Monitoring Study On Alpine Meadow Responseto Freezing-Thawing Events In The Nagqu River Basin

Alpine meadows are a special vegetation that grow only in alpine regions,which are very important for soil and water conservation,soil remediation,animal husbandry development and ecosystem services.The freezing-thawing cycle is an important feature for the frozen soil ecosystem in the alpine region,and effects the growth and survival of alpine meadows.As the climate continues to warm,glaciers and frozen soil are melting,and the characteristics of freezing-thawing cycle also changes significantly.This series of changes will definitely affect the growth and survival of alpine meadow.Therefore,the evolution of freezing-thawing cycle under climate change,the influence mechanism of freezing-thawing cycle on alpine meadows,and coping strategies have always been concerned.In this article,the Nagqu River Basin of Tibetan Plateau is the study area.Monitoring site layout,sample collection and measurement,biomass measurement,and root ecological monitoring experiments were completed.Types of data such as soil temperature and moisture,soil chemical property,soil microbial property,alpine meadow biomass,and root distribution were observed and analyzed.The influenced mechanism of freezing-thawing cycle on alpine meadow in study area was illustrated,as alpine meadows and soil properties were considered.Additionally,the evolution trend of freezing-thawing cycle and the evolution principle of alpine meadows in Nagqu River Basin were analyzed,and effective countermeasures were proposed.The main achievements and results are as follows:(1)An evaluation index for freezing-thawing cycle in alpine regions is constructed.The freezing-thawing cycle frequency(FTCF)indicator is constructed by calculating the ratio.The results are as follows: The freezing-thawing cycle modes in Nagqu River Basin are the high frequency mode(HFM)(FTCF>1.0)and the low frequency mode(LFM)(FTCF<1.0).The Bange Bridge(BG)is the boundary line,the north of BG is LFM,and the south of BG is HFM.With the influence of climate change,the LFM is gradually shifting to the HFM.(2)The effect of freezing-thawing cycle mode on alpine meadow biomass and root distribution is analyzed.With the increase of FTCF,the abundance of Bradyrhizobium decreased by 2.85%,the abundance of Mesorhizobium decreased by 54.03%,the abundance of Pseudomonas decreased by 81.13%,the abundance of Bacillus increased by 6.5 times,the symbiotic nitrogen fixation ability and disease resistance ability of alpine meadows are weakened,the moisture and nutrient content in each layer of soil is significantly reduced,the aboveground biomass of alpine meadows is reduced by 35.89%,the underground biomass is reduced by 22.74%,the range of root distribution is reduced from 0-40 cm to 0-20 cm,the content of fine roots with a diameter of 0-0.5 mm increases and its specific gravity increases.(3)The influence mechanism of freezing-thawing cycle on alpine meadow growth is identified.With the increase of FTCF,the damage to the root system of alpine meadows also increase.Frequent freezing-thawing cycle can kill Bradyrhizobium,Mesorhizobium and Pseudomonas,the symbiotic nitrogen fixation and disease resistance of alpine meadows may be reduced.Meanwhile,the increase of FTCF accelerates the process of nutrient loss and freezing,the increase of FTCF also destroys the soil structure,and the water content and nutrient content in the soil is reduced.With the combined effects of these series of effects,the biomass of alpine meadows in Nagqu River Basin is reduced,and the growth and survival of alpine meadows is threatened.In LFM,the soil moisture gradually increases from the surface layer to the deep layer.With the increase of FTCF,the soil moisture is higher in the surface layer(0-20 cm)and the deep layer(40-60 cm),and the soil moisture is lower in the middle layer(20-40 cm).In order to absorb more water,root distribution range changed from 0-40 cm to 0-20 cm,and the growth mode changed from vertical extension to transversal divergence.With the increase of FTCF,the soil moisture and nutrient content in each layer are significantly reduced,and the root content of 0-0.5 mm diameter increased to improve the absorption of water and nutrients in the soil.(4)The response measures of alpine meadow under the influence of freezingthawing cycle are put forward.In order to protect the growth of alpine meadow,the melting water of glacial and frozen soil should be used efficiently,nitrogen and potassium fertilizers should be reasonably added.In addition,the planting method and frequency should be adjusted,the vegetation coverage of alpine meadows should be improved,and the vegetation with smaller root diameter should be planted.This study can better understand the direct and indirect effects of freezing-thawing cycles on the growth and survival of alpine meadows.It also provides a clear research direction and important theoretical basis for studying the response of frozen soil to climate change.It has very important theoretical and practical values for ecological construction in alpine regions.