| The Qinghai-Tibet plateau?QTP?,which is one of the highest and largest geographical feature on earth,and were very sensitive to global climate change or management practices at different spatial scales.The Qinghai-Tibet plateau is also usually called “water of Asia” due to it's active water cycle.This area has one of the world's most extensive and highest alpine meadow ecosystems,and its ecological function is also significant.This plateau is known as the“third pole”of the earth and has a significant role in maintaining the ecological security of China.Over the past century,especially in the recent decades,this region has undergone severe grassland degradation due to overgrazing,excessive farming and vegetation devastation by fuelwood gathering.To date,the primary landscape of grassland has been degraded to a certain extent,and shrubs have become the most significant dominant plant species.The existence of shrublands help improve the physical and chemical properties of soil and the local microclimate,as well as provide suitable habitats for creatures compared with herbaceous plants which result in higher biomass,composition of soil microorganisms and herbaceous plants growing beneath shrubs compared to outside shrubs.Hence,Shrubs usually plays an important role in the degraded grassland ecosystems.All these works were designed to provide theoretical and methodological bases for policy making and optimizing ecosystem management in grasslands.The study was conducted in Hezuo,Gannan Tibetan Autonomous Prefecture,Gansu Province,on the eastern part of the Tibetan Plateau.The vegetation is alpine meadows,and H.rhamnoides was a key woody shrub species in the study area.Based on the field sampling and the laboratory analyses,the spatial distribution of soil nutrients in different plant communities were studied.The results showed that:?1?Averaged over the five depths of 0–0.2,0.2–0.4,0.4–0.6,0.6–0.8 and 0.8–1.0 m,the root C,N and P contents in shrubs were 424.85 g kg–1,18.86 g kg–1,1.28 g kg–1,respectively,contrasted to 394.71 g kg–1,12.29 g kg–1,1.05 g kg–1 in grasses,respectively.The results showed that the root C,N and P contents were greater in shrubs relative to the adjacent grasses,which indicate differences in the plant nutritions between shrubs and grasses were determined by intrinsic physiological characteristics.?2?The biomass of ground litter present comprised 279.16 g m?–2? beneath the shrubs,comparedwith 80.13 g m?–2? beneath the grasses.The mean groundlitter biomass beneath the shrub canopies were increased by 71% compared to those of the adjacent grasses land.The mean values of litter C,N and P contents for shrubs were 456.92 g kg–1,12.47 g kg–1,0.92 g kg–1,respectively,while for grasses,they were 364.69 g kg–1,16.44 g kg–1 and 0.23 g kg–1,respectively.Without question,a strong bond existed between the ground litter and soil.Ground litter may induce dramatic changes in soil nutrient cycling in alpine meadows.?3?The leaf C,N,P concentration of Hippophae rhamnoides were 470.84 g kg–1,35.28 g kg–1,3.16 g kg–1,respectively.The C,N,P concentrations in leaves were 435.68 g kg–1,20.22 g kg–1,2.12 g kg–1,respectively,in grasses.Analysis showed that the leaves of shrubs contain significantly higher C,N,P concentration than that of grasses.The results revealed that shrubs had a greater capacity to acquire nutrients and use the nutritional elements from soils than the herbaceous plant species.In addition,shrubs accumulated significantly more organic matter in soil.?4?Summed over the entire 1.0m soil profile,the root C,N and P stocks were 6586.76 g m?–2?,276.08 g m`?–2?,19.86 g m?–2?,respectively,for shrubs,while for grasses,they were 3295.50 g m?–2?,99.03 g m?–2?,8.51 g m?–2?,respectively.Root C,N and P stocks in soils beneath shrubby areas were remarkably higher compared to the adjacent herbaceous plant species.The increases in root C,N,P stocks were mainly due to the increase in the root biomass?extending deeper?in the soils under shrubs.It might also be related to the differences in soil environments,as well as to their morphological traits.?5?Significantly higher contents of SOC,nutrients,and microorganisms were accumulated under shrub canopies creating ‘‘fertility of islands' ',this effect was particularly evident in the 0–20cm soil layer.Shrubs caused the largest increases in SOC,total N and inorganic N at 0–20 cm,by 21.98%,15.10%,66.65% compared with the grasses,respectively.Here we found that Shrubs can induce the spatial heterogeneity of soil properties,and have been seen to markedly change the microclimatological conditions and nutrient input beneath their canopy,Hence,this resulted in an accelerated nutrient cycling in soils.?6?There were higher P concentrations above 0.4 m depth,but lower concentrations below 0.4 m,under the shrubs,compared to the herbaceous plant species.The results indicated that shrubs had a greater capacity to translocate P via root uptake from the lower to the upper soil depths than the herbaceous plant species.?7?In shrubby species plot,the proportions of MBC in SOC?% MBC?was 0.79 %,while MBN in total N?% MBN?and MBP in TP?% MBP?were1.50 %,2.16 %,respectively.But in the herbaceous species plot,they were 0.68%,1.14%,1.10%,respectively.Under shrub canopies,the soil organic matters in the upper soil layers were higher than in the herbaceous areas.Accumulation of SOM under shrub canopies could enhance microbial growth.?8?Under both woody species and herbaceous species,soil phosphatase activities decreased with the increase of depth throughout the soil profile,in particular,it was significantly more abundant in the surface soils.In our study,we found that the higher Ac PME and Al PME activities in the soil profile down to 1.0 m depth under the shrubs,compared with the herbaceous plant species,which followed the same trend as microbial biomass.Overall,the results revealed that the shrubby encroachment in the alpine meadows on the Qinghai–Tibetan Plateau has dramatically influenced and altered the spatial distribution of nutrients in soils,which favor growth of shrubby species and may have exacerbated shrubby plant encroachment into grass-dominated ecosystems. |
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