Question

Which one of the following elements cycles the LEAST (mass per year) through the atmosphere?

• A. Carbon
• B. Nitrogen
• C. Phosphorus
• D. Sulphur

Hint:

Ask which element has no common gas form; carbon, nitrogen and sulphur all move through the atmosphere as gases, but phosphorus mainly cycles through rock, soil and water.

Answer 1

The Correct Option is C

Biogeochemical cycles differ a lot in how much of the element passes through the atmosphere each year, and that mostly comes down to whether the element has a common gas form.

1. Carbon: cycles heavily through the air as carbon dioxide, moving in and out of the atmosphere constantly via photosynthesis, respiration, decay and burning. This is one of the largest atmospheric fluxes of any element.
2. Nitrogen: makes up most of the atmosphere itself as $N_2$ gas, and even the smaller side reactions, like fixation and denitrification, still move enormous quantities of nitrogen gas each year.
3. Phosphorus: has no ordinary gas phase at all. Its entire cycle runs on solid and dissolved forms, moving through rock weathering, soil and water as phosphate ions, entering the air only as dust, which carries a very small mass compared to a true gas cycle.
4. Sulphur: does have gas forms, such as sulphur dioxide from volcanoes and fuel burning and hydrogen sulphide from decomposition, so it still moves a meaningful mass through the atmosphere each year, even if usually less than carbon or nitrogen.

Since three of the four elements have real gas cycles and only phosphorus is stuck moving as dust and dissolved phosphate, phosphorus is by far the smallest atmospheric mover of the four.

Let's summarize:

• Having a common gas phase drives a large atmospheric flux; carbon, nitrogen and sulphur all have one.
• Phosphorus has no significant gas phase, so its atmospheric cycling is the smallest by far.

So phosphorus is the element that cycles the least mass per year through the atmosphere.