| With global climate and environment change,temperate desert ecosystems,mainly located in Eurasian inland,could be more and more sensitive,with significant changes in rainfall,temperature,and nitrogen(N)deposition.Climate change(e.g.precipitation and temperature)and elevated N deposition are important driving factors for the change of productivity and biodiversity of desert ecosystems.In addition,climate change and N deposition are also related to the global carbon and N cycle and plants C:N:P stoichiometry.Some studies have been carried out to test the effects of precipitation and N deposition on the structure and function of temperate desert ecosystems in China.However,there were no systematic studies considering the effects of precipitation,temperature and N deposition on temperate desert ecosystems in China simultaneously.It's important for accurately forecasting the structure and function response of temperate desert ecosystems under global climate change.Therefore,a multi-factorial experiment of water(W),nitrogen(N)and temperature(T)at the southern edge of the Gurbantunggut desert in the field observation station of the Fukang Desert Ecosystem of the Chinese Academy of Sciences was established during 2015 and 2017.The field experiment consisted of ten treatments of various W,N and T combinations(W0N0,W0N1,W0N2,W1N0,W1N1,W1N2,and W0N0T0,W0N0T1,W1N1T0,and W1N1T1).In addition,an investigation of desert sample zone and controlled laboratory experments were conducted to quantify how water and N and other environmental factors affect desert plants.During 2015-2017,desert ecosystem biomass,diversity,N fate in plant-soil system,and stoichiometry characteristics were evaluated systematically via field and laboratory experiments as well as investigation of desert belt zone.The main results were summarized as follows:1.Height of herbaceous layer,aboveground biomass,belowground biomass,species number,density,and coverage of herbaceous communities in temperate desert ecosystem were significantly varied among years.Extra water could significantly increase aboveground biomass,species number,density and coverage of herbaceous plant communities.However,exogenous N addition and warming did not significantly alter herb height,aboveground biomass,belowground productivity,species number,density and coverage of herbaceous community.2.With 15N(15NH415NO3)as a tracer,we quantified the two cumulative years of added 15N flux in a Haloxylon ammodendron dominated ecosystem in the Gurbantunggut Desert of Northwest China from 2015-2016 and its response to water and N addition.Herbaceous plants were a significantly larger sink for added 15N than the H.ammodendron trees,the fate of the applied 15N accumulated between 4.11 to 9.05 kg N ha-1 in the herbaceous species and from 2.6 to 6.13 kg N ha-1 in the H.ammodendron.The retention of added 15N varied within the components of H.ammodendron,with the stems retaining most,followed by the root and assimilating branches.Soil was the dominant sink for added 15N,in which the topsoil and subsoil respond differently to water and N addition over the two-year period.N relative recovery percentage in the whole ecosystem ranged from 43%to 61%,significantly increased with water addition but decreased with enhanced N deposition.3.Malcolmia Africana(ephemeral)and Salsola affinis(annual)were chosen to determine the effect of different 15N addition rates(double-labeled 15NH415NO3 added at 0,15,30,60 and 120 kg N ha-1 yr-1)on their growth and 15N retention in a pot experiment under sufficient water supply(soil moisture content to be controlled at 70%of water-filled pore space).Species-specific and rate-dependent effects of N deposition on plant growth and plant N retention were found.The maximum aboveground and belowground biomass of ephemeral and corresponding threshold N addition rate were significantly lower than those of the annual.15N retention of ephemeral was more sensitive to N addition than annual,and the threshold N addition rate for the maximum aboveground(6.43 kg N ha-1)and belowground(0.89 kg N ha-1)retention of ephemeral was 42.1 and 37.3 kg N ha-1,significantly lower than those for the maximum retention(21.56 and 2.03 kg N ha-1 for aboveground and belowground maximum retention,respectively)of annual(114.9 and 60.7 kg N ha-1,respectively).However,the average plant-soil system recovery rate of the ephemeral was 70%,significantly higher than the annual with an average value of 50%.4.Except N content,there were significant differences in the stoichiometric characteristics of leaves desert plants with different life type.The leave carbon(C),phosphorus(P)content and C:N ratio of the ephemerals were significantly greater than those of annuals and small trees.Conversely,the C:P and N:P of the ephemerals was significantly smaller than that of annuals and small trees.N addition significantly increased N content and N:P in desert plants leaves,but significantly reduced the C:N in desert plants leaves.Water addition and warming did not significantly alter the stoichiometric characteristics of desert plants.There were significant interactions between species and warming treatment,in which warming can significantly increase N content in annual and small tree leaves.5.Mean C,N and P content and C:N,C:P and N:P were 337,19.2,and 1.64 mg g-1,and 19.8,229 and 13.2,respectively.Elemental contents and stoichiometry were correlated with geography,climate,soil and taxonomy,with taxonomic factors explaining 54-66%of the variation.C and P contents and stoichiometric ratios significantly differed between Chenopodiaceae and Brassicaceae,with taxonomic factors explaining most of the variation of Chenopodiaceae and edaphic factors for Brassicaceae.In summary,the study answered the effects of precipitation,temperature,and N deposition increasing on the structure and function of temperate desert ecosystems in Xinjiang.Increased precipitation can significantly increase aboveground biomass,species number,density,and coverage of herbaceous plants in desert ecosystems.Simultaneous increases in precipitation and N deposition can significantly improve N recovery rate in desert ecosystems.N addition significantly decreased the N recovery rate of both ephemeral and annual,and the N recovery rate of ephemeral was significantly higher than that of annual.Increased N deposition can significantly increase leaf N content and N:P ratio of desert plants,and reduce leaf C:N ratio of desert plants.The stoichiometric characteristics of C,N,and P in desert plants of Gurbantunggut are mainly affected by taxonomy and soil properties. |
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