14.2 Water and Nutrient Cycles in Earth Systems

The Water Cycle and Weather

The water cycle (hydrologic cycle) is the continuous movement of water among the ocean, atmosphere, land, and living things. The sun provides the energy; the ocean is the largest reservoir, holding about 97% of Earth's water. Key processes:

• Evaporation — solar energy converts liquid water to vapor, mostly from the ocean.
• Transpiration — plants release water vapor through stomata; a single oak can transpire hundreds of liters per day. Evapotranspiration combines the two.
• Condensation — rising vapor cools and condenses into liquid droplets that form clouds.
• Precipitation — droplets grow heavy enough to fall as rain, snow, sleet, or hail.
• Infiltration — water soaks into the ground and recharges aquifers.
• Runoff — water that flows over the surface into streams and rivers, eventually reaching the ocean.

Link to Weather

Weather is a short-term expression of the water cycle. When warm, moist air rises and cools, water vapor condenses, releasing latent heat that fuels storms. Texas thunderstorms along a dryline in spring, Gulf Coast hurricanes in late summer, and Pacific atmospheric-river floods in winter all illustrate the water cycle interacting with atmospheric dynamics. The TExES framework expects candidates to connect a weather event to a specific water-cycle step, not just label the storm type.

The Carbon Cycle

Carbon moves among four main reservoirs: the atmosphere (as CO₂), the biosphere (in organic molecules), the hydrosphere (dissolved CO₂ and bicarbonate in the ocean), and the geosphere (carbonate rocks, fossil fuels). The major processes:

• Photosynthesis — producers convert CO₂ and water into glucose, moving carbon from atmosphere to biosphere.
• Respiration — organisms break down glucose for energy, releasing CO₂ back to the atmosphere.
• Combustion — burning biomass or fossil fuels rapidly oxidizes carbon to CO₂.
• Decomposition — decomposers break dead organic matter into simpler compounds; some carbon returns to the atmosphere as CO₂ or methane (CH₄).
• Ocean uptake — the ocean dissolves atmospheric CO₂; marine organisms build shells of calcium carbonate (CaCO₃), which eventually form limestone, locking carbon in the geosphere for millions of years.
• Fossil-fuel formation and combustion — ancient organic matter buried and heated becomes coal, oil, or gas; humans burn these fuels, returning carbon to the atmosphere far faster than burial removes it.

The carbon cycle has a fast carbon path (years to centuries: photosynthesis, respiration, exchange with the ocean surface) and a slow carbon path (millions of years: weathering of silicate rocks, formation and subduction of carbonate rocks). Human activity has accelerated the fast path by burning fuel stored on the slow path — the central cause of recent climate change.

The Nitrogen Cycle

Nitrogen is essential for proteins and nucleic acids, but most organisms cannot use atmospheric N₂ directly. The nitrogen cycle moves nitrogen through several chemical forms, each step mediated mainly by bacteria:

• Nitrogen fixation — bacteria such as Rhizobium in legume root nodules, plus free-living Azotobacter and cyanobacteria, convert N₂ to ammonia (NH₃); industrial Haber-Bosch fertilizer production also fixes nitrogen.
• Nitrification — bacteria such as Nitrosomonas oxidize ammonia to nitrite (NO₂⁻), and Nitrobacter oxidize nitrite to nitrate (NO₃⁻), the form most plants absorb.
• Assimilation — plants take up nitrate or ammonium and build amino acids; animals get nitrogen by eating plants or other animals.
• Ammonification — decomposers convert nitrogen in dead organic matter back to ammonia.
• Denitrification — anaerobic bacteria in waterlogged soils and sediments convert nitrate back to N₂, returning nitrogen to the atmosphere and closing the cycle.

Without bacteria, the cycle would stall — a point the TExES framework emphasizes because it connects Earth systems to the microbial material in Domain III.

Carbon and Nitrogen Cycles Compared

Human and Natural Effects on the Cycles

Human activity now rivals natural processes in several cycles. Burning fossil fuels releases about 40 billion metric tons of CO₂ per year, raising the atmospheric CO₂ concentration from about 280 ppm before 1850 to over 420 ppm today and driving global warming. Deforestation removes the photosynthetic pump that pulls CO₂ out of the air, compounding the fossil-fuel problem. Fertilizer runoff delivers excess nitrate to rivers; when it reaches the Gulf of Mexico, it fuels algal blooms whose decomposition creates a dead zone — a hypoxic region that the 2024 NOAA survey placed at about 6,700 square km, a Texas-relevant example for classroom discussion.

Natural processes also alter the cycles. Volcanic eruptions inject CO₂ and sulfur dioxide; on short timescales the sulfates cool climate, on long timescales the CO₂ warms it. El Niño-Southern Oscillation shifts patterns of evaporation and precipitation, redistributing water and carbon fluxes between the Pacific Ocean and the atmosphere. Wildfires, both natural and human-set, rapidly move carbon from biomass to atmosphere.

Dynamic Interactions Among Cycles and Spheres

The cycles are coupled, not independent. The water cycle transports dissolved carbon and nitrogen through rivers and groundwater, linking the hydrosphere to the biosphere. Photosynthesis ties the carbon and water cycles together: stomata open to take in CO₂ and water vapor escapes at the same time, so drought stresses both cycles simultaneously. Decomposition connects nitrogen, carbon, and water: soil moisture controls microbial activity, which controls how fast nitrogen ammonifies and carbon returns to the atmosphere. The ocean is the meeting ground — it stores carbon, hosts nitrogen-fixing cyanobacteria, and drives the water cycle through evaporation.

A TExES 4-8 teacher who can trace one molecule through the water, carbon, and nitrogen cycles — for example, a nitrogen atom from fertilizer running off a Texas farm, traveling down the Brazos River, feeding a Gulf algal bloom, and returning to the atmosphere as N₂ from a denitrifying bacterium — demonstrates the systems thinking that Competency 017 rewards.