread limestone dissolution sinkholes, fractured recharge zones, and high permeability, the Edwards Aquifer responds rapidly to rainfall recharge but is exceptionally vulnerable to surface pollutants.
• Surface Runoff & Watershed Dynamics: When precipitation rates exceed the infiltration capacity of soil (or when urban landscapes are sealed with impermeable asphalt and concrete), excess water flows overland under gravity as surface runoff. Runoff collects into rills, gullies, streams, and rivers within a watershed (drainage basin)—a geographic land area that channels all rainfall and snowmelt into a common river outlet, lake, or bay.

2. Latent Heat & Earth's Thermodynamic Energy Transfer

The water cycle functions as Earth's premier thermodynamic heat engine:

1. At tropical latitudes, intense solar radiation drives immense surface evaporation, absorbing vast quantities of thermal energy into the vapor phase as latent heat.
2. Atmospheric wind belts and low-pressure systems transport this moist, warm air poleward and vertically into the troposphere.
3. Upon reaching cooler altitudes, water vapor condenses, releasing that latent heat directly into the upper atmosphere.
4. This continuous phase-change cycle redistributes solar energy from the equator toward the poles, mitigating planetary temperature extremes.

---

The Rock Cycle as an Earth System Cycle

While the water cycle operates over hours, days, and years, the rock cycle cycles geological materials over millions of years through interactions among the lithosphere, atmosphere, and hydrosphere:

COUPLING: HYDROLOGIC & ROCK CYCLES

Atmospheric CO2 + Rainwater ──► Carbonic Acid (H2CO3) ──► Chemical Weathering of Silicates
                                                              │
                                                              ▼
Sediment Deposition in Basins ◄── River Transport of Ions ◄── Leached Calcium & Bicarbonate
            │
            ▼
Compaction & Cementation ──► Sedimentary Limestone & Shale ──► Subduction & Metamorphism

• Weathering & Hydrosphere Coupling: Rainwater enriched with atmospheric carbon dioxide forms weak carbonic acid, chemically dissolving continental rocks and leaching calcium, magnesium, and carbonate ions into runoff.
• Oceanic Sedimentation: Rivers transport billions of tons of dissolved ions and weathered clastic sediments to continental shelves and ocean floors. Marine organisms (corals, foraminifera, mollusks) extract dissolved calcium and carbonate to construct shells of calcium carbonate ($\text{CaCO}_3$). When these organisms perish, their skeletal remains accumulate as thick marine oozes that compact into limestone.
• Tectonic Recycling: Plate tectonic subduction drags ocean-floor sedimentary rocks deep into the asthenosphere, where elevated heat and pressure cause metamorphic alteration or partial melting into magma. Volcanic eruptions subsequently return carbon dioxide and water vapor to the atmosphere, completing a multimillion-year geological loop.

---

The Biogeochemical Carbon Cycle

Carbon is the fundamental structural backbone of all organic biomolecules (carbohydrates, lipids, proteins, nucleic acids). The carbon cycle tracks the continuous biogeochemical flux of carbon among four interconnected planetary reservoirs:

THE GLOBAL CARBON CYCLE RESERVOIRS & FLUXES

                      ┌────────────────────────┐
                      │  ATMOSPHERIC RESERVOIR │
                      │  (CO2 and CH4)         │
                      └───┬────────▲───────▲───┘
           Photosynthesis │        │       │ Combustion of Fossil Fuels
           (Fixation)     │        │       │ & Deforestation
                          ▼        │       │
    ┌────────────────────────┐     │ Respiration &   ┌────────────────────────┐
    │  BIOSPHERIC RESERVOIR  ├─────┤ Decomposition   │ LITHOSPHERIC RESERVOIR │
    │  (Living Flora/Fauna,  │     │                 │ (Coal, Oil, Gas,       │
    │   Soil Organic Humus)  │     │                 │  Limestone Rock)       │
    └─────────────┬──────────┘     │                 └───────────▲────────────┘
                  │ Buried Biomass │                             │ Millions of Years of
                  │ in Anoxic Beds │                             │ Heat and Pressure
                  └────────────────┼─────────────────────────────┘
                                   ▼
                      ┌────────────────────────┐
                      │ HYDROSPHERIC RESERVOIR │
                      │ (Dissolved CO2, HCO3-, │
                      │  Carbonate Shells)     │
                      └────────────────────────┘

