| As the transition zone between steppe and desert,desert steppe is an important part of grassland ecosystem.The desert steppe of Inner Mongolia Autonomous Region,accounting for 18%of China's total grassland area,act as an important ecological security barrier in northwest China due to its proximity to the source of wind and sand.Darhan Muminggan Joint Banner,a typical desert steppe region,is chosen as the study area,the temporal and spatial dynamics of vegetation fractional coverage(VFC)and the correlations between the VFC and its influential factors is analyzed in pixel scale,the dominant factors of the VFC is identified based on that.The precipitation pulse patterns and their spatiotemporal variation characteristics in the desert steppe is revealed.Response of soil water content and actual evapotranspiration on precipitation pulses is explored by a continuous and synchronous observation of multi-water sources in the field.The soil-vegetation-microbial C,N,P and their stoichiometric characteristics in different rainfall zones of desert steppe are investigated and analyzed.Finally,an ecological suitability evaluation system of desert steppe based on ecological suitability index is established,and an optimal layout plan of the existing farmland in the farming-pastoral intersection zone in the study area is proposed.The main conclusions of this paper are as follows:(1)Spatiotemporal analyses of vegetation fractional coverage dynamics and its dominant factors in desert steppe.The study area is dominated by medium-low VFC grassland.The total and grassland of different types show an increasing trend during the 14 years of the study period.A northwest moving trend of the VFC centre is evident in the observed period at a speed of 88 m/a westwards and 62 m/a northwards.The pixel-level correlation coefficients between VFC and precipitation,temperature,groundwater table,available soil water content,and DEM is 0.81,-0.76,0.04,0.23,and 0.66,respectively.Rainfall and temperature are the two dominant factors of VFC.Among the precipitation and temperature factors,the dynamics of the VFC centre are mainly affected by precipitation in the observed years.(2)Precipitation pulse pattern and its spatiotemporal variation in desert steppeAffected by the topography,there is a significant precipitation gradient from south to north in the study area.The fluctuations in precipitation amount are significantly greater than fluctuations in the number of precipitation days.And overall,as the annual precipitation decreased,the fluctuation in the number of precipitation days and the precipitation amount in the study area gradually increased.The study area is dominated by small precipitation events of 0-5mm,and the difference in the occurrence of precipitation events>10 mm is the main reason for dramatic interannual variations in precipitation.Both the amount of precipitation and the number of precipitation days of different precipitation events showed an increasing trend in different precipitation zone of the study area.It shows a significant increasing trend of small precipitation events of 0-5mm in both Bailingmiao and Mandula regions.However,in the Xilamuren region,there has been a shift in the internal precipitation structure from large and medium precipitation events to small precipitation events.The annual and growing-season precipitation amount and number of precipitation days have 2.84?4.5 a main periods.(3)Response of soil water content and evapotranspiration on precipitation pulses in desert steppeA single small precipitation event has a limited impact on SWC and provides no obvious increase in the SWC within the top 10 cm soil layer.The precipitation interval ratio(P/I)is suitable for comparing water stresses of different drying-wetting cycles,and correlations between soil layers are found to be closer in humid years than in dry years.The maximum daily ETa of-5.78 mm/d occurred in August during the growing season of 2016.In 2016,the amount of ETa was greater than that of precipitation for the same period during the whole growing season,and this water deficit was related to previous soil water storage and condensation water.According to continuous ETa observations,three modes of interpulse ETa in the desert steppe have been obtained,which is a stable ETa mode under a water-sufficient condition,an attenuation ETa mode,and a stable ETa mode under extreme drought conditions.(4)Soil-vegetation-microbial C,N,P and their stoichiometr:ic characteristics in different rainfall zones of desert steppeSoil C,N and P in different rainfall zones in the study area shows a decreasing trend with the decrease of precipitation gradient.The mean soil C:N:P ratio is 28.9:2.7:1 in the study area,which is mainly controlled by P.The mean MBC:MBN:MBP ratio of soil microbial biomass is 108.6:5.6:1 in different rainfall zones,which shows a C enrichment.The mean plant C:N:P ratio is 117.4:6.7:1?which shows a limitation of C and N or a enrichment of P.The climatic reasons caused a relatively high P content in the environment of the study area.A significant positive correlation at P<0.01 is found between C and N contents in soil,which has a correlation coefficient of 0.98.The plant N and P show a significant positive correlation(P<0.05)with their correlation coefficient of 0.90.Among soil,plant and microbial biomass in the study area,soil N has a significant positive correlation(P<0.05)and a significant negative correlation(P<0.05)with plant C(correlation coefficient 0.84)and P(correlation coefficient-0.82),respectively.The rest do not reach the significant level.(5)An optimal layout plan of the existing farmland in the farming-pastoral intersection zone based on ecological suitability index in desert steppeThe main changes in the landscape pattern in the study area in recent decades is the reduction of grassland and the increase of the farmland,which led to the weakness of the overall connectivity,and the aggravation of the degree of fragmentation and discretization of the landscape.The development of farmland concentrated between 1980 and 2000 is the main reason for the variation of landscape pattern in the short term.The ESI of the study area decreases from southeast to northwest with a average ESI of 0.39.The ecological suiability condition in desert steppe of the study area could be well reflected by ESI,which has a correlation coefficient of 0.84 with VFC.In terms of different types of grassland,the ESI of the high,medium,and low coverage grassland is 0.14,0.37,and 0.66,respectively.The optimized farmland has the best hydrothermal conditions among the existing farmland in the study area with its ESI of 0.72,which could adapt to the climate conditions more.The ESI-based optimized farmland layout plan could obviously improve the economic,environment and landscape ecological benefits in the farming-pastoral intersection zone of desert steppe region. |
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