SCIENCE · INVESTIGATE AND EXPLAIN

Follow nitrogen across a plot boundary

Track elemental nitrogen across four pools and external accounts.

Learning outcome: Close a nitrogen budget while distinguishing internal transformations from external transfers.

Starting recap

Every number is kg of nitrogen atoms, not kg of nitrate or fertilizer product.

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Try

At fertilizer 10 and nitrate export 6, final pools total 96 kg N; 100 + 14 = 96 + 18.

Open the experiment

Predict

Does plant uptake remove nitrogen from the soil-and-plant boundary?

Your prediction and reason

Experiment

Capture the middle case, then change fertilizer alone and export alone. Trace the same atom mass through every step.

Trace nitrogen across a plot boundary

Prescribed nitrogen transfers · elemental mass accounting · Original prescribed-transfer ledger over one unspecified accounting interval. Four plot pools start with 100 kg elemental N; nine supplied events include fixed uptake, transformations and external transfers. Control amounts are not reaction rates, rainfall responses or recommended fertilizer doses. No yield, concentration, kinetics or ecological impact is predicted.

Supplied ammonium fertilizer: Add 10 kg N fertilizer Stipulated nitrate export: Export 6 kg nitrate N

Reset keeps your prediction, captures and practice history.

Follow nitrogen atoms, not whole fertilizer or plant mass

Predict which pool changes if you add 10 kg N of fertilizer while holding every other transfer fixed. Then compare captures. This original teaching ledger begins with 100 kg N across soil and plants. All numbers measure the mass of nitrogen atoms: 1 kg nitrate N is not 1 kg of nitrate ions or 1 kg of fertilizer product.

The plot boundary includes organic soil, residues and microbes; ammonium; nitrate; and living plants. Atmospheric fixation and fertilizer enter. Water export, gaseous export and harvested material leave. The controls stipulate 10 kg N of ammonium fertilizer and 6 kg N of nitrate export. Neither is a measured rate or recommended dose.

Whole-plot account: 100 initial + 14 inputs = 96 final stores + 18 outputs, in kg N. Residual: 0 kg N. An exported atom is accounted for outside the plot; it has not disappeared.

One initial state and nine accounting events. Pool columns and totals are kg N.
Step / event	Transfers (kg N)	Organic soil, residue and microbial N	Ammonium N	Nitrate N	Living plant N	Stored total	Cumulative inputs	Cumulative outputs
0. Initial plot stores	Initial state; no transfer	40	10	20	30	100	0	0
1. Net biological fixation and assimilation	Atmospheric N₂ input → Organic soil, residue and microbial N: 4	44	10	20	30	104	4	0
2. Ammonium fertilizer addition	Fertilizer input → Ammonium N: 10	44	20	20	30	114	14	0
3. Mineralization	Organic soil, residue and microbial N → Ammonium N: 8	36	28	20	30	114	14	0
4. Nitrification	Ammonium N → Nitrate N: 12	36	16	32	30	114	14	0
5. Plant uptake	Ammonium N → Living plant N: 6 Nitrate N → Living plant N: 10	36	10	22	46	114	14	0
6. Litter and residue return	Living plant N → Organic soil, residue and microbial N: 5	41	10	22	41	114	14	0
7. Dissolved nitrate export	Nitrate N → Water-exported N: 6	41	10	16	41	108	14	6
8. Denitrification	Nitrate N → Gaseous N export: 4	41	10	12	41	104	14	10
9. Harvest	Living plant N → Harvested N: 8	41	10	12	33	96	14	18

At every row, 100 + cumulative inputs = stored total + cumulative outputs. Plant uptake is one event with two transfers: 6 kg N from ammonium and 10 kg N from nitrate. Neither is an external output. Zero in the fertilizer or export event means a stipulated zero transfer for that packet, not a missing observation.

What the arrows mean

Fixation plus assimilation is a net atmospheric N₂-to-microbial-organic-N transfer; the ammonia intermediate is not shown. This does not mean plants directly absorb atmospheric N₂. Mineralization releases organic N into ammonium. Nitrification changes ammonium N into nitrate N, omitting the nitrite intermediate. Uptake incorporates inorganic N into plants; litter returns plant N to the organic pool.

Denitrification sends nitrogen to the external gas account. Its products need not all be N₂; the model supplies no N₂O fraction or climate effect. Water export does not calculate concentration or recipient harm. Harvested N is distinct from the mass of harvested food.

The supplied event order is bookkeeping over one unspecified interval, not a kinetic model or a universal chronology. Real processes overlap and respond to conditions. Uptake remains fixed in these packets; extra fertilizer therefore remains as ammonium here. This is not evidence that fertilizer cannot change uptake or yield in nature. Other exchanges, including immobilization, volatilization and deposition, are omitted.

