Nitrogen is taken up by plant roots in the form of

Nitrogen is an essential nutrient for plant growth and development. However, plants cannot directly use atmospheric nitrogen gas. Instead, they take up nitrogen through their roots in the form of nitrate ions (NO3-) or ammonium ions (NH4+). Nitrate ions are the most common form of nitrogen taken up by plants from the soil. They are formed through the oxidation of ammonium by soil bacteria, and can be easily absorbed by plant roots. Therefore, the correct answer to the question is "nitrate ion".