Abstract: Global climate change makes snow pattern of terrestrial ecosystems change in the middle and high latitude areas, which is critical to soil nitrogen cycling, and it can lead to changes of intensity and frequency of soil freezing-thawing cycles. The freezing-thawing cycles could affect soil nitrogen cycles through microbial biomass and community. The study was carried out in Dahuofang experimental forest watershed. The effect mechanism of freezing-thawing cycles on inorganic nitrogen content of soil was revealed, which could provide the references for assessing the risk of N loss in the riparian ecosystem. The results show that soil inorganic nitrogen content is in an increasing trend with the increasing times of freezing-thawing cycles. Different freezing temperature has the significant effect on inorganic nitrogen. The inorganic nitrogen contents are 34.9±0.9 mg/kg and 37.2±0.8 mg/kg after 10 freezing-thawing cycles with the -5℃/+5℃ and -20℃/+5℃ treatments, which are 1.21 and 1.41 times of initial inorganic nitrogen content of soil samples, respectively. Furthermore, freezing-thawing cycles and its temperature difference have the significant effect on soil NH₄ ⁺-N content (P<0.01). After 10 freezing-thawing cycles, soil NH₄ ⁺-N content is 4-10 times of soil samples for +5 ℃ treatment. Freezing-thawing cycles have the significant influences on soil NO₃- -N content (P< 0.05), while its temperature difference has not the significant influences on NO₃-N content (P>0.05). The response of soil microbial biomass nitrogen content to freezing-thawing cycles is significant (P<0.01). Thus, freezing-thawing changes increase the inorganic nitrogen content of soil. Because of less absorption of inorganic nitrogen by vegetation in early spring, the risk of soil nitrogen loss may increase with snow melting in study area.

Key words: Freezing-thawing change, Inorganic nitrogen, Microbial biomass nitrogen of soil, Riparian zone, Small watershed