| We selected a typical grassland-shrubland mosaics formed by the anthropogenic introduction of shrub in the desert grasslands of eastern Ningxia as our research object,the four transition sites in the mosaic,desert grassland(DG),grassland edge(GE),and shrubland edge(SE),shrubland(SL),were selected based on the ecological boundary theory.We conducted the field investigation,different rainfall gradient control experiments,different water pulse and litter addition in the laboratory test to study the vegetation changes and soil respiration and soil microbial response characteristics(quantity,biomass,functional diversity,microbial enzymes)in different microhabitats(vegetation patch and bare interspace)of each site in the process of the anthropogenic transition from desert grassland to shrubland with an aim to understand the direct and indirect effects of vegetation transition and global precipitation change on soil respiration and soil microbial enzyme activities.The main conclusions were obtained as follows:1.Soil moisture,organic carbon and total nitrogen all showed a decreasing trend,soil porosity and water holding capacity showed a trend of rising and then falling,and the soil bulk density,total phosphorus and pH decreased first and then increased with the four sites transition.The soil physical and chemical properties were significantly lower in the shrubland than that in the desert grassland,and most of the soil physical and chemical properties were higher in vegetation patches than in bare interspaces in each site,especially in the shrubland.Except soil bulk density and pH had the negative correlations with other soil factors,other soil physical and chemical properties were positively intercorrelated.With the transition the aboveground and underground biomass and the total amount of litter biomass increased gradually,and the difference between shrubland and other sites was significant(P<0.05).The aboveground biomass and the total amount of litter in the herbaceous layer were the highest in the grassland edge,and the underground biomass of herbs was the highest in the desert grassland.2.Soil microbial quantity,microbial biomass carbon and microbial biomass nitrogen showed a trend of decrease with the transition,which were higher in vegetation patches than in bare interspace at each site,and soil microbial biomass carbon and nitrogen in the shrubland were significantly lower than those in the desert grassland(P<0.05).the average color change rate(AWCD)of soil microbial utilization of carbon sources in each transition site showed a trend of decreasing first and then rising,and the utilization degree of microbial carbon source in desert grassland and grassland edge were higher than that in shrubland edge and shrubland,while it at 96h was the lowest at the shrubland edge.soil microbial richness,uniformity,and dominance index are significantly different in different microsites in each site(P<0.05),and microbial community diversity index in desert grassland and grassland edge was relatively higher and was postively correlated with actinomycetes,microbial biomass.3.Catalase activity,urease activity,sucrase activity and alkaline phosphatase activity all showed a decrease trend,and these enzyme activities in shrubland decreased by 55.6%,54.7%,7.8%and 49.4%,respectively,compared with desert grassland.Except alkaline phosphatase activity had no significant differences between microsites in desert grassland and grassland edge,there were significant difference of all enzyme activities in two microsites of all sites(P<0.05).And the six kinds of soil extracellular enzymes also showed a decreased trend in the transition,among which cellobiohydrolase decreased the least by 28.6%,and other extracellular enzyme activities decreased by 42.6%?83.0%.Except the activity of ?-1,4-N-acetylglucosaminidase in shrubland was significantly higher in bare interspaces than in vegetation patches(P<0.0 5),other extracellular enzyme activities in each sites were higher in vegetation patches than in bare interspaces by 10.7%?42.7%.With the transition process the response of the hydrolase activities were not sensitive to bacteria and fungi,while the extracellular enzyme activities responsed more positively to soil microbial biomass carbon,nitrogen and total nitrogen.4.,The daily maximum of soil respiration in desert grassland occured from 10:00 to 12:00,while that of the shrubland was from 12:00 to 14:00 in the growing season.Soil daily respiration and soil moisture and temperature were correlated siginificantly,especially in desert grassland,grassland edge,the liner regression relationships between soil respiration and soil moisture,temperature were more significant(P<0.05).The monthly maximum value of soil respiration in four transition sites all occured in July,and soil respiration in the shrubland sites were significantly higher than other sites(P<0.05).Soil respiration in shrubland responded more positively to rainfall changes,and During the growing season,the increased rainfall has a significant effect on soil respiration in every sites in growth season.In August,the maximum of soil respiration in desert grassland,grassland edge and shrubland were 52.7%,52.9%and 57.5%under+40%increased precipitation,which were higher than embient rainfall,but the maximized soil respiration of shrubland edge rising 52.9%at+20%increased precipitation level.5.The water(+40%,-40%)and litter(grass,shrub)addition experiment showed that the interaction between sites and rainfall level and litter had a more significant effect on soil microbial activity.Soil microbial respiration increased significantly in early stage of the cultivation and then decreased.Soil respiration decreased slowly under low water addition,but the addition of litter significantly increased soil microbial respiration.Under high water condition,the soil microbial community in desert grassland was relatively stable and had no significant response to shrub litter addition,and although the microbial biomass and enzyme activity under litter addition were higher than that of the no-litter group,the difference between the litter addition of grass and shrub was not significant.However,with the transiton,the indirect effect of shrub litter on soil microbial activity is strengthened,especially under low moisture conditions,soil microbial community in the desert grassland responded more positively to shrub litter,indicating that in the future precipitation decrease,the structure and function of soil microbial community in desert grassland would actively respond to the transiton from desert grassnland to shrubland. |
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