The biogeochemical cycle best described by the list of terms and processes is the nitrogen cycle.
Here's how each term relates to the nitrogen cycle:
- Fixation by lightning: Nitrogen in the atmosphere can be fixed into reactive forms through lightning strikes.
- Biologically inert in the atmosphere: Molecular nitrogen (N₂) makes up about 78% of the Earth's atmosphere and is relatively inert and cannot be used directly by most living organisms.
- Complex; many chemical forms: The nitrogen cycle involves various forms of nitrogen, including N₂, ammonia (NH₃), ammonium (NH₄⁺), nitrate (NO₃⁻), and nitrite (NO₂⁻).
- Ammonium: Ammonium is a key component in the nitrogen cycle, produced through the process of ammonification and is utilized by plants.
Thus, the correct answer is nitrogen.