SCIENCE · INVESTIGATE AND EXPLAIN

Track nitrogen through an ecosystem

Account for uptake, litter and mineralization without creating matter.

Learning outcome: Reconstruct a simultaneous nutrient update and explain how changing one transfer affects several pools.

Starting recap

A pool is an amount stored; a flux transfers an amount per interval. Grams of nitrogen measure an element, not whole plant biomass or energy.

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Try

A system starts with soil 40, plants 30 and detritus 30 g N. Uptake moves 8, litter 3 and mineralization 3 g N. Where does each amount go?

Open the experiment

Predict

Predict the first two updates at uptake 20% and mineralization 10%. Predict what changes when mineralization stops.

Your prediction and reason

Experiment

Capture the default endpoint and inspect the first two ledger rows. Hold uptake at 20% and compare mineralization 0% and 30%. Check total nitrogen in each case, then explain why changing one transfer changes multiple pools.

Follow nitrogen through three pools

Worked teaching model · synthetic values · Initial nitrogen: soil mineral pool 40 g, plant pool 30 g, detritus pool 30 g. Ten simultaneous updates move selected fractions of preceding soil/detritus pools; 10% of preceding plant nitrogen enters detritus each step. Fractions are teaching assumptions, not fitted rates or days. No atmospheric input, leaching, microbial biomass or chemical species are tracked.

Soil-to-plant uptake fraction: 20 % per step Detritus mineralization fraction: 10 % per step

Reset keeps your prediction, captures and practice history.

State and transfer ledger

Uptake moves nitrogen from soil to plants; litter moves it from plants to detritus; mineralization returns it to the soil mineral pool. The local boundary includes these three pools. Export crosses that boundary and remains in a separate account.

Step 0 shows the pools after any initial export. Transfer columns at later rows describe the update that produced that row; zeros at step 0 mean no update has occurred.

Update ledger (g N), displayed to three decimals
Step	Soil	Plants	Detritus	Uptake	Litter	Mineralized
0	40.000	30.000	30.000	0.000	0.000	0.000
1	35.000	35.000	30.000	8.000	3.000	3.000
2	31.000	38.500	30.500	7.000	3.500	3.000
3	27.850	40.850	31.300	6.200	3.850	3.050
4	25.410	42.335	32.255	5.570	4.085	3.130
5	23.553	43.184	33.263	5.082	4.234	3.225
6	22.169	43.576	34.255	4.711	4.318	3.326
7	21.161	43.652	35.187	4.434	4.358	3.426
8	20.447	43.519	36.034	4.232	4.365	3.519
9	19.961	43.257	36.782	4.089	4.352	3.603
10	19.647	42.923	37.430	3.992	4.326	3.678

Displayed rounding can cause tiny differences in a hand sum. Calculations use unrounded values. No microbial processes, organisms or sites were measured to produce this ledger.

Calculated values on the same fixed scale
Row	Quantity	Value (g N)
1	Soil mineral nitrogen	19.6472
2	Plant nitrogen	42.9232
3	Detritus nitrogen	37.4296
4	Exported nitrogen outside local system	0

After 10 updates: local nitrogen 100.000 g N; exported 0.000 g N; combined ledger 100 g N.

All transfers use the preceding pools. Recycling changes location and chemical form; it does not create nitrogen or replace exported matter. Plant nitrogen is not plant count, height, biodiversity or proof of ecological recovery.

Observation to attach to a capture

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

Understand

Trace an element, not a vague amount of “food”

Nitrogen atoms occur in amino acids and nucleic acids. Organisms need usable nitrogen compounds, not simply an atmosphere containing N₂. Nitrogen fixation converts N₂ to biologically usable forms; mineralization releases inorganic nitrogen from organic matter; nitrification and denitrification change nitrogen compounds through different microbial processes. These transformations belong to a larger cycle than the three pools modeled here. Background: biogeochemical cycles.

The workbench tracks grams of nitrogen, abbreviated g N. Thirty grams of plant nitrogen is not thirty grams of plant biomass. The soil box combines mineral forms rather than distinguishing ammonium and nitrate. The detritus box represents nitrogen in dead material and waste. Microbial biomass and its own nutrient demands are omitted, so the return flow is a simplified mineralization rule.

Reconstruct one simultaneous update

At the default settings, initial pools are 40 g N in soil, 30 in plants and 30 in detritus. In the first step:

Uptake = 20% of 40 = 8 g N, from soil to plants.
Litter transfer = 10% of 30 = 3 g N, from plants to detritus.
Mineralization = 10% of 30 = 3 g N, from detritus to soil.

The new pools are soil 40 − 8 + 3 = 35, plants 30 + 8 − 3 = 35, and detritus 30 + 3 − 3 = 30 g N. Total nitrogen remains 100 g. Use the old pools for all three fluxes; using the already-updated plant pool for litter would apply a different rule.

Predict the second step: calculate each transfer from 35, 35 and 30 before opening the ledger.

Check the second update

Uptake is 7, litter is 3.5 and mineralization is 3 g N. The pools become 31, 38.5 and 30.5 g N, again totaling 100. A changing pool can reflect the difference between several flows; plant accumulation does not create nitrogen.

Change a process and explain the result

Keep uptake at 20% and set mineralization to zero. Capture the endpoint and inspect intermediate rows. Detritus can receive nitrogen but cannot return it to soil in this altered model. Increasing uptake cannot make the isolated detritus nitrogen immediately available.

Then set uptake to zero while restoring mineralization to 10%. Predict which pool accumulates nitrogen and explain why there is still plant-to-detritus transfer. The rules describe transfers from existing stores, not purposeful decisions by organisms.

Energy does not follow the same closed material ledger. Energy enters ecosystems, changes form and leaves as heat; a nitrogen atom can circulate among pools. Recycling matter is not recycling all usable energy.

Identify what the model omits

There is no atmospheric fixation, deposition, leaching, erosion or denitrification loss. A real site’s budget can be open. No step duration is calibrated, and fixed fractional rates ignore temperature, moisture, microbial activity and plant demand. The model is useful for testing accounting and dependencies, not predicting fertilizer needs or a field recovery date.

Apply

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

Trace a nitrogen atom through the three pools, identify two omitted pathways, and explain why a conserved nitrogen ledger is not an energy budget or field forecast.

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: Compare observational groups without inventing causation Next: Distinguish recycling from ecological recovery

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