Answer: The fact that nitrogen is recycled and its constant amount is maintained on earth.
Step-by-step explanation:
Nitrogen is present in abundant amount in the atmosphere in the form of nitrogen oxide, nitrogen dioxide and nitric acid when combined with water.
The nitrogen is fixed by nitrogen fixing bacteria and is used by plants for synthesizing food and other metabolic processes.
Animals eat plants and get nitrogen from there. The waste from the animals are excreted out by urine and excreta. Rest of the nitrogen is also released when the animals die. Again the nitrogen is available in the atmosphere.
This is how nitrogen is recycled and shows conservation of matter.