Which element can be taken up by plants either as an anion or a cation?

Nitrogen is unique among the options provided because it can be absorbed by plants in both anionic and cationic forms. As an anion, nitrogen primarily takes the form of nitrate (NO3-), which is directly absorbed by plant roots from the soil. Additionally, nitrogen can exist as ammonium (NH4+), a cation that plants can also take up. This versatility in its chemical forms allows nitrogen to be a crucial nutrient for plant growth, as it is a fundamental component of amino acids, proteins, and nucleic acids.

Other options like sulfur primarily enter plants as sulfate (SO4^2-), making it an anion, while calcium is absorbed solely in its cationic form (Ca2+). Manganese is also taken up primarily as a cation (Mn2+). Therefore, nitrogen's ability to be utilized in multiple forms gives it a distinct role in plant nutrition.