When running a nitrogen crop response model with 100kgN/ha fertilizer input I found that the amount of nitrogen in the top 90cm of the soil decreased compared to when zero nitrogen was added (66kgN/ha vs 93kgN/ha). No other variables in the model were changed (ie organic matter content, rainfall etc). the other results given by the model were:
For 100kgN/ha added - crop yield 3.9t/ha with %N content 3.52, N leaching below 90cm 94 kgN/ha
For 0kgN/ha added - crop yeild 1.99 t/ha with %N content 2.17, N leaching below 90cm 94 kgN/ha
I understand why the crop yeild and % N content went up in response to the added nitrogen but why did the amount of stored nitorgen in the soil go down - what am I missing about the nitrogen cycle that would explain this?
Nitrogen dynamics in soil can be complicated by a number of factors. Plants only take up inorganic forms of nitrogen (NO3-, nitrate, and NH4+, ammonium), and other sources of nitrogen must be converted into one of these forms for use by plants. The conversion of ammonium to nitrite (NO2-) and then to nitrate is called nitrification. This process is carried out by soil microoranisms; all forms of nitrogen are eventually converted into nitrate. The ammonium form of nitrogen is held in the soil due to attraction between its positive charge and negatively charged clay particles and organic matter, and is obtained by plants by ion exchange with H+. The nitrate form is water soluble and is carried with soil water, so it can be lost if water percolates below the active root zone of the crop.
Your model estimates suggest that an ...
Nitrogen's effect on soil is discussed.