1. Major Carbon Reservoirs

• Lithosphere (Crust & Upper Mantle): By far the largest planetary carbon reservoir, containing over 99.9% of Earth's total carbon. Most is locked in sedimentary rocks—predominantly limestone (calcium carbonate, $\text{CaCO}_3$) and dolomite—with a smaller fraction stored as fossilized organic hydrocarbons (coal, crude oil, oil shale, and natural gas).
• Hydrosphere (Oceans): The second-largest reservoir, storing carbon as dissolved inorganic carbon (carbon dioxide gas, carbonic acid, bicarbonate ions $\text{HCO}_3^-$, and carbonate ions $\text{CO}_3^{2-}$), as well as marine biomass.
• Biosphere (Terrestrial & Marine Organisms): Carbon stored within the cellular tissues of living organisms (primarily terrestrial forests) and decomposing soil humus.
• Atmosphere: Carbon stored in gaseous forms, primarily carbon dioxide ($\text{CO}_2$) and methane ($\text{CH}_4$). Although quantitatively smaller than the lithosphere or ocean, atmospheric carbon exerts disproportionate control over global climate via the greenhouse effect.

2. Biological & Geochemical Carbon Fluxes

• Photosynthesis (Biological Uptake): Terrestrial green plants, algae, and marine cyanobacteria/phytoplankton capture atmospheric or dissolved carbon dioxide, utilizing solar photons to synthesize energy-rich glucose while releasing oxygen:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{solar energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Cellular Respiration (Biological Release): Autotrophs and heterotrophs break down organic carbohydrates in mitochondria to generate cellular ATP energy, returning carbon dioxide to the surrounding atmosphere or aquatic environment:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP energy}$

• Decomposition: Fungi and heterotrophic bacteria decompose dead organic carcasses and leaf litter. Under aerobic conditions, decomposers release $\text{CO}_2$. Under anoxic conditions (such as waterlogged bogs, wetlands, and ruminant digestive tracts), methanogenic archaea release methane ($\text{CH}_4$).
• Fossilization & Lithification: Organic matter buried rapidly in anoxic sedimentation basins escapes complete microbial decomposition. Over millions of years of tectonic burial, geothermal heat and confining pressure transform buried plant matter into coal, and marine phytoplankton/zooplankton into petroleum and natural gas.
• Combustion (Anthropogenic & Natural): Rapid chemical oxidation of organic fuels or ancient biomass in the presence of oxygen, releasing thermal energy, water vapor, and carbon dioxide. While natural forest fires are a regular ecosystem reset, the industrial combustion of ancient fossil fuels extracts carbon locked in the lithosphere for hundreds of millions of years and injects it directly into the atmosphere within decades.
• Ocean Acidification: Oceans absorb approximately 25-30% of anthropogenic atmospheric $\text{CO}_2$. When carbon dioxide dissolves in seawater, it forms carbonic acid ($\text{H}_2\text{CO}_3$), which dissociates into hydrogen ions ($\text{H}^+$) and bicarbonate ($\text{HCO}_3^-$):

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

Elevated concentrations of free $\text{H}^+$ ions lower oceanic pH (ocean acidification) and react with ambient carbonate ions ($\text{CO}_3^{2-}$), depleting the carbonate pool required by marine calcifiers (corals, oysters, clams, pteropods) to construct protective calcium carbonate shells ($\text{CaCO}_3$).

---

The Nitrogen Cycle Fundamentals

Although nitrogen comprises approximately 78% of Earth's atmosphere by volume as diatomic nitrogen gas ($\text{N}_2$), it is completely inaccessible to plants and animals in this form because the two nitrogen atoms are held by an extremely strong triple covalent bond ($N \equiv N$). The nitrogen cycle relies on specialized microorganisms to transform inert $\text{N}_2$ into reactive biological compounds:

