Abstract: Seasonal freeze-thaw cycle and its change pattern under the scenarios of climate warming might exert strong effects on the soil nitrogen mineralization in alpine forests. In this paper, intact soil cores were collected from the subalpine/alpine forests along an altitudinal gradient in west Sichuan, and an incubation test was conducted to study the soil  net nitrogen mineralization rate and the amount of soil mineralized nitrogen in the forests during growth season and seasonal freeze-thaw period under simulated scenarios of global warming. In the test soils, the NH4+-N and NO3--N contents both showed a clear tendency of decreased in the period from growth season to the onset stage of freezing, increased at deep freezing stage, and decreased again at the early stage of thawing. The soil net nitrogen  mineralization rate and the amount of soil mineralized nitrogen were significantly lower in freeze-thaw period than in growth season, and the
soil inorganic nitrogen was obviously immobilized. The soil nitrogen immobilization was stronger at middle altitudes but weaker at high altitudes, as compared with that at low altitudes, possibly due to the variation of soil temperature and its induced different freeze-thaw cycle. During growth period, the soil  net nitrogen mineralization rate and the amount of soil mineralized nitrogen showed an obvious increasing trend with the decrease of altitude, and the soil nitrogen mineralization was the strongest at low altitudes, implying that under the scenarios of climate warming, the increase of soil temperature promoted the soil nitrogen mineralization during growth season, and affected the soil nitrogen mineralization rate by increasing the frequency of freeze-thaw cycle and shortening the time period of freeze-thaw. Soil micro-environment could also affect the soil nitrogen mineralization in alpine forest regions.

Key words: seasonal freeze-thaw, soil nitrogen mineralization, alpine forest, climate warming