Effects Of Nitrogen Addition And Mowing On The Temporal And Spatial Stability Of Aboveground Primary Productivity In A Meadow Steppe

The stability of productivity refers to the ability of a community to maintain stable productivity in face of temporal and spatial changes,which could be quantified as temporal stability and spatial stability.Human activities have dramatically enhanced reactive N deposition in many terrestrial ecosystems since the industrial revolution.Meadow steppe is the most productive type of natural grasslands in China and provides important ecosystem services.Nitrogen（N）deposition,an important global change factor,threatens biodiversity,affects plant community composition and structure as well as ecosystem stability.As a common management and utilization measure across global grasslands,mowing can partly reduce the N-induced eutrophic effect and improve the light condition of plant communities by removing biomass from the ecosystem and reducing N accumulation.Understanding the mechanisms underlying the effects of nitrogen enrichment and mowing on temporal and spatial stability of primary productivityis crucial to predict the changes of forage provision under the scenarios of increasing atmospheric N deposition.This study was conducted in a Hulunbuir Meadow steppe and based on the N addition and mowing experiment platform of Eerguna Forest-Steppe Ecotone Ecosystem Research Station,Institute of Applied Ecology,Chinese Academy of Sciences.There were four treatments,including control,N addition(5 g N m^-2yr^-1),mowing（annualy at each growing season）,and combined N addition and mowing.I examined the effects of N addition and mowing on soil physical and chemical properties,soil nutrient concentrations,plant nutrient concentrations,species diversity,aboveground primary productivity and its temporal and spatial stability.The main aim of this study was to reveal the mechanisms associated with soil and plant community ecology underlying the variaitons of temporal and spatial stability of aboveground net primary productivity.The main results from this study are as follows:（1）Nitrogen addition significantly affected the soil physical properties and nutrient concent.Nitrogen addition reduced soil p H,leading to soil acidification,and accelerated the loss of soil Ca²⁺and Mg ²⁺.While mowing did not affect all the soil physicochemical parameters examined in this study,it did modulate the impacts of N addition on soil nutrient concentrations.Mowing strengthened the negative effect of N addition on soil potassium,but weakened the negative effect of N addition on soil Mg concentration.（2）Nitrogen addition and mowing altered the relative biomass and species diversity of different plant functional groups.Nitrogen addition significantly increased the relative biomass of rhizomatous grass,while it decreased the relative biomass of sedges and legumes.Mowing reduced the positive effect of N addition on the relative biomass of rhizomatous grasses,and alleviated the negative effects of N addition on the relative biomass and richness of non-legume forbs.Nitrogen addition significantly increased community aboveground net primary productivity and functional group evenness,but reduced species richness.Plant species richness was much higher under mowing conditions,with positive association between species richness and community aboveground net primary productivity when comnined nitrogen addition and mowing.（3）Nitrogen addition significantly increased the N concentrations of rhizomatous grasses,bunchgrasses,and non-legume forbs,and thus resulted in an increase in community-level N concentrations.The effects of N addition on community-level plant P concentration varied across different years.Mowing did not affect plant nutrient contents,but significantly increased the temporal stability of P concentration.The community-level N concentration was quite stabile across different years.The spatial stability of community-level N and P concentrations were relatively stable under each treatment,indicating high resistance ability of plant communities to perturbations with respect to nutritional traits.（4）Nitrogen addition significantly reduced the temporal stability community aboveground net primary productivity.Nitrogen addition and mowing did not affect the spatial stability of aboveground primary productivity.Nitrogen addition reduced the dominance of legumes and non-legume forbs,whereas mowing had an opposite impact with N addition.Mowing reduced functional group asynchrony,whereas N addition did not affect functional group asynchrony.The compensation effect among different functional groups did not contribute to the stability of community-level primary productivity.Functional group dominance was the main driver of changes in community temporal stability.Nitrogen addition decreased the temporal stability of community-level aboveground primary productivity by reducing species diversity and altering the dominance of plant functional groups.Nitrogen addition and mowing did not affect the spatial stability of community-level aboveground primary productivity.Taken together,results from this study showed that N addition can change soil environment,reduce plant species diversity,and threatens the temporal stability of community-level aboveground primary productivity.Mowing was able to modulate the effects of N addition on species diversity and the temporal stability of aboveground primary productivity.The spatial stability of aboveground primary productivity was conservative in response to N addition and mowing.This study deepens our understanding how would N addition and mowing affect the temporal and spatial stability of community aboveground primary productivity,and thus provides a theoretical basis for protecting biodiversity and ecosystem stability of meadow steppe in the scenarios of increasing N deposition.