Nitrogen is the main constituent of Earth's atmosphere and a key component of the biosphere, but it is a trace element in the major silicate reservoirs. The relatively low concentrations (parts per million level) complicate efforts to constrain the nitrogen speciation and abundance in the mantle and crust. In most silicates, nitrogen occurs as NH 4 + (substituting for K +), whereas its speciation in hydrous fluids and in silicate melts can vary widely depending in large part on redox conditions. Current knowledge of nitrogen isotope fractionation among relevant mineral and fluid/melt phases, and thus more quantitative applications of nitrogen isotopes to crust-mantle problems, is limited by the lack of experimental data to confirm theoretical predictions of these fractionations. Understanding of modern and long-term nitrogen cycling on Earth is limited by uncertainty in the sizes and isotopic compositions of the major crust and mantle nitrogen reservoirs.