Most commonly used isotope in plant is N15 for determination of nitrogen uptake, but why not N14? Is there any possibility to use both isoptopes at same time??
The use of nitrogen isotopes typically uses both 14N and 15N. The ratio of 15N/14N as expressed as a ratio against the standard ratio in atmospheric air which is arbitrarily set at 0 permil.
A not to recent literature review can be found in:
A preliminary study of the carbon and nitrogen isotopic biogeochemistry of lacustrine sedimentary rocks from the Green River Formation (Collister and Hayes, 1991)
The use of nitrogen isotopes typically uses both 14N and 15N. The ratio of 15N/14N as expressed as a ratio against the standard ratio in atmospheric air which is arbitrarily set at 0 permil.
A not to recent literature review can be found in:
A preliminary study of the carbon and nitrogen isotopic biogeochemistry of lacustrine sedimentary rocks from the Green River Formation (Collister and Hayes, 1991)
...Also, for a relatively new comprehensive book that describes all things related to the use of N isotopes in agronomic research, Please see the reference cited by Dr. Euan Kevin James in the link below. This a link for a current discussion on a similar question.