Effects Of Seasonal Freeze-Thaw On Soil Physicochemical Properties Under Climate Change

We first looked at how seasonal freezing and thawing affect the stability of soil aggregates in Sygera Mountain,and the duration of freeze-thaw and depth of freeze-thaw at different altitudes（3500 m,3900 m,4300 m）were measured by meteorological instruments,so as to determine the different degrees of freeze-thaw.We did this by comparing changes in soil water content（VWC）,0.25 mm water-stable aggregate content（WSA）,mean mass diameter（MWD）,mean geometric diameter（GMD）,the fractal dimension D,and the erodibility index K value.The second goal is to understand how soil aggregates at different altitudes are different in space and how this difference affects the soil’s physical and chemical properties.Examine the relationship between soil physical indicators（soil bulk density,capillary porosity,non-capillary porosity,total porosity,and water content）and chemical indicators（carbon（SOC）,total nitrogen（TN）,available nitrogen（AN）,ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,total phosphorus（TP）,available phosphorus（AP）,p H）,and soil aggregate characteristics.Explore the impact of various treatments on the stability of forest soil aggregates in southeast Tibet,as well as the responses of several soil aggregate indicators to nitrogen and phosphorus additions.In natural conditions,temperatures increase,cooling occurs,and nitrogen and phosphorus fertilization experiments are conducted.The experiment involving the addition of nitrogen and phosphorus was conducted in a random manner.Three nitrogen addition treatments(5,10&15 g N m^-2 yr^-1)were established,and each treatment level was set to four repetitions.There were a total of three phosphorus addition treatments(3,6&9 g P m^-2 yr^-1)and four duplicates for each treatment level.With the combination of nitrogen and phosphorus,a total of three treatments(P3N5,P6N10&P9N15 m^-2 yr^-1)and three duplicates were established for each treatment level.Explain the impact of temperature increases,cooling,and the addition of nitrogen and phosphorus on the stability of soil aggregates.Using multivariate analysis of variance and Pearson correlation,the data were compared.In order to understand the correlation between spatial heterogeneity of soil aggregates and soil physical and chemical properties at different elevations,and to explore the effects of different treatments on the stability of soil aggregates in the forests of southeast Tibet,and the response of soil aggregate indexes to nitrogen and phosphorus addition.The following are important study findings:（1）Seasonal freeze-thaw effect leaded to a certain cracking effect on soil aggregate structure.In each soil depth layer,the distribution of large-size aggregate structure decreased,while the distribution of small-size aggregate content increased.At the altitude of 4300 m,the soil stability decreased gradually with the increase of sampling depth,and the anti-erosion ability also showed a gradual weakening trend.In the three soil layers,WSA,MWD and GMD increased first and then decreased with the increase of altitude,soil erodibility K and fractal dimension D decreased first and then increased,and soil stability increased first and then decreased with the increase of altitude.Correlation analysis results showed that WSA,MWD and GMD were negatively correlated with D and K values in the three elevations.At the altitude of 3900 m and 4300m,the grain size contents between 10 mm and 0.5 mm had no significant correlation with VWC,WSA,MWD,GMD and D value and K value.In the diameter range between 0.5 mm and 0.25 mm,the grain size contents were significantly negatively correlated with MWD and GMD value,and significantly positively correlated with K value.The critical point is 0.5 mm grain size.（2）At the same elevation,bulk density and capillary porosity increased with the increase of soil depth,while non-capillary porosity showed inconsistent change trends at the three elevations,and soil nutrients showed a decreasing trend（except TP and p H）.With the increase of altitude,the bulk density generally increased,and the porosity increased first and then decreased.With the increase of soil elevation,soil nutrients firstly decreased and then increased（except AP and AN）.At 3500m,there was no significant correlation between physical and chemical indexes and aggregate indexes;at 4300 m,NCAP is significantly positively correlated with MWD and GMD,while significantly negatively correlated with K and D.In 3900 m,AP has a significant positive correlation with MWD and GMD and a significant negative correlation with fractal dimension D.At 4300 m,AP had a significant negative correlation with D,NH₄⁺-N,TN,SOC,AN had a significant positive correlation with MWD and GMD,and a significant negative correlation with K and D.The altitude had a certain regulation effect on the soil physical indexes,soil nutrients and soil aggregates,and the increasing of altitude strengthened the interaction between soil aggregates and soil nutrients.（3）The mechanism of nitrogen and phosphorus addition to enhance the stability of soil aggregates was mainly to improve the stability of soil aggregates in deep soil rather than in surface soil.The effect of nitrogen addition on the surface layer was not significant at 3500 m elevation,but the effect on the deep soil was significant.The effect of P addition and N-P mixture addition on the surface depth was significant.After cooling treatment at 3500 m elevation,the effect of N and P addition on the surface soil was not significant,but the effect on the deep soil was significant.The mixed addition of N and P had significant effects on the deep surface soil.Nitrogen addition had significant effect on the surface soil stability,but not on the deep soil.Phosphorus addition had no significant effect on the surface and deep soil stability.The effects of nitrogen and phosphorus mixed addition on the deep surface soil were significant.The effects of nitrogen and phosphorus addition and nitrogen and phosphorus mixed addition on the stability of surface and deep soil were significant at 4300 m elevation.The stability of soil aggregates was enhanced by increasing temperature and applying nitrogen.The main conclusions are as follows:1)The soil aggregate structure was broken due to seasonal freeze-thaw effect,and 0.5 mm particle size was an important critical point.2)Altitude had a certain regulation effect on soil physical indexes,soil nutrients and aggregates,and the interaction between soil aggregates and soil nutrients was strengthened with the increase of altitude.3)The mechanism of nitrogen and phosphorus addition to enhance the stability of aggregates is mainly to improve the stability of deep aggregates rather than surface aggregates.The stability of soil aggregates was enhanced by increasing temperature and applying nitrogen.This study aims to investigate the spatial heterogeneity of soil physical and chemical properties and aggregate characteristics at various altitudes,to investigate and provide data support for the potential mechanism of high-altitude areas affecting soil stability,and to provide insights into the potential erosion of permafrost and global climate change.Soil safety serves as a theoretical foundation.To establish a scientific and theoretical foundation for the building and preservation of the ecological environment in alpine regions comparable to Sygera Mountain in southeast Tibet.