| Global warming and atmospheric nitrogen deposition have caused significant impacts on terrestrial ecosystems,which have attracted widespread global attention and research.Grassland system productivity and ecological stoichiometry participate in the nutrient cycle of the ecosystem,and are important factors to maintain the stability of the ecosystem structure.Previous studies on the response of plant root traits and ecostoichiometric characteristics to warming and nitrogen addition were mostly based on a single region,lacking multi-point studies on different environmental gradients in large-scale transects.Therefore,this experiment selected 9 research sites along the drought gradient in Inner Mongolia of China,including desert steppe,meadow steppe and typical steppe.The root shape index and C,N,P concentration data were obtained by using the methods of transect investigation and experimental treatment.The research contents include:（1）changes of root traits and stoichiometric characteristics with drought gradient in grassland ecosystem;（2）effects of temperature and nitrogen addition on root traits and stoichiometric characteristics;（3）the effect of drought degree on warming and nitrogen addition.The following research results were obtained:（1）On the natural transect gradient,the average variation range of root biomass is727-2273 g m-2,the average variation range of root productivity is 176-407 g m-2,and the average variation range of root turnover rate is 0.211-0.312 yr-1.Root traits were negatively correlated with drought,while root turnover was not correlated with drought.The effect of drought on root carbon concentration varied from year to year.Drought inhibited root nitrogen concentration,but had no effect on phosphorus concentration;Drought promoted root C:N and C:P while inhibiting N:P.This indicates that drought has an impact on root ecostoichiometry（stoichiometry）by changing soil nutrient concentration and the concentration effect of root biomass on nutrients.（2）The decrease in soil moisture caused by warming increased the root cap ratio by an average of 121%in dry years,and decreased root productivity by an average of28.6%and root turnover rate by an average of 27.9%in wet years.This inhibited root carbon input in grassland ecosystems.It also inhibits nutrient cycling in grassland ecosystems.Increasing temperature increased the root carbon concentration of typical grasslands by 6.2%,increased the root nitrogen concentration of meadow grasslands by an average of 6.5%in wet years,and increased the root phosphorus concentration of meadow grasslands by an average of 7.6%in dry years;Increasing temperature had no effect on C:N,N:P,and increased root N:P by an average of 3.3%only in typical grassland drought years.This indicates that warming changes the physical and chemical properties of soil,directly or indirectly affecting the concentration of carbon,nitrogen,and phosphorus（ratio）in roots.Nitrogen addition had no significant impact on root productivity,but increased root turnover rate by an average of 13.2%in wet years.Nitrogen addition had little effect on root carbon concentration,increasing root nitrogen concentration by 5.6%on average,while nitrogen addition reduced root phosphorus concentration by 10.9%in typical grasslands in dry years,but had no significant impact on desert grasslands and humid grasslands;Nitrogen addition decreased C:N and C:P by an average of 6.0%and 3.3%,but increased N:P by an average of 8.9%in dry years and 9.7%in wet years in meadow grasslands.This indicates that nitrogen addition has an impact on root ecostoichiometry（stoichiometric ratio）by changing soil physical and chemical properties.In addition,the addition of warming nitrogen showed an antagonistic effect on C:N,a synergistic effect on C:P,and an antagonistic effect and synergistic effect on N:P in desert grasslands and typical grasslands in dry years,respectively.（3）Under drought conditions,warming treatment increased root productivity and increased its turnover rate,but inhibited root productivity and turnover rate under nitrogen addition treatment.Drought has no significant effect on changes in root carbon concentration under increasing temperature or nitrogen,which reflects the stability of root carbon concentration in plants.Under warming or nitrogen addition,root nitrogen and phosphorus concentrations are negatively regulated by drought;Drought promoted the changes of root C:N and N:P in dry years under warming or nitrogen addition treatments,but inhibited root C:P and N:P in wet years under nitrogen addition treatments.（4）The results of this study show that the effects of temperature increase and nitrogen addition on ecosystem underground productivity and root carbon,nitrogen,and phosphorus are mostly inconsistent in transect surveys and control experiments,reflecting differences in the adaptability of grassland root productivity and ecological stoichiometric characteristics to short-term rapid response and long-term changes.Therefore,it is necessary to conduct large-scale and long-term control experiments and combine them with transect surveys.To sum up the above research results,this study,through control experiments and transect surveys,elaborates on the changes in root traits and stoichiometric characteristics of plants in natural transects,as well as the impact of controlled trials of warming and nitrogen addition on root traits and stoichiometric characteristics,and combines two methods to elaborate the impact of drought on warming and nitrogen addition on root traits and ecological metrological characteristics of grassland ecosystems on a larger regional scale.The results showed that warming and nitrogen addition changed soil physical and chemical properties,microbial activity,and plant growth,affecting root traits and ecological stoichiometric characteristics of grassland ecosystems.The short-term rapid response of root traits and ecostoichiometric characteristics to environmental factors in control experiments is inconsistent with the results of long-term changes in transect surveys.Therefore,it is necessary to conduct long-term large-scale control experiments combined with transect surveys. |
