Legacy Effects Of Nitrogen Inputs On Mineral Nutrition Of Soil And Plant In A Typical Steppe Of Inner Mongolia

Mineral nutrients play a key role in maintaining the growth and development of plants and animals as well as affect many ecosystem functions.In the last few decades,the rapid increase of atmospheric reactive nitrogen deposition caused by human activities has led to substantial changes in plant biomass and mineral nutrition by affecting soil physical and chemical properties and promoting the nitrogen interception in vegetation canopy.Homeostasis mechanism of plants can regulate their nutrient absorption,which can facilitate the persistence of plants under conditions with nutrient imbalance.At present,with the implementation of global economic transformation and environmental protection policies,nitrogen（N）deposition is decreasing or has been declined in some countries.However,the research on the responses of plant mineral nutrition to such decline trend of N deposition is still in its infancy.In addition,the legacy effects of historical N inputs on soil nutrient supply would make it to be more complicated to explore the responses of plant mineral nutrition to further N inputs in regions with different backgrounds of historical N deposition.What are the effects of historical N deposition on mineral nutrients of soil and plants in grassland ecosystems?How would N deposition background drive the responses of plant mineral nutrition to future N inputs?There are no definitive answers to these questions.In this study,nine different N addition levels(0,1,2,3,5,10,15,20,50 g N m^-2yr^-1)were set up in a typical steppe in Inner Mongolia from 2008 to 2014 to form different background gradients of historical N deposition.After the cessation of N inputs in 2014,different historical N input gradients were formed.Since 2016,part of the plots continued to receive N addition with the same amount of 5 g N m^-2yr^-1,while other part received no external N addition.From 2016 to 2018,the concentration of potassium（K）,calcium（Ca）,magnesium（Mg）,iron（Fe）,manganese（Mn）,copper（Cu）,zinc（Zn）in soil and plants were determined by using two kinds of plots with historical nitrogen addition levels of 0,2,5,10,20 g N m^-2yr^-1,The aims of this study were to explore the legacy effects of historical N inputs on mineral elements concentration in soil and plants and the homeostatic response of plants and to explore the role of historical N input backgrounds in the response of mineral elements in soil and plants to future N inputs.During the three years examined after the cessation of N addition（2016-2018）,there was a significant negative correlation between extractable Ca concentration in topsoil（0-10 cm）and historical N addition amounts.The concentrations of extractable Mg,Fe,Mn and Cu increased significantly with the increases of historical N addition amounts.Those results indicated that historical N inputs had significant legacy effects on the availability of Ca,Mg,Fe,Mn and Cu in soils.There were legacy effects of historical N addition on community-level plant K,Mg,Fe and Mn concentrations,and the magnitudes of which were weakened with increasing duration of the cessation time of N addition.The legacy effects of N addition on plant mineral nutrition varied across different functional types.The K concentrations in short bunchgrass and forb had significant positive correlations with historical N addition amounts,while that of tall rhizomatous grass and tall bunchgrass did not correlate with historical N addition amounts.Except for the significant correlation between Ca concentration in short bunchgrass and Mg concentration in forb and the corresponding soil available nutrient concentrations,the concentration of Ca and Mg in other functional types and the concentration of K,Cu and Zn in four functional types did not correlate with the change of soil extractable nutrient concentration.In other words,K,Ca,Mg,Cu,and Zn in these plant functional types have high homeostasis.Plant Fe and Mn concentrations were significantly correlated with soil extractable nutrient concentrations,indicating low homeostasis of Fe and Mn in plants.The response of soil extractable K,Ca,Mg,Fe,Mn and Cu to subsequent N addition(5 g N m^-2yr^-1)was significantly affected by N deposition history.The availability of K,Mg,Fe,Mn and Cu significantly increased while that of Ca decreased with the increases of historical N addition amounts.There was no significant correlation between the extractable Zn concentration in soil and the background of historical N inputs,indicating that the background value of historical N inputs did not affect the response of soil extractable Zn to subsequent N addition.The responses of community-level plant Ca,Fe,Mn,Cu and Zn concentration to subsequent N inputs were dependent on historical N inputs background,whereas K and Mg concentrations in plant communities were not affected by historical N inputs background.Under the background of N inputs,mineral nutrition of different plant functional types showed varied responses to subsequent N inputs,probably due to varied sensitivities of different plant functional types to subsequent N inputs.In conclusion,N inputs had legacy effects on plant mineral nutrition and soil nutrient availability in the temperate steppe.With the increases of the duration after the cessation of N inputs,such effect will be gradually weakened.All mineral elements except Fe and Mn in the four plant functional types had high homeostasis,which may be an important reason for maintaining the ecological stoichiometric balance of mineral nutrients in plants.In all,results from this study highlight the importance of N input history in driving plant mineral nutrition,not only for its legacy effects,but also for its determination on the responses of soil and plants to further N inputs.This study help improve our understanding about the long-term impacts of increasing N inputs on plant mineral nutrition in grasslands.