Characters Of Soil Nitrogen Mineralization Processes Of Different Community To Seasonal Freeze-thaw In The Forests Of Tianshan Mountains

The nutrient balance and biogeochemical cycle of forest ecosystems are affected by the freeze-thaw period,but the process of soil gross nitrogen mineralization in the freeze-thaw period requires a quantitative description.Previous studies on the effects of freeze-thaw on soil gross N mineralization are often limited to indoor experiments on shorter time scales,and are mostly directed at temperate forests,grasslands,etc.,and rarely cover the gross N mineralization dynamics of forest soil on a seasonal scale in arid regions.Therefore,this paper sets up different types of vegetation plots in the northern slope of Tianshan mountains,and analyzes the relationship between soil N mineralization dynamics and water and thermal environmental factors based on the measured data.The response of N gross mineralization dynamic process and conversion volume to the freeze-thaw period is not clear.In this paper,¹⁵N pool dilution technique with situ and indoor incubation methods was used to analyze the dynamic process and accumulation of gross N mineralization during the freeze-thaw period of the three community types of soil in woodland,bushland,and grassland in Tianshan mountains.The relationship between soil gross N mineralization rate and soil temperature,moisture content and microbial biomass N（MBN）during freeze-thaw.The freeze-thaw reaction process has an important impact on the changes of the N-transformation of forest ecosystems.The qualitative and quantitative research on the N-transformation mechanism during the process can improve the understanding of the ecosystem N process.This paper can lay a theoretical foundation for the ecological process of forests in arid areas,and provide basic data and research processes for subsequent research.The results showed that:（1）The cumulative gross ammonification accumulation of woodland,bushland,and grassland communities in the whole period（2017.11.01²018.04.15）was 163.9,88.3 and 103.4 kg N ha^-1 a^-1,and the ratio of ammonium accumulation during freeze-thaw period accounted for about 66%of that accumulated in the whole period;the gross nitrification is 55.4,41.6 and 47.1 kg N ha^-1 a^-1,and the ratio is about 77.4%;（2）gross N mineralization rate,net N mineralization rate and MBN rate have been affected by the freeze-thaw process and the community types significantly（P<0.01）;gross N mineralization rate in autumn and spring freeze-thaw period was higher than the constant frozen period;and soil gross N mineralization rate and MBN rate in woodland were greater than bushland and grassland;（3）gross N mineralization rate,net N mineralization rate,and MBN rate have been affected by soil temperature and moisture significantly.As soil temperature increases,gross ammonification rate（only woodland and bushland）,net ammonification rate（woodland）and net N mineralization rate（woodland）increase,while net nitrification rate（bush）,net nitrogen mineralization rate（grassland）and MBN rate（woodland and grassland）decreased,which was significantly correlated with soil temperature（P<0.05）;when soil moisture content increased,net ammonification rate（bushland）,net nitrification rate（bushland）and net N mineralization rate（bushland）decreased,which was significantly negatively correlated with soil moisture content（P<0.05）.It can be seen that the gross N mineralization（gross ammonification rate and gross nitrification rate and the cumulative amount of conversion）process is responsive to the freeze-thaw process and closely related to temperature,moisture,and MBN,which provides a theoretical basis to explain the biogeochemical processes in the forest soil of Tianshan mountains.