which istope of nitrogen is more commonly found, nitrogen-14 or nitrogen-15?

Answer 1

Wiki:

"There are two stable isotopes of nitrogen: 14N and 15N. By far the most common is 14N (99.634%), which is produced in the CNO cycle in stars and the remaining is 15N. Of the ten isotopes produced synthetically, 13N has a half life of nine minutes and the remaining isotopes have half lives on the order of seconds or less. Biologically-mediated reactions (e.g., assimilation, nitrification, and denitrification) strongly control nitrogen dynamics in the soil. These reactions typically result in 15N enrichment of the substrate and depletion of the product.

The molecular nitrogen in Earth's atmosphere is 0.73% comprised of the isotopologue 14N15N and almost all the rest is 14N2."

Answer 2

N-14 is the common one

Answer 3

N-14 is the most common


http://en.wikipedia.org/wiki/Nitrogen#Isotopes