Final answer:
The second-largest reservoir in the carbon cycle is the oceans, which store large amounts of dissolved inorganic carbon, exceeding the amounts stored in the atmosphere, soil, and plants.
Step-by-step explanation:
The second-largest reservoir in the carbon cycle is the oceans.
The oceans play a critical role in storing carbon, primarily as dissolved inorganic carbon, which includes species such as bicarbonate ions, carbonate ions, and carbon dioxide.
Carbon in the atmosphere is a significant reservoir as well but does not hold as much carbon as the oceans.
The other reservoirs mentioned, including soil and plants, although important, contain less carbon than the oceans and the atmosphere.