Environmental Science
Nitrogen gas returns to the atmosphere by the action of

Denitrifying bacteria
Nitrate fertilizers
Nitrifying bacteria
Nitrogen fixing bacteria

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Environmental Science
The aurora is produced by:

Reflections of sunlight by polar ice fields
Burning oxygen caused by the intense sunlight at high altitude
The combination of molecular and atomic oxygen to form ozone
Fast-moving charged particles colliding with air molecules
Scattering of sunlight in the upper atmosphere

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