The model was developed using AnyLogic software
This N model contains six stocks:
1) Atmospheric (gaseous) dinitrogen, N2
2) Soil Inorganic N (ammonia (NH3), ammonium (NH4) and nitrate (NO3))
Crop N, which at harvest time is split into
3) Yield N (harvested portion) and
4) Detrital N (residue returned to Soil Organic N);
5) Soil Organic N
6) Stream N
Plus, an optional flow to Riparian buffer strips.
The model contains seven flows:
- N Fixation. By this process, N flows from the atmosphere to the soil. Lightning or microbes hosted in legumes such as soybean and alfalfa transform non-reactive atmospheric N2 into reactive ammonia in air and ammonium within the Soil Inorganic N stock. This is the only natural input of N to the system. Within the soil, ammonium is converted to nitrate by the process of nitrification by microbes (not depicted in the model); nitrate is also contained in the Soil Inorganic N stock. In this model, N fertilizer additions by people are treated as an event, not a flux, but the amount of N fertilizer added is included as an input in the N budget calculations.
- Assimilation (uptake by crops) occurs when N flows from the Soil Inorganic N stock to Crop N, mostly in the form of nitrate or ammonium.
- Crop N flows to Harvested Yield at harvest time.
- The remainder of the crop, Detrital N, flows to the Soil Organic N stock.
- Immobilization occurs when Soil Inorganic N is taken up by microbes and becomes part of the Soil Organic N stock.
- N Mineralization (= Ammonification) occurs when Soil Organic N is decomposed into Soil Inorganic N. N ‘fixed’ during N fixation can meet at least part of the crop N uptake requirement. Other crops, such as corn and oats, rely entirely on inorganic N that becomes available from N mineralization, or on fertilizer additions.
- Denitrification, also mediated by microbes, is the process by which nitrate (Soil inorganic N) is converted to nitrous oxide or to atmospheric N2. Nitrous oxide is a greenhouse gas, an air pollutant. 8) N leaching loss occurs when soil water leaches nitrates into groundwater and streams, polluting water onsite and downstream, all the way to the Gulf of Mexico.
The model contains several Management factors
- Type of cropping system
- Amount & timing of N fertilizer addition in kg/ha/yr
- Other practices, including tillage type (Tilled or No-till), Cover crops, Buffer strips
- A combination of No-till, Cover crops and Buffer strips. The model also contains a means for experimenting about climate change. To experiment, one can adjust temperature and precipitation values relative to historical means over the last 20 years.
Although not explicitly depicted, the model flows are controlled by these factors: precipitation, temperature, and crop growth stage and N fixation capacity. Climate data are from: https://mesonet.agron.iastate.edu/. Because the water cycle is so integral for N cycling, this model includes a water sub-model in which precipitation and temperature control various processes. The model is calibrated for a silty clay loam soil in a conventional agricultural system in north central Iowa from data from Russell et al. (2005, 2009).