Nitrogen or phosphate?

Question

If I wanted to track the movement of nutrients between organisms, I thought I should grow plants with either radio-labeled nitrogen or phosphate which would then be incorporated into plant tissues and then animal tissues when it is eaten.

Which would be better, N or P and why?

Nitrogen incorporates into proteins and DNA anything else?

Phospate incorporates into DNA anything else?

Answer 1

I believe radioactive phosphorus has been used to study the flow of nutrients through an ecosystem.  Phosphorus would also be incorporated into ATP, plasma membranes, and RNA.

Part of the decision could be based on the half-life of the each of the isotopes you would be using.  You would need to select the isotope that lasted long enough to complete your study of the transfer.

Another factor to consider is how you would introduce the isotopes into the food chain.  You could fertilize plants with either radioactive phosphorus or nitrogen in the appropriate form to the plant and follow the transfer by monitoring animals that you trap in the area.

You could do a literature search using keywords to get the protocols for doing such research.  Certainly you would need to check federal permits needed for conducting the research.

Answer 2

Nitrogen 15 is used where NMR can be used to track the molecules and avoids the problems of radioactive isotopes. This is used in agriculture to follow the fixation of nitrogen from the atmosphere to the plants.

Mass spec can be used to determine the ratios of carbon or nitrogen isotopes; Carbon12/Carbon13 or Nitrogen14/Nitrogen15. This is used in diet analysis. Different foods contain distinctive isotopes or isotope ratios that can be used to determine the origin of the food. Plants vary how they fix carbon so the isotope ratio is distinctive for the plant. "C 3-photosynthesizing plants (dicotyledonous trees, shrubs, and forbs) have C13 values consistently lower than do C 4-photosynthesizers (grasses)."

http://www.scienceinafrica.co.za/2005/ju...

Plants can be labeled by rowing them in air tight conditions with carbon-13 dioxide.

http://www.ars.usda.gov/research/publica...

Plants use low levels of nitrogen with a carbon to nitrogen ratio of 40:1. This is concentrated in animals who need a carbon to nitrogen of 14:1 so meat has more nitrogen 15 than plants. These isotopes can be tracked through the food chain subsequently.