Why would adding nitrogen fertilizer reduce the amount of nitrogen in the top soil layers?
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?
