The Distribution,Formation Mechanism Of Soil Cracking And Its Ecological Impacts On Alpine Rangelands In Eastern Tibetan Plateau

Soil cracking is an important process and feature of terrestrial ecosystem degradation because it affects soil structure,soil properties,soil moisture migration,plant growth,and community species distribution.The research of soil cracking on alpine rangelands on the Tibetan Plateau has rarely been reported previously.This study is based on the observation of soil cracking in alpine rangeland in 2012.From 2013 to 2019,we laid out the framework for a comprehensive cross-scale approach to indicates the relative importance of potential cracking for the entire Tibetan plateau and the detailed relationship between the crack phenomena and the related variables considering grazing by integrating three scales simultaneously.At the landscape scale,we conducted a survey in the alpine meadow,alpine steppe and alpine swamp on the Tibetan Plateau for knowing the spatial distribution of the cracks,and we measured the potential environmental variables in these survey areas for explaining the relationships among soil types,climatic factors and soil cracks.Based on the survey at large scale,we selected an alpine meadow with grazing in Tianzhu County,north-eastern Tibetan Plateau as a studying area at a small scale and established a long-term fixed studying site for clarifying the correlation between the crack parameters and the vegetation and soil properties.Morover,on the micro-scale cracked-soil mosaic,we evaluated the effects of soil cracks on redistribution of water and nutrient as well as plant distribution patterns at the mosaic crack patches.This study is of great significance to understand the response of the degradation and restoration process of alpine rangeland ecosystem to overgrazing.The results are as follow:?1?The soil cracks occurred only in alpine meadow with the annual temperature of0.25±1.04°C and the annual rainfall of 550.11±60.02 mm.The soil would be cracked when the actual stocking rate in the area was greater than 1.59 SU/ha,the plant biomass was less than 49.56 g/m² and the community height was less than 2.43 cm.When the plant biomass was greater than 63.12 g/m² and the community height was greater than 5.31 cm,there were not soil cracks in alpine meadow.Therefore,the residual biomass with 65 g/m²and the community height with 6 cm should be a safeguard threshold for preventing alpine meadow cracking.?2?The soil cracks in alpine meadow has temporal regularity in a whole year.The period of soil cracking and healing are from November to May next year,and from June to October,respectively.During the period of soil cracking,the length,width and depth of the cracks increased firstly and then decreased.In March every year,these values of length,width and depth were greatest in a whole year.However,the length of the cracks increased and then decreased along the month extension and the greatest value was in August,the average length was 43.44±7.01 m and the depth of the healed cracks was 1.81±0.14 cm,which was stable.?3?The soil cracking and healing had the time consistency with the soil temperature was at 0?.The variability of soil temperature in the soil crackling area was higher than that in the non-cracking area.The soil temperature in the non-cracking area was significantly higher than that in the soil crackling area.The annual average soil temperature in the 0-10cm layer and 10-25 cm layer were 1.41°C and 1.08°C higher in the non-cracking areas than in the cracking areas,?4?Soil compaction and soil moisture were the most important ecological factors that cause the formation of soil cracks.Based on multivariate analysis between soil crack factors and environmental factors,and comparison analysis of soil compaction and soil moisture between non-cracking areas and cracking areas.The variance of soil compaction and soil moisture together accounted for 89.0%of the total variance in the crack characteristics with extremely high significance.The soil compaction of the layer between 0cm to 20cm and between 20cm to 40cm,and the soil liquid content of the layer between 0cm to 10cm,10cm to 20cm and 20cm to 30cm in cracking areas were significantly higher than those in non-cracking area.?5?The total number of the seeds was influenced by the mosaic of crack patches.The seeds number in the healed cracks was significantly greater than that in the raised area for different soil layer?0-2 cm,2-5 cm,5-10 cm?.In the soil layer between 0 to10 cm,the soil seed number inside the healed cracks(3738.60±885.17 seeds m^-2)was 2.37 times than that in the raised areas of the crack patches(1575.38±450.16 seeds m^-2).In the soil depth of 0-2cm,2-5cm and 5-10cm,the number of healed cracks was 1.89,3.34 and 3.5 times of the number of soil seeds in the raised areas of the crack patches,respectively.The seeds were mainly distributed in the soil layer of 0-2 cm,and the seeds were moving down deep in soil profile in the healed cracks.Moreover,the dicotyledon plants account for the vast majority proportion of the all seeds?>90%?for both healed cracks and the raised areas.?6?The formation of soil crack had the spatial heterogeneity at micro-patch scale.The soil temperature in the healed cracks was lessen influenced by environmental factors,and the variability of soil temperature in the healed cracks was significantly lower than that in the raised areas.the soil nutrient contents of organic carbon,total nitrogen,available P,and available K in the healed cracks was significantly higher than that in raised areas for all soil layers.The variability of soil available P in the healed cracks was highest among all the soil chemical variables.The soil compaction and bulk density in the healed cracks was significantly lower than that in raised areas,and the soil porosity and mass water content in the healed cracks was significantly higher than that in raised areas for all soil layers.?7?The formation of soil crack had the redistribution of water at micro-patch scale.the soil compaction of the layer between 0 and 30 cm in the raised areas was 2.33 times than that in the healed cracks.The porosities of the healed cracks at the 0-10 cm,10-20 cm and20-30 cm layers were significantly higher than that of same layer in the raised areas.The initial infiltration rate and the steady infiltration rate of the healed cracks were significantly higher than that of the raised areas.The cumulative infiltration time for the monthly total rainfall in the healed cracks was 7.79 times than the raised areas.?8?The formation of soil crack had the significant impact on the plant community.The aboveground biomass and belowground biomass in the healed cracks were significantly higher than that in raised areas.the cumulative rate of the important value of plant species in the raised areas was significantly higher than that of healed cracks,which means the distribution of plant species was more uneven in raised areas.The cover of plant community was stable during the studying period,however,the cover of plants in raised areas was significantly low.The species richness of healed cracks was significantly higher than that of raised areas.The results of non-metric multidimensional scaling?NMDS?revealed a strong change of plant spatial distribution.CCA analysis and variation partitioning analysis further showed that the soil parameters together explained 71%of the variance in the plant community composition on the crack mosaics.The interaction effect of soil physical property and chemical property explained 52.4%of the total variance in the plant community composition.In conclusion,this thesis firstly reports a study of soil crack occurred in alpine meadow with overgrazing on the Qinghai-Tibet Plateau,and explained the causes of soil crack formation as well as the possible ecological consequences of cracked soil.More importantly,the study provided a threshold index for preventing soil crack in alpine meadow,which a residual biomass greater than 63.12 g/m² and a height greater than 5.31 cm,and the soil compaction should be lower than 1044.26±188.88 kPa.The stage of soil cracking in alpine meadow is a turning point,which indicate the health alpine meadow is degradation.Finally,the study suggests that rangeland managers should pay more attention to soil crack occurrence in alpine meadow and prevent soil cracking by reducing the stock rate.