Effects Of Nitrogen And Phosphorus Addition On Stoichiometry And Homeostasis Of Typical Carex Wetland In Poyang Lake

Due to the continuous intervention of human activities,the input of exogenous nitrogen and phosphorus in the ecosystem is increasing,which has a profound impact on the structure and function of the ecosystem.The research on the response of wetland to nitrogen and phosphorus addition is very important to protect and maintain the function of wetland ecosystem.However,the current understanding of the response law and mechanism of wetland plant communities under the dramatic change of environmental nutrients is still unclear.In this study,a typical Carex wetland in Poyang Lake Nanji Wetland National Nature Reserve was selected for nitrogen and phosphorus fertilization experiment.The effects of nitrogen and phosphorus addition on wetland soil,plant nutrients and plant biomass were analyzed by setting up control,three nitrogen addition levels(5,10 and 30 g N·m^-2·a^-1)and three phosphorus addition levels(0.6,1.2 and 3.0 g P·m^-2·a^-1),Combined with the theory of ecological stoichiometric internal stability,the response mechanism of Carex cinerea community to exogenous nitrogen and phosphorus was elucidated.The main results are as follows（1）Nitrogen and phosphorus addition significantly increased the content of soil nitrogen,phosphorus and effective nitrogen（ammonium nitrogen and nitrous nitrogen）,effective phosphorus content.Compared with the control group,high nitrogen addition increased soil total nitrogen content by 29%and 25%in March and April,respectively（P<0.01）,while high phosphorus addition only increased soil total phosphorus content by 107%in April（P<0.01）.Compared with the total amount of nitrogen and phosphorus,the response of soil available nitrogen and phosphorus content to the corresponding exogenous nutrient elements was more sensitive,and the effect of treatment in April was more obvious than that in March.In April,ammonium nitrogen and nitrate nitrogen increased by 1.6 times and 2.5 times respectively under high nitrogen treatment,while available phosphorus increased by 100%under high phosphorus treatment.（2）Compared with the control group,high-level nitrogen addition significantly increased the aboveground and underground biomass by 53%（P<0.01）and 36%（P<0.05）,respectively,while high-level phosphorus addition increased the aboveground and underground biomass by 51.4%（P<0.01）and 66.4%（P<0.01）,respectively.Although the biomass of Carex community increased significantly with the increase of nitrogen and phosphorus,the effects of nitrogen and phosphorus on the root shoot ratio of Carex were different.Nitrogen addition decreased the root shoot ratio of Carex community,while phosphorus addition increased the root shoot ratio of Carex community.（3）The effects of different treatments on aboveground carbon,nitrogen and phosphorus contents of Carex communities in spring（March and April）were basically the same as those of underground parts.There was no significant difference in total carbon content between aboveground and underground parts of Carex communities under different nitrogen and phosphorus treatments;The total nitrogen content increased significantly with the increase of nitrogen addition level,and the high nitrogen treatment increased 29%and 41%of the total nitrogen content of aboveground and underground parts in April,respectively;Phosphorus addition significantly increased total phosphorus content,and high phosphorus treatment increased 65%and57%of total phosphorus content of aboveground and underground parts in April,respectively.（4）The Carbon-Nitrogen ratios of aboveground and underground parts of Carex communities decreased significantly with the increase of nitrogen addition level.The Carbon-Nitrogen ratios of aboveground and underground parts of Carex communities under high nitrogen treatment in April were about 76%and 75%of the control group,respectively.Phosphorus addition significantly reduced the Nitrogen-Phosphorus ratio of aboveground part（63%of the control group in April）,but the Nitrogen-Phosphorus ratio of underground part increased with the increase of nitrogen addition level（1.2%of the control group in April）,and decreased with the increase of phosphorus addition level（66%of the control group in April）.The ratio of nitrogen and phosphorus in the aboveground and underground parts reached the maximum under high nitrogen treatment（about 1.3 and 1.5 times of the control group in April）,and the minimum under high phosphorus treatment（about 58%and 64%of the control group in April）.（5）The contents of Nitrogen,Phosphorus and Nitrogen-Phosphorus ratios in the aboveground and underground parts of Carex communities showed significant internal stability,and the internal stability of nitrogen and phosphorus was stable or weak stable.The internal stability of nitrogen content in aboveground and underground parts of Carex communities was higher than that of phosphorus content and nitrogen phosphorus ratio,and the internal stability of nitrogen content in aboveground parts was higher than that in underground parts.In conclusion,the study shows that the soil of Poyang Lake wetland and the dominant plant moss are sensitive to nitrogen and phosphorus addition.The internal stability of nitrogen content in the above-ground and underground parts of plants was significantly higher than that of phosphorus,and the internal stability of the nitrogen content of the above-ground parts was higher than that of the underground parts.The study predicts that in the future,in the context of the continued increase in anthropogenic nitrogen and phosphorus,especially phosphorus,Carex cinerea in the study area may occupy the niche of associated species,further reduce plant biodiversity,affect wetland ecological function and aggravate wetland degradation.