The molecule, which the plant is most likely synthesizing using the extra nitrogen is PROTEIN.
Plants and animals do not have the capacity to use atmospheric nitrogen as nutrient. Atmospheric nitrogen has to be converted to forms that are usable by plants and animals before they can utilize it. Nitrogen fixing bacteria are the ones that are responsible for converting atmospheric nitrogen to the forms such as nitrate and ammonium, which can be used by plants. Animals get their own portion of nitrogen when they eat plants or other animals. Nitrate is the form of nitrogen that is mostly used by plants. Excess ammonium in plants are usually used directly to synthesize proteins. Nitrogen is a very important component of chlorophyll, which is essential for photosynthesis. Nitrogen is also found in the genetic materials of plants and it is needed for their growth and development. Animals used nitrogen to synthesize proteins, nucleic acid and other biological compounds that contain nitrogen.