Which of the following correctly pairs each major biogeochemical cycle with a key form of its nutrient?

• A. Carbon cycle—CO2; Nitrogen cycle—N2/NH4+/NO3-; Water cycle—H2O; Phosphorus cycle—PO4^3-
• B. Carbon cycle—CO2; Nitrogen cycle—NO2-; Water cycle—H2O; Phosphorus cycle—P
• C. Carbon cycle—CO3^2-; Nitrogen cycle—N2; Water cycle—H2; Phosphorus cycle—P
• D. Carbon cycle—CO2; Nitrogen cycle—N2/NH4+/NO3-; Water cycle—HO2; Phosphorus cycle—PO4^3-

Answer: (A)

Think of each biogeochemical cycle as moving its nutrient through air, water, soil, and living organisms using the forms that are most biologically and chemically important. For the carbon cycle, carbon dioxide (CO2) is the primary inorganic form that drives uptake by plants for photosynthesis and that connects atmospheric and aquatic reservoirs, with other forms like dissolved inorganic carbon and organic carbon following in the system. For nitrogen, the cycle operates through several main forms that organisms actually use, including N2 gas in the atmosphere and inorganic forms such as ammonium (NH4+) and nitrate (NO3-), which microbes transform among these states during processes like fixation, nitrification, and assimilation. In the water cycle, the essential substance is water itself (H2O), cycling through evaporation, condensation, precipitation, and runoff. In the phosphorus cycle, phosphorus moves primarily as phosphate (PO4^3-), since phosphorus doesn’t exist as a gas and must be taken up as phosphate by organisms after weathering rocks. The other options mix forms that aren’t the central, readily usable forms for driving these cycles—carbonate (CO3^2-) is only part of carbon chemistry in some contexts, nitrite (NO2-) is a less representative form of nitrogen, hydroperoxyl (HO2) isn’t a meaningful player in the aquatic water cycle, and elemental phosphorus isn’t the main form taken up by organisms. Therefore, pairing CO2 with the carbon cycle, N2/NH4+/NO3- with the nitrogen cycle, H2O with the water cycle, and PO4^3- with the phosphorus cycle best reflects the key nutrient forms.