Effect Of Grassland Reclamation And Highway Traffic On Grassland Soil Degradation In Tibetan Plateau

The cultivation of crops on grassland has a long history on the Tibetan Plateau,which has promoted the settlement of human beings on the plateau.The content of soil organic matter decreased significantly after reclamation,which is generally believed to be related to the decrease of organic carbon input and mineralization acceleration due to tillage disturbance.However,if the metabolic synthesis of soil microorganisms changes during this process,the decrease of soil organic matter content is related to the decrease of the accumulation of microbial synthetic materials,which has not been reported before.With the increase of economic activity since the reform and opening up,the transportation in Tibetan Plateau has been developing very fast,and there is little research on the effect of highway traffic on the accumulation of heavy metals in grassland soil.This thesis we take soil non-cellulosic hexoses(mainly by microbial synthesis)and pentose sugar(mainly plant sources)and the proportion of the content of amino sugars as the indicator,to study the grassland reclamation effects on soil microbial metabolism synthesis product accumulates.At the same time,we studied the accumulation of heavy metals in grassland soil and the influence of soil profile distribution on the high altitude and distance of highway.Conversion of grassland to cropland affects microbial transformation of plant derived organic substances and accumulation of microbially synthesized products in soil.We investigated long-term effects of agricultural use[more than 50 years'rotation with barley(Hordeum vulgare)and rapeseed(Brassica rapa)]after conversion from grasslands(grazed in the winter season)on the contents and composition of non-cellulose sugars and amino sugars in a broad range of soils on the Tibetan Plateau.Soils from two types of croplands(barley or rapeseed)were compared to reference grassland soil by detailed analyses of plant vs.microbial and bacterial vs.fungal bio-markers.The results show:(1)Long-term cultivation decreased total soil organic matter(SOM),light SOM,heavy SOM and total non-cellulose sugar contents in the 0–20 cm soil by 27%,60%,24%and 57%,respectively,reflecting decreased root biomass compared to grassland.(2)The ratios of(galactose+mannose)/(arabinose+xylose)and(rhamnose+fucose)/(arabinose+xylose)were both 26%smaller in cropland than in grassland soils,while these ratios of hemicelluloses in shoots or roots were similar between native and cultivated plants.(3)Consequently,net transformation of plant substances to microbially synthesized polysaccharides decreased in cultivated soils.The total amino sugars(muramic acid,glucosamine,mannosamine,galactosamine)in cropland soils decreased by 42%and their contribution to the SOM by 22%compared to grassland soils,but the ratio of glucosamine/muramic acid in cropland soils doubled compared to grassland soil.(4)All the above results were verified in the other seven extensive sampling pointsThe effects of human activities on heavy metal pollution in soil have been less investigated on the Tibetan Plateau.The present study was designed to assess the effects of highway traffic on Cu,Zn,Pb,and Cd enrichment in the 0?60cm soil profile in the eastern Tibetan Plateau.Soils were sampled at four transects(with an altitude range of 2643?2911 m)across the G212 highway and five transects(3163?3563 m)across the G213 highway.(1)Background concentrations of Cu,Zn,Pb,and Cd to the 60 cm soil depth(measured at each transect 400maway from highways)varied greatly among transects and between highways.However,this spatial variation in the heavy metal concentrations was not related to the altitude of the investigated areas.(2)On each the left and right sides of G212 or G213,Cu,Zn,and Pb concentrations to the 60 cm depth,at 5,10,20,and 50 m away from the highway,were all generally greater than the respective metal background concentrations.(3)Cd concentrations to the 20 cm on G212 or 60 cm soil depth on G213increased prominently within a distance of 20maway from the highways,compared to background values in different depths.From the curb to 400 m away from highways,concentrations of Cu,Zn,Pb,and Cd were generally higher in the upper than in the lower soil layers.(4)This may suggest that other factors such as atmospheric deposition were also contributable to the accumulation of heavy metals in soil.(5)The contamination factor(C_f)calculation showed that roadside soils to the60-cm depth,within a distance of 50 m from the curbs of both G212 and G213,were moderately(1?C_f<3)contaminated with Cu,Zn,and Pb.The contamination from Cd mainly occurred(1?C_f<8)in the top 20 cm soil with a closer distance from the highways.Compared with the natural grassland soil,in the cultivated soil,the microorganisms converted the plant compounds into the net conversion rate of polysaccharide.The number of bacteria declines faster than the fungus.Long-term farming significantly reduced the net conversion rate of plant compounds and the cumulative amount of microbial synthetic compounds.In the soil along the national highway along the eastern Tibetan Plateau,the concentration of heavy metals in the soil profile is at least 60 cm deep and 50 m far away.To evaluate the degree of soil heavy metal contamination in mountainous terrain,the background value must be measured in the sampling area.