  Context. The nitrogen reservoir in planetary systems is a long standing
problem. Part of the N-bearing molecules is probably incorporated into the ice
bulk during the cold phases of the stellar evolution, and may be gradually
released into the gas phase when the ice is heated, such as in active comets.
The chemical nature of the N-reservoir should greatly influence how, when and
in what form N returns to the gas phase, or is incorporated into the refractory
material forming planetary bodies.
  Aims. We present the study the thermal desorption of two ammonium salts:
ammonium formate and ammonium acetate from a gold surface and from a water ice
substrate.
  Methods. Temperature-programmed desorption experiments and Fourier transform
infrared reflection spectroscopy were conducted to investigate the desorption
behavior of ammonium salts.
  Results. Ammonium salts are semi-volatile species releasing neutral species
as major components upon desorption, that is ammonia and the corresponding
organic acid (HCOOH and CH3COOH), at temperatures higher than the temperature
of thermal desorption of water ice. Their desorption follows a first-order
Wigner-Polanyi law. We find the first order kinetic parameters A = 7.7 $\pm$
0.6 $\times$ 10$^{15}$ s$^{-1}$ and E$_{bind}$ = 68.9 $\pm$ 0.1 kJ~mol$^{-1}$
for ammonium formate and A = 3.0 $\pm$ 0.4 $\times$ 10$^{20}$ s$^{-1}$ and
E$_{bind}$ = 83.0 $\pm$ 0.2 kJ~mol$^{-1}$ for ammonium acetate. The presence of
a water ice substrate does not influence the desorption kinetics. Ammonia
molecules locked in salts desorb as neutral molecules at temperatures much
higher than previously expected that are usually attributed to refractory
materials.
  Conclusions. Ammonia snow-line has a smaller radius than the water snow-line.
As a result, the NH3/H2O ratio content in solar system bodies can be a hint as
to where they formed and subsequently migrated.
