  Polymeric nitrogen with single bonds can be created from the molecular form
at high pressure and due to large energy difference between triple and single
bonds it is interesting as energetic material. Its structure and properties
are, however, still not well understood. We studied amorphous nitrogen by
\textit{ab initio} simulations, employing molecular dynamics and evolutionary
algorithms. Amorphous nitrogen was prepared at pressure of 120 GPa by quenching
from hot liquid, by pressure-induced amorphization of molecular crystal and by
evolutionary search. All three amorphous forms were found to be structurally
similar. We studied in detail the structural evolution of the system upon
decompression from 120 GPa to zero pressure at 100 K. At pressures above 100
GPa, system consists mainly of 3-coordinated atoms (80 \%) connected by single
bonds while some short chains made of 2-coordinated atoms are also present.
Upon decompression, the number of 3-coordinated atoms rapidly decreases below
60 GPa and longer chains are created. At 20 GPa the system starts to create
also N$_2$ molecules and the ultimate structure at $p=0$ contains molecules
inside a polymeric network consisting dominantly of longer chains made of
2-coordinated atoms. Besides structure, we also study vibrational and
electronic properties of the system and estimate the amount of energy that
could be stored in amorphous nitrogen at ambient pressure.
