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 NH4+ (substituting for K+), whereas its speciation in hydrous fluids and 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 is limited by the lack of experimental data to confirm theoretical predictions of these fractionations. Modeling of modern and long-term nitrogen cycling on Earth will be advanced by better constraints on the sizes and isotopic compositions of the major crust and mantle nitrogen reservoirs.