Nutrient Cycling Lab
Model nitrogen cycling in a soil ecosystem over a full growing season. Adjust fixation, nitrification, denitrification, and fertilizer input rates. Add perturbation events like a fertilizer pulse or heavy rain. Verify mass balance and explore how agricultural practices affect nutrient pools.
Guided Experiment: Nitrogen Balance in Agricultural Soil
If you add fertilizer to a soil system, what do you predict will happen to the NH₄⁺ and NO₃⁻ pool sizes over a growing season? Will the total nitrogen in the system increase, decrease, or stay the same?
Write your hypothesis in the Lab Report panel, then click Next.
Time Series
Click Run to start the simulation
Controls
Results
Data Table
(0 rows)| # | Day | NH₄⁺(kg/ha) | NO₃⁻(kg/ha) | Organic N(kg/ha) | Total N(kg/ha) | N Input(kg/ha) | N Output(kg/ha) |
|---|
Reference Guide
Nitrogen Cycle Processes
Nitrogen-fixing bacteria (Rhizobium, Azotobacter) convert atmospheric N₂ into NH₄⁺ (ammonium), making it available to plants. Nitrifying bacteria then oxidize NH₄⁺ to NO₃⁻ (nitrate) in two steps.
Denitrifying bacteria in anaerobic conditions convert NO₃⁻ back to N₂ gas, removing nitrogen from the soil. Mineralization (ammonification) converts organic nitrogen from dead matter back to NH₄⁺, completing the cycle.
Mass Balance
Mass balance is a fundamental principle: the total nitrogen entering a system minus the total nitrogen leaving must equal the change in stored nitrogen.
Inputs include biological fixation, fertilizer application, and atmospheric deposition. Outputs include denitrification to gas, crop harvest removal, and leaching to groundwater. Any imbalance means the soil is either accumulating or losing nitrogen.
Nutrient Pools and Fluxes
Soil nitrogen exists in three main pools. Organic N in humus is the largest reservoir (typically 2000-5000 kg N/ha in topsoil). Mineral pools of NH₄⁺ and NO₃⁻ are smaller but biologically active, supplying nutrients to plants.
Flux rates depend on temperature, moisture, soil pH, and microbial activity. Nitrification is fastest in warm, well-aerated, near-neutral pH soils. Denitrification increases in waterlogged conditions.
Human Impact on the N Cycle
The Haber-Bosch process produces roughly 120 Tg of reactive nitrogen per year for fertilizer, more than all natural fixation combined. This has doubled the rate of nitrogen entering terrestrial ecosystems.
Excess nitrogen causes eutrophication in waterways (algal blooms and dead zones), contributes to greenhouse gas emissions (N₂O is 265 times more potent than CO₂), and acidifies soils. Managing fertilizer timing and amounts is critical for reducing nitrogen losses.