| External input of nitrogen and phosphorus can directly change the soil nutrient content and availability,thus changing the state of individual plant resource utilization,and causing changes in community growth and species diversity.Plant functional traits are important indicators of plant resource utilization strategies in grassland.The influence of external resource supply changes on plant growth is mainly reflected in the change of plant functional traits.Therefore,it is necessary to reveal the effects of nitrogen and phosphorus deposition on grassland plant community and plant functional traits through nitrogen and phosphorus addition control experiments,which will help deepen the understanding of the changes in grassland ecosystems and plant adaptation strategies under the background of increased nitrogen and phosphorus deposition and different nutrient management measures in the future.In this study,a typical grassland formed by abandonment in the hilly region was selected as the research object,a single factor randomized block experimental design was adopted.Four concentration levels of nitrogen and phosphorus were set respectively(nitrogen supplemental levels were 1,2,4,8 g N/(m~2·a)and phosphorus supplemental levels were0.5,1,2,4 g P/(m~2·a)),and no nutrient was used as the control.Through field investigation and sampling analysis,plant functional traits and soil physical and chemical properties were determined,the response characteristics of grassland plant communities to nitrogen and phosphorus addition were defined,the influence of nutrient addition on soil physical and chemical properties was analyzed,and the response rules of functional traits of three key species to nitrogen and phosphorus addition were emphatically explored.The synergistic change mechanism of functional traits of communities and typical plants and soil nutrients under nutrient addition was also revealed.The main results are as follows:(1)A total of 50 plant species belonging to 19 families were found in the study area,among which Compositae(13 species of 9 genera),Gramineae(11 species of 9 genera)and Legumes(7 genera 7 species)were the three major families in the study area.The number of grassland community species,individual number and coverage of the community increased significantly with the addition of nitrogen and phosphorus,and the richness index of Shannon-Wiener and Gleason also increased.Compared with P treatment,N addition can promote the increase of aboveground biomass and plant litter.(2)There were significant differences in the response degree and direction of functional traits of the three dominant species(Lespedeza davurica,Phragmites australis and Artemisia sacrorum)to N and P addition.The ratio of leaf length to width of Ph.australis decreased with nutrient addition,but the change of leaf length to width of L.davurica and A.sacrorum were not significant.Under N addition,the specific leaf area of L.davurica increased,while the stem to leaf ratio decreased,indicating that L.davurica adopted a"competitive strategy"under N addition.On the contrary,the specific leaf area of Ph.australis and A.sacrorum decreased,while the stem to leaf ratio increased,indicating that Ph.australis and A.sacrorum showed a"resource-conservative strategy"under nitrogen addition.The stem nitrogen content of L.davurica increased and then decreased with increasing N concentration,while the leaf nitrogen content did not change significantly.Compared with Ph.australis and A.sacrorum,the plasticity index of functional traits of L.davurica was higher after nutrient addition,indicating that it was more sensitive to nutrient addition.(3)Nitrogen additions significantly decreased soil pH,and both N and P additions significantly changed soil carbon,nitrogen,phosphorus and other nutrients and their stoichiometry.With the increase of nitrogen concentration,soil organic carbon,total nitrogen,nitrate nitrogen,carbon to phosphorus ratio and nitrogen to phosphorus ratio decreased first and then increased,while the content of available phosphorus and dissolved organic carbon increased.Soil organic carbon,available phosphorus,carbon to nitrogen ratio and carbon to phosphorus ratio all showed a significant increase trend with the increase of P addition concentration,4 g P/(m~2·a)P treatment increased by 75.65%,524.74%,132.70%and 63.03%compared with CK control,respectively.Soil total nitrogen and nitrogen to phosphorus ratio showed a decreasing trend under P addition.(4)There was significant relationship between plant functional traits and soil physicochemical properties after nutrient addition.According to Mantel test results,soil pH under nitrogen addition is the key factor regulating leaf structure changes of the three key species.Leaf nitrogen to phosphorus ratios indicate that L.davurica is phosphorus-limited,whereas Ph.australis and A.sacrorum are mainly nitrogen-limited.The functional traits of L.davurica,especially stem and leaf nutrient indicators,were more sensitive to P addition,and had significant correlation with phosphorus related indexes of soil available phosphorus and C/P.The functional traits of Ph.australis and A.sacrorum,especially the stem to leaf ratio,were more plastic under nitrogen addition treatment,and were positively correlated with soil ammonium nitrogen and nitrate nitrogen,which were consistent with the results reflected by stoichiometric characteristics.In summary,the effects of nutrient addition on grassland community structure,plant functional traits of key species and soil physical and chemical properties were clarified in this study,and the differentiated responses of functional traits of different types of plants to nutrient addition were revealed,providing a theoretical reference for the management of grassland in semi-arid region. |
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