Effects Of Freeze-thaw On Soil Nitrogen Dynamics And Nitrogen Mineralization Rate In Tianshan Mountains

Seasonal freeze-thaw regimes are the main factors influencing nitrogen(N)transformations in the temperate forest of northern China and one of the key processes in the soil N cycle.The dynamics of nitrogen transformation in forest soil is affected by factors such as hydrothermal conditions and microbial communities.In the process of gradual falling of temperature and snow in autumn and winter and the gradual melting of snow and snow in spring,the freeze-thaw phenomena of soil in high latitudes and high altitudes of the North frequently occur.The fragmentation of soil aggregates,litter decomposition,microbial death and group changes in soil freezing and thawing can promote the accumulation and release of soil nitrogen.This part of soil nitrogen can be directly used by soil microorganisms and plants or leaching with snow melting.Freeze-thaw processes can affect the dynamics of nitrogen transformation in soil nutrient mineralization,nutrient turnover and so on,and then regulate the ecological processes,such as litter decomposition,root growth,community structure change and system function stability.More attention has been paid to the soil freezing and thawing processes and ecological effects of farmland and other ecosystems in the northern forestland,grassland and mid latitudes,but there are still some unknown problems in the aspects of soil nitrogen composition,transformation and loss in the process of freeze-thaw.In this study,we conducted experiments to understand the effect of soil freeze-thaw on N transformations in soils(0-15 cm)from different coenotypes(woodland,grassland,and shrub)in Tianshan Mountains forest by simulating extreme soil freeze-thaw in different natural snow covers formed by canopy shading.The purpose of this study was to clarify the effect of seasonal freeze-thaw and snow on the soil nitrogen in the Tianshan forest area,and the difference in the response of soil nitrogen dynamics to the freezing thawing process in different types of communities.The results show that:(1)The content of ammonium nitrogen(NH4+-N)and microbial biomass nitrogen(MBN)in the soil of different types of communities were positively correlated with the temperature of soil(5 cm),and the content of ammonium nitrogen in the soil of the deep frozen forest was lower than that of the other community types,while the nitrate nitrogen content was higher than that of the other community types.(2)Nitrate nitrogen(NO3--N)is the main body of mineral nitrogen in Tianshan forest area during seasonal freezing thawing period,accounting for 78.4%.(3)The risk of nitrate nitrogen in shrub soil was greater.At the end of thaw stage,the nitrate nitrogen content in shrub soil decreased by 64.6%.(4)In the study area,the whole nitrogen mineralization rate was significantly affected by freeze-thaw period,and the community type had a significant effect on ammonification rate.(5)Soil nitrogen in Tianshan forest area is mainly nitrogen fixation during freezing period.