Check a worked ledger and a new boundary case

For fertilizer 10 and nitrate export 6, final pools are organic 41, ammonium 10, nitrate 12 and plant 33 kg N, totaling 96. The balance is 100 + 4 fixation + 10 fertilizer = 96 stored + 6 water export + 4 gas export + 8 harvest. After uptake the stores total 114, just as immediately before uptake: changing which pool holds N did not export it.

Paper transfer: another plot starts with 60 kg N, receives 9, exports 7 to water and harvests 12. With no other external exchanges, it ends with 50 kg N. Internal uptake or mineralization does not change that whole-boundary result. A nitrate-only measurement would leave the other pools unmeasured.

Use two matched captures to show where additional N went, then name one observation required before interpreting the ledger as a real field explanation. USDA NRCS nitrogen guidance explains environmental processes; it does not supply these original transfer amounts.

Final plot stores 96 kg N; external inputs 14 kg N; external outputs 18 kg N.

100 initial + 14 inputs = 96 stored + 18 outputs (kg N). Internal uptake and transformations are not external losses. Fixed uptake is a teaching assumption, not evidence that fertilizer never changes plant growth.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Follow atoms, not just a fertilizer label

A plot can gain fertilizer while exporting nitrogen to water or harvest. To explain the change, draw a boundary around both its soil and living plants. This fictional accounting packet starts with 100 kg N: 40 in organic soil/residue/microbial material, 10 in ammonium, 20 in nitrate and 30 in plants. Every quantity means kilograms of nitrogen atoms. A kilogram of nitrate includes oxygen mass as well; it is not a kilogram of nitrogen.

Choose fertilizer 10 kg N and water export 6 kg N. Predict whether uptake removes nitrogen from our whole plot. Follow the displayed snapshots before checking the final balance. The interval has no specified duration: these transfers are amounts, not daily rates or a growing-season forecast.

Distinguish chemical transformations and boundary crossings

Biological fixation brings 4 kg N from atmospheric nitrogen into microbial organic material. This is net fixation plus assimilation; an ammonia intermediate is not resolved. It does not mean plants directly absorb atmospheric nitrogen gas. Fertilizer then adds the selected amount to ammonium.

Mineralization transfers 8 from organic material to ammonium. Nitrification transfers 12 from ammonium to nitrate; the nitrite intermediate is omitted. Uptake transfers 6 from ammonium and 10 from nitrate to plants. Litter returns 5 from plants to organic material. Each internal transfer subtracts from one pool and adds the same nitrogen mass to another.

Water export removes the selected nitrate-N amount. Denitrification removes 4 to a gaseous-N account; its nitrogen-gas/nitrous-oxide composition is not supplied. Harvest removes 8 from plants. Those three transfers cross our boundary. Label their destinations instead of describing exported nitrogen as destroyed.

NRCS nitrogen guidance explains environmental nitrogen forms and transformations. Real processes overlap and depend on conditions. The displayed order is an accounting sequence, not a universal chronology. Immobilization, volatilization and deposition are examples of processes a wider study may need; they are not hidden numerical losses here.

Close an intermediate and final budget

At fertilizer 10, after fixation, fertilizer and mineralization, the four pools are 36, 28, 20 and 30 kg N. Their total is 114, equal to initial 100 plus external inputs 4 + 10. Nitrification and uptake rearrange that mass. After uptake the pools are 36, 10, 22 and 46, still totaling 114.

With water export 6, final pools are 41, 10, 12 and 33, totaling 96. External outputs are water export 6 + gaseous export 4 + harvest 8 = 18. Check initial + inputs = final stores + outputs: 100 + 14 = 96 + 18. Do not add uptake again as an external loss.

Capture this case. Change only fertilizer to 20: final ammonium increases by 10 under the prescribed transfers. That controlled result does not demonstrate that fertilizer never changes uptake in nature. Next restore fertilizer 10 and raise water export to 12. Final nitrate falls by 6; the exported account rises by 6. Neither change predicts a recipient concentration or ecological effect without water volume and further evidence.

Connect cycling with energy

Nitrogen and carbon atoms can move among organisms, soil and external reservoirs. Revisit the plant carbon ledger to compare its different boundary. Energy in a trophic pathway is transferred and dissipated rather than recycled in the same way as atoms. Decomposition connects material recycling with biological energy use; it does not create nitrogen.

Transfer: repair a boundary mistake

A separate plot starts with 60 kg N, receives 9, exports 7 to water and harvests 12. Uptake moves 11 from soil to plants inside the boundary. Calculate final total. Can a nitrate-only measurement check the entire budget?

Check the nitrogen transfer

Final total is 60 + 9 − 7 − 12 = 50 kg N. Internal uptake does not change that total. Nitrate is only one pool; verifying the whole plot requires the other stores and relevant boundary transfers. This is public accounting practice, not an agronomic recommendation.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Identify which transfers cross the boundary and explain why fixed uptake is a teaching assumption.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Distinguish habitat amount from arrangement Next: Connect soil functions with food and water

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