| We selected a typical grassland-shrub mosaic formed by the anthropogenic introduction of shrub(Caragana Korshinskii)in the desert grassland of the eastern Ningxia as our research object,the four transition sites in the mosaic,i.e.,desert grassland(DG),grassland edge(GE),and shrubland edge(SE)and shrubland(SL)were selected based on the ecological boundary theory.We conducted the field investigation,in-situ mineralization under different rainfall gradients and indoor water(+40%,-40%)and litter(Agropyron mongolicum,Caragana korshinskii)addition experiments in laboratory to carry out research on.vegetation,soil microbes and their mineralization characteristics in the process of anthropogenic transitionn from desert grassland to shrubland,and aim to further explore the characteristics of variation in soil and vegetation with the anthropogenic vegetation transition from desert grassland to shrubland and the impact of vegetation change on soil microbial mineralization under global climate change,providing a theoretical basis and data for the restoration sustainability of desert grassland.The main results are as follows:(1)With the grassland-shrubland transition,there was no significant difference in richness,diversity and uniformity between the desert grassland and the two marginal sites,and these indexes of the shrubland was significantly lower than other sites(P<0.05),and was significantly lower 37.0%,42.4%,and 35.2%than that of the desert grassland and the two edge sites,respectively.The above-ground herbious biomass of grassland increased first and then decreased with the highest biomass at the grassland edge,while the shrub biomass showed a gradual increase and the highest shrub biomass was obtained in shrubland.(2)With the grassland-shrubland transition,the soil moisture in the 0-200 cm soil layer showed a decrease with the sites transition,i.,e the desert grassland>grassland edge>shrubland edge>shrubland.The seasonal dynamics of soil moisture at the desert grassland edge and grassland actively responded to rainfall changes,while Shrub edge and shrubland decreased with seasonal changes,resulting from the deep-seated water consumption was mainly supplemented in autumn;The soil moisture physical properties of the 0-100 cm soil layer at each sites show a boundary effect with fluctuation at the two edge sites.Except for bulk density,the soil saturated water holding capacity,capillary porosity,capillary water holding capacity,non-capillary porosity,and field water holding capacity all were higher at the desert grassand than those at shrubland.(3)With the grassland-shrubland transition,the soil organic carbon,total nitrogen,and total phosphorus in the 0-60 cm soil layer at the desert grassland and the grassland edge were significantly higher than that of the shrub edge and shrubland,while there were slight differences in the 60-100 cm soil layer;The variation of soil fractal dimension of each sites was between 1.02-2.58,and the shrubland was significantly lower than other sites,accompanied with the coefficient of variation of fractal dimension of shrub edge and shrubland is higher than that of desert land,and the edge of grassland,and leading to soil erosion resistance gradually declines with transition.(4)With the grassland-shrubland transition,the number of soil bacteria showed no significant difference between desert grassland and shrubland(P>0.05),which was higher than that in the two marginal plots(P>0.05).The number of fungi showed significant difference between the shrubland edge and shrubland,and was higher than that at desert grassland and grassland edge.The number of actinomycetes shows a gradual decrease trend with the transiton,among which the desert grassland is the highest,reaching 293.3 10-3 cfu/g;there is no significant difference between the transition sites(P>0.05),while soil Catalase,urease,alkaline phosphatase,microbial biomass carbon,and nitrogen all showed a gradual decrease trend.Among them,shrubland was significantly lower than that of desert grassland(P<0.05),and decreased by 55.6%,54.7%,49.4%,39.7%,76.4%,respectively.(5)With the grassland-shrubland transition,the NH4+-N content under vegetation patches and bare interspaces in each site showed no significant difference,while NO3--N content showed a gradually increasing trend,and the inorganic nitrogen content under vegetation patches were higher than that of bare interspaces.Compared with desert grassland,the net nitrification rate,net ammonification rate,and net mineralization rate of shrubland decreased by 142.2%,226.5%,and 593.2%,respectively.The soil NH4+-N content at the desert grassland showed a increasing trend,and there was no significant difference of NH4+-N in the soil between the two edge sites(P>0.05),and the soil NH4+-N at the shrubland first decreased and then increased.With rainfall gradients increase there was no significant difference in the NO3--N content of soil at the desert grassland,grassland edge,and shrub edge(P>0.05),while shrubland shows an increase-decrease-increasing trend.The average range of changes in the net nitrification rate,net ammonification rate,and net mineralization rate in the soil of the four transition sites were-0.084-0.022 mg kg-1 d-1,-0.010-0.010 mg kg-1 d-1,-0.094-0.029 mg kg-1 d-1,respectively,in which these soil mineralization indexes in the shrubland were significantly lower than desert grassland(P<0.05),and reached to the lowest in four tansition sites under the decreased 20%rain graiden in whole incubtion period(P>0.05).(6)The conducted experiment of the addition of water(+40%,-40%)and litter(Agropyron mongolicum,Caragana korshinskii)concluded that:The net mineralization rate of soil at four transition sites was not significantly different between high and low water addition,and the net mineralization rate under high water and litter addition was significantly lower than the control,but the shrub litter addition under the low moisture accelerated the more net mineralization rate than under the herb addition and the control.During the transition from desert steppe to shrubland,the interaction between water,litter,and transition sites jointly drives soil microbial mineralization process,especially under drought conditions,the Caragana korshinskii litter was more acclimationing to improving the mineralization of soil microorganisms in desert steppe. |
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