THE NITROGEN CYCLE PATHWAYS

                   ┌────────────────────────┐
                   │   ATMOSPHERIC N2       │
                   └─────┬────────────▲─────┘
                         │            │ Denitrification
   Biological Fixation   │            │ (Pseudomonas bacteria in
   (Rhizobium in legumes │            │  anoxic/waterlogged soils)
   & Azotobacter)        │            │
                         ▼            │
                   ┌──────────┐       │
                   │ AMMONIA  │       │
                   │  (NH3)   │       │
                   └─────┬────┘       │
       Protonation in    │            │
       moist soil        ▼            │
                   ┌──────────┐       │
                   │ AMMONIUM │       │
                   │  (NH4+)  │       │
                   └─────┬────┘       │
                         │ Nitrification: Nitrosomonas bacteria
                         ▼
                   ┌──────────┐
                   │ NITRITE  │
                   │  (NO2-)  │
                   └─────┬────┘
                         │ Nitrification: Nitrobacter bacteria
                         ▼
                   ┌──────────┐       Assimilation
                   │ NITRATE  ├────────────────────────► Plant Roots
                   │  (NO3-)  │                          (Proteins & DNA)
                   └─────┬────┘                                │
                         ▲                                     ▼
                         │ Ammonification              Herbivore Consumption
                         │ (Decomposition of wastes)           │
                         └─────────────────────────────────────┘

Nitrogen Cycle Steps

1. Nitrogen Fixation: The conversion of gaseous atmospheric $\text{N}_2$ into bioavailable ammonia ($\text{NH}_3$) or ammonium ($\text{NH}_4^+$).
   • Biological Fixation: Accomplished by specialized diazotrophic bacteria possessing the nitrogenase enzyme. Some are free-living in soil (Azotobacter), while others form mutualistic symbiotic relationships with the root nodules of legumes (such as clover, alfalfa, beans, peas, and peanuts) belonging to the genus Rhizobium. The plant provides carbohydrates and an oxygen-shielded microenvironment, while the bacteria provide fixed ammonium.
   • Atmospheric Fixation: Lightning strikes provide extreme thermal energy to break $\text{N}_2$ triple bonds, combining nitrogen with oxygen to form nitrates that wash to soil in rainfall.
2. Nitrification: A two-step aerobic process performed by specialized chemolithoautotrophic soil bacteria:
   • Step 1: Nitrosomonas bacteria oxidize ammonium ($\text{NH}_4^+$) into toxic nitrite ions ($\text{NO}_2^-$).
   • Step 2: Nitrobacter bacteria rapidly oxidize nitrite ($\text{NO}_2^-$) into nitrate ions ($\text{NO}_3^-$).
3. Assimilation: Plant root hairs absorb dissolved nitrates ($\text{NO}_3^-$) and ammonium ($\text{NH}_4^+$) from soil water, incorporating the nitrogen into cellular amino acids, proteins, ATP, and nucleic acids (DNA/RNA). Consumers assimilate nitrogen by eating producers or other consumers.
4. Ammonification (Mineralization): When organisms excrete nitrogenous wastes (urea, uric acid) or die, decomposers (heterotrophic bacteria and fungi) break down biological nitrogen compounds, converting them back into ammonia ($\text{NH}_3$) and ammonium ($\text{NH}_4^+$).
5. Denitrification: In waterlogged, anoxic soils and aquatic sediments, anaerobic bacteria (such as Pseudomonas) utilize nitrates as electron acceptors instead of oxygen for respiration, reducing $\text{NO}_3^-$ back into gaseous diatomic nitrogen ($\text{N}_2$), which escapes to the atmosphere, completing the global cycle.

---

Comparison of Biogeochemical Cycles

---

Classroom Instructional Strategies & Scenario Application

Overcoming Common Student Misconceptions

Exemplary Classroom Inquiry Scenario

Classroom Context: Mr. Henderson is guiding a 5th-grade science inquiry investigation on how energy from the Sun drives the water cycle.

Investigation Activity: Student lab pairs construct a "Water Cycle in a Bag". Each team draws an ocean line, clouds, and the sun on a sealable plastic sandwich bag using permanent markers. They add $50\text{ mL}$ of water dyed with blue food coloring, seal the bag securely, and tape it to a sunny classroom window.

Observations & Guided Formative Questions:

1. Thermal Absorption: As solar energy warms the water, students notice that the blue liquid evaporates, but the rising vapor leaves the blue dye behind on the bottom, demonstrating that evaporation naturally purifies water.
2. Condensation: As the invisible vapor cools against the cooler plastic near the top of the bag, microscopic droplets coalesce into visible fog and clouds.
3. Precipitation: As droplet size increases, gravity overcomes surface tension, causing water droplets to stream down the inside walls as precipitation, collecting back in the simulated ocean.
4. Pedagogical Extension: Mr. Henderson connects their closed baggie models to global dynamics, asking students to explain why rain falling over the salty Gulf of Mexico or Pacific Ocean is always fresh water rather than saltwater.