Response Of Artemisia Ordosica Community To Water And Nitrogen Addition In The Mu Us Desert

Arid and semiarid ecosystems are the most widespread ecosystem type in the Earth's land surface These ecosystems are usually characterized by ecological vulnerability,and extremely sensitive to global changes.Global changes project that precipitation and atmospheric nitrogen deposition in dryland ecosystems of China will elevate in the future.These changes are expected to profoundly impact the structure and function of these ecosystems.However,the responses of desert ecosystems to increased precipitation and nitrogen deposition are paid fewer attentions for long time,and the impacts of these changes on typical sand-fixed shrub communities remain poorly understoodTo address these uncertainties,a three-year(2015-2017)field experiment simulating three levels of precipitation(ambient,+20%,+40%)and with six levels of nitrogen deposition(0,5,10,20,30,and 60 kg N ha-1 yr-1)was conducted in an Artemisia ordosica shrubland in the Mu Us Desert of northern China Soil environmental factors,plant productivity,plant diversity and community structure were determined,to examine the patterns and underlying mechanisms of them in response to increased precipitation and nitrogen deposition.The key findings are as follows(1)Water and nitrogen input significantly improved soil water and nitrogen availability,but had limited influences on soil pH,organic carbon and total nitrogen.In addition,nitrogen enrichment significantly decreased soil shallow moisture in under-shrub patch(average change rate 15%),probably due to water consumption following nitrogen-accelerated plant growth.Water and nitrogen addition had limited effects on microbial diversity,but significantly shifted microbial community composition.Water addition tended to promote flourishing of dormant(oligotrophic)bacterial taxa(e.g.,Acidobacteria and Planctomvcetes)and depress drought-adapted fungal taxa(e.g.,the orders Pezizales,Verrucariales,and Agaricales).Nitrogen enrichment had no impacts on bacterial community composition,but significantly altered fungal community composition with depressing oligotrophic fungal taxa(e.g.,the orders Agaricales and Sordariales).(2)Increased precipitation and nitrogen remarkably increased herbaceous aboveground productivity.Water input mainly promoted the productivity of perennial forbs(42%)and annual plants(117%),while nitrogen enrichment primarily improved the productivity of perennial grasses(43%).Current environmental factors and previous plant growth had considerable influences on current herbaceous productivity,and their roles in influencing plant productivity were varied with the type of functional group.For annual plants,both current and previous factors impacts on their productivity were mainly through altering individual density and height.For perennial herbs,the effects of current and previous factors on their productivity were primarily via changing individual height and density,respectively.(3)The responses of shrub productivity to precipitation and nitrogen enrichment were varied yearly.In 2015,both water and nitrogen addition remarkably increased shrub productivity,with average increased rate of 292%compared to control treatments.However,in 2016 and 2017,nitrogen enrichment suppressed water-induced increases in shrub productivity.The declines of shrub productivity under water-nitrogen interaction treatments were attributed to perennial grasses-driven decreases in topsoil moisture.Previous shrub growth had much larger legacy effects on current shrub productivity than current environmental factors,which causing the lagged responses of shrub productivity to environmental changes.The legacy effects of previous shrub growth occurred primarily through changes of twig density.(4)Water and nitrogen addition significantly altered community diversity,but their impacts on herbaceous diversity were dependent on shrub-patch type.At the community level,water addition increased plant diversity(13%),while nitrogen addition had no effects.The responses of inter-shrub herbaceous diversity were similar to those of community diversity.In under-shrub patch,water addition increased herbaceous diversity(25%)in the first two years,while nitrogen enrichment declined it(21%)in the last year of the experiment.Nitrogen-induced declines in under-shrub herbaceous diversity was due to the loss of annual plant species and rare species,caused by perennial grasses-driven decreases in topsoil moisture and understory light.(5)Water and nitrogen enrichment significantly shifted plant community structure,but their impacts on herbaceous community structure were dependent on shrub-patch type.At the community level,water and nitrogen addition tended to increase the relative abundance of herbs,but decrease that of shrubs.The declines in shrub abundance were more likely driven by enhanced negative feedback effects of herbs on shrubs.At the patch level,the under-shrub herbaceous community shifted from forb-dominated to grass-dominated community;while the inter-shrub community shifted from annual-dominated to annual and grass-codominated community.Nitrogen-induced enhancement in the competitive effects of perennial grasses on annual plants were the probable mechanism underlying the alteration of herbaceous community structure.Taken together,the results in this thesis suggest that increased precipitation and nitrogen tended to shift Artemisia ordosica shrubland to favor herbs over more shrubs,which implying that desert shrub communities in northern China were not stable,and may transition into a new state of grass communities under future global change scenario.This information contributes to advance a potential mechanistic understanding of the responses of desert plant communities to global environmental changes,and can be used as guide to manage the shrublands for desertification control.