I was watching the RI xmas lectures yesterday ( ), and it said that heavy nitrogen isotopes accumulate in animals, wheres this does not happen so much in plants. Since nitrogen isotopes behave the same chemically, and any nitrogen would have come from a primary consumer anyway, how is this possible?

Good question, the answer isn`t complicated but it IS a bit long, I`ll do my best to explain though

 

you`re correct in saying that all nitrogen comes from Plants in the 1`st place, we`re not capable of "Fixing" nitrogen directly like a plant can.

in the air 78% of it is Nitrogen, 75% is N14 the rest is N15 (the isotope). and like all isotopes, they decay over time.

no if someone eats only veg, they`ll have alot more N15 in them than say a Meat eater or anything high up in the food chain, the simple reason is that the nitrogen in the meat gets passed on from animal to animal right up the meat eating food chain to US, of course during all this time, the nitrogen isotopes are decaying, and by the time it reaches out plate, there`s very little N15 left, it`s mostly decayed back down to N14

 

I hope that helps a little