Trace carbon and nitrogen through Earth’s interacting spheres

Learning goal: Follow conserved atoms through changing chemical forms and reservoirs, explain biological pathways that connect gases with organisms, and use an input-output account to reason about a changing atmospheric reservoir.

An atom can move through air, water, organisms and geological material while becoming part of different compounds. A biogeochemical cycle describes these linked biological, geological and chemical processes. “Cycle” does not mean that each atom follows one circular route in the same time. Some reservoirs exchange material rapidly; others retain it much longer. Matter can cycle while the quantity stored in a particular reservoir changes.

Carbon dioxide contains one carbon atom and two oxygen atoms in each molecule. During photosynthesis, plants and other photosynthetic organisms use carbon dioxide and water to make organic matter with energy supplied by light. The carbon becomes part of sugars and other substances; it does not turn into sunlight. Feeding can transfer that carbon to consumers. Cellular respiration by plants, animals and many microbes returns carbon dioxide while transferring energy from chemical stores to processes that support life.

Plants both photosynthesize and respire. A model showing plants only removing carbon dioxide misses an important pathway. Decomposers use dead organic material and wastes, returning some carbon to the atmosphere through respiration while other carbon remains in soils or other reservoirs. Combustion also transfers carbon from fuel into atmospheric compounds. Ordinary chemical reactions rearrange atoms rather than create new carbon atoms from nothing.

The hydrosphere exchanges carbon with air, and aquatic organisms incorporate carbon into living material. Some carbon becomes part of sediments and carbonate rocks through longer-term processes. Weathering, geological transformations and volcanic activity connect geological reservoirs with the rest of the system. Distinguish these routes from the faster exchange between a leaf and surrounding air. A single arrow cannot represent a universal travel time for all carbon.

Nitrogen is needed for proteins and nucleic acids. Plants and animals cannot use atmospheric nitrogen gas, N₂, directly as their nitrogen source for building those molecules. Nitrogen-fixing microorganisms convert nitrogen gas into biologically usable forms; some of these bacteria live in root nodules of plants such as beans and clover and supply those plants with usable nitrogen. Other microbes transform nitrogen compounds in soils and water, including transformations between ammonium, nitrite and nitrate. Plants take up suitable nitrogen compounds, and animals obtain nitrogen by eating organisms or their products.

Death, waste and decomposition return nitrogen compounds to soil and water. Denitrifying microbes can return nitrogen to the atmosphere as gas. Dissolved compounds may move with water, linking the nitrogen cycle to runoff and groundwater. These pathways change chemical form without changing nitrogen atoms into carbon atoms. An animal breathing air is not thereby obtaining all the nitrogen needed to build its proteins; food and digestion provide the relevant organic nitrogen route.

Human activities alter transfer rates. Burning carbon-rich fuels transfers stored carbon to the atmosphere. Fertilizer use can add available nitrogen to soils, while excess nitrogen can move to waterways. These are changes in location and chemical form, not manufacture of new element identities. Evaluate the amount, timing and pathways before judging an effect; the presence of a naturally cycling substance does not make every added quantity inconsequential.

The diagram uses an invented carbon-account model in arbitrary units. The atmospheric store begins with 100 units. During one interval, 12 enter and 9 leave, ending with 103. A second scenario begins with the same 100 but has 12 enter and 12 leave, ending with 100. Both scenarios contain ongoing exchange. Only the second has no net storage change during this interval. Balanced arrows do not mean atoms stopped moving.

This arithmetic identifies what a mass account can and cannot show. If measured inflows and outflows do not explain the observed storage change, investigate omitted pathways, mismatched periods and measurement uncertainty. Do not label the difference “new atoms.” An atmospheric measurement alone also does not identify which individual reservoir supplied every atom. Combining multiple kinds of observations provides a stronger account of transfers among Earth's spheres.

Worked reservoir calculation: A store of 100 model units receives 12 and loses 9. Its ending store is 100 + 12 − 9 = 103, a net gain of 3. If the same net gain recurred for four identical intervals, the model would reach 112. That projection assumes unchanged transfers; it is not a forecast of actual atmospheric concentration.

Worked nitrogen pathway: Follow one nitrogen atom from atmospheric N₂ through microbial fixation, a soil compound, root uptake, plant protein, animal food and decomposition. A later microbial transformation may return it to the atmosphere. Name both reservoirs and processes; “air → plant → animal” omits the crucial route that makes atmospheric nitrogen usable.

Supported modeling: Use counters representing atoms to model one carbon reservoir receiving and losing material over three intervals. Keep an external supply and receiving tray so every moved counter is accounted for. Record initial amount, inputs, outputs and final amount; explain why the counters represent atoms rather than energy.

Independent performance: Create separate carbon and nitrogen pathway maps, each connecting atmosphere, organisms, water and geological material. Label transformations, branching and one potentially long storage interval. Explain how human respiration belongs in the carbon account and how food supplies usable nitrogen. Add a numerical reservoir scenario and identify a measurement that would test one claimed change. Check the map against the NASA sources and correct one missing pathway.