Study The Reason Of Soil Crack Formation And Its Ecological Impact Assessment In Alpine Meadow

Qinghai-Tibet Plateau is one of the hotspots in global biodiversity and geology research.The alpine meadow is the most important ecosystem in the plateau.At present,the degradation of rangeland ecosystem in this region is becoming more and more serious because of human disturbance and climate change.Grazing is one of human disturbance in alpine meadow system and it significantly affected the composition of plant community and soil structure.Based on the long-term observation in field,the author found that the soil of alpine meadow crack under overgrazing.As soil cracks is a new research field.Its mechanism is not yet clear.This project try to explain the reasons of soil crack in alpine meadow and its possible ecological consequences.Firstly,we investigated the soil crack in typical alpine swamps,alpine meadows and alpine steppe located in the eastern Qinghai-Tibet Plateau,the purpose was to determine the occurrence of the soil cracks in the different climate area.Based on the results of the survey at landscape scale,three sample sites in overgrazing areas and the non-cracked areas in moderate grazing were set up in the alpine meadow system.The sample sites were determined by random sampling method in each sampling area.The soil and vegetation samples were measured.The samples were fixed to clarify the causes of soil cracks in alpine meadow.In each sample site in the overgrazed area,nine patches were selected to measure the vegetation and soil data at the corresponding healing fracture and the protruding parts inside the fracture plaque.From the aspects of soil and vegetation in alpine meadow,the possible ecological consequences of healing fractures were discussed.The results showed:1.The investigation at the landscape scale indicated that the soil cracks only occurred at the annual mean temperature of 0.25 ± 1.05 ℃ and the average annual rainfall of 550.11 ± 60.60 mm in alpine meadow.Temperature and rainfall mainly cause soil cracks.In alpine meadow,the soil cracking and healing time in the year was unchanged.At the end of October each year,soil began to crack,the following year in March to reach the maximum fracture(width of up to 3cm,depth of up to 30cm),soil began to heal in early May,the location of the crack was occurred at same place.The cracking area of the soil in August accounted for 17.96 ± 1.15% of the total sample area.DCA ordination showed that the location of the cracked region in the sorting axis has been significantly different from that of the non-cracked region.The height,coverage and aboveground biomass of the plant community in the cracked area were significantly higher than those in the non-cracked area,and the relative important value of Cyperaceae and Compositae were higher in the cracked soil.The temperature of the surface soil(0-25cm)in the non-cracked area was higher than that in the cracked zone in every month.The difference of soil organic carbon between the cracked and non-cracked areas was not significant difference.The soil bulk density and soil compaction in non-cracked areas were significantly lower than those in crack area,and the soil water content was significantly higher than that in crack area.The content of aggregates with 0.25-2mm in soils underground 20-30 cm in non-cracked area was significantly higher than that in the cracked area.The comprehensive analysis of RDA showed that the formation of soil fractures was caused by the combined effect of soil compaction with depth of 10-40 cm,soil moisture with depth of 10-30 cm and aboveground biomass.2.To know the ecological consequences of the crack,we performed a comparative analysis of healing crack grooves and protrusions.It was found that the soil in the healing crack groove caught a large amount of germplasm resources and was 3.15 time in the same area.In the soil of 0-25 cm soil layer,the temperature was higher than that of protruding area from May to October,and the soil temperature of 0-10 cm soil layer was lower than that of protrusion at the time from October to January.The soil temperature of 0-25 cm soil layer was higher than that of protruding part.The soil compaction and bulk density of the healing crack groove were significantly lower than those of the protruding parts,while the capillary porosity and soil moisture were significantly higher than the protruding parts.The soil aggregates content(<1 mm)in soil layer with 10-30 cm was significantly higher than that of protruding area.The contents of soil organic matter,total N,available P and total K in the healing crack groove were significantly higher than those in the protrusions.This provides the basis foundation for plant community colonization and transformation.The further analysis showed that the species richness,height and coverage of the plant community were significantly higher than those of the protruding parts.The plant distribution also showed obvious differentiation,and the plants tend to transfer to the cracked groove.The plant communities with the dominant species of Kobresia capillifolia,Kobresia humilis,Poa crymophila are distributed in a large number of the healing crack groove.CCA sequencing showed that the formation of healing cracks leads to a significant decrease in soil compaction and bulk density below to the protruding sites,which is the most important environmental factor that causes a significantly difference about the plant community structure between healing cracks and the protruding sites,also providing a new microhabitat.The formation of soil crack with large area in alpine meadows is due to the increase of soil compaction and bulk density because of overgrazing in the Qinghai-Tibet Plateau.When the temperature drastically decreases,the volume of solid particles in soil is almost constant,but the soil moisture increases due to icing,which leads to soil movement.As a result,soil was cracking.However,after the temperature rises,the cracks are re-healed.Due to the micro-topographic,the water and fertilizer are enriched in the place.The soil temperature is high and the temperature difference is small,which forms the suitable habitat for vegetation growth,and eventually leading to differences in vegetation distribution.