Abstract: Soil samples were collected from a 17-year fertilization experiment site in southern Loess Plateau to study the effects of different fertilizations and fallowing on the characteristics of soil nitrogen mineralization, and the ransformation of exogenous NO₃^--N in sterilized and non-sterilized soil samples. The results showed that a combined application of manure and chemical NPK and long-term fallowing increased the contents of soil organic matter, total N, and mineralized N, as well as the N mineralization rate significantly, compared with the control. Applying chemical NPK increased soil inorganic N content, but had less effects on the contents of soil organic matter, total N and mineralized N, as well as the N mineralization rate. Autoclaved sterilization resulted in a significant increase of soil NH₄⁺-N content, while soil NO₃^--N content remained relatively constant whether sterilization or not, suggesting that under the conditions of our incubation, abiotic and biological factors had no significant effects on the transformation of exogenous NO₃^--N.

Key words: density, sapwood, Beta regression, heartwood, Larix olgensis plantation