Abstract: From June 2008 to September 2009, a field experiment was conducted to study the change characteristics of the nitrogen fractions in horse dung during its decomposition on the typical steppe of Inner Mongolia. Two treatments were installed, i.e., deposited the horse dung on the ground and buried the horse dung in soil. In the two treatments, the concentrations of ammonia nitrogen (NH₃-N), amino acid nitrogen (AAN), and aminosugar nitrogen (ASN) in residual horse dung were maintained at a higher level in the earlier period of horse dung decomposition (0-90 d), but declined obviously in the later period (330-450 d). The concentrations of acid-hydrolysable unknown nitrogen (AHUN) and nonacidhydrolysable unknown nitrogen (UUN) presented an increasing trend with the decomposition of horse dung, and increased to a higher level in the later period. In fresh horse dung, NH₄⁺-N was the main fraction of inorganic nitrogen, and its concentration in residual horse dung decreased with the decomposition of horse dung. The concentration of NO₃^--N in fresh horse dung was lower, its leaching loss from the residual horse dung was weak, and it was accumulated in the residual horse dung with the decomposition of horse dung. The horse dung buried in soil had significant effects on the gaseous ammonia volatilization of NH₄⁺-N, but minor effects on the other nitrogen fractions. The main nitrogen sources of nitrogen mineralization in the earlier period of horse dung decomposition were NH₃^--N, AAN and ASN, and those in the later period were AHUN and UUN. The bioavailability of NH₄⁺-N was mainly revealed in the earlier period of horse dung decomposition, while that of NO₃^--N was in the later period.