Using density functional theory, the mechanism of surface segregation of N to the [001] Fe surface was studied. The formation and migration energies were decomposed into chemical bonding and strain energy components. While the segregation energy was determined to be -1.1 eV, the bonding and strain energy components for segregation were -0.5 and -0.6 eV, respectively. The results indicate that strain energy relaxation plays a major role in surface segregation, and that there is approximately a 7-layer transient region, which separates bulk and surface environments. The role of the strain energy on interstitial migration barriers was also critically evaluated.