Effects Of Nitrogen And Phosphorus Addition On Plant Community Strcture And Seasonal Dynamics In Typical Grassland Of Inner Mongolia

Since the industrial revolution,human activities have greatly influenced the process of nitrogen entering the biosphere from the atmosphere.Nitrogen is the limiting element of various ecosystems on the earth.The increasing nitrogen input in terrestrial ecosystems has significantly changed plant community productivity,biodiversity,community structure and composition.Phosphorus is also one of the essential elements for plant growth and development.Many studies of the effects of nitrogen and phosphorus additions on plant communities biomass and species diversity in various ecosystems,but the seasonal dynamics of plant community have been lessly investigated.Here we studied the effects of nitrogen(N)and phosphorus(P)enrichment on net plant aboveground net primary productivity(ANPP)and seasonal dynamics of plant community in a long-term(8 years)fertilizzation experiment in a typical grassland of Inner Mongolia.The main results are as follows:(1)N addition alone with different levels had no significant effects on ANPP in two years,while the addition of nitrogen and phosphorus together significantly increased the ANPP in relatively wet year(i.e.,2018).(2)The addition of nitrogen and phosphorus had no significant impacts on the species richness and diversity index of plant communities in 2017,however the species diversity under the high level nitrogen and nitrogen,phosphorus interraction was significantly declined in 2018.(3)In addition,we found that the relative biomass of perennial grasses increased after longterm N loading,while the proportion of annual and biennial herbs increased in phosphorus enrichment plots,and showed stronger response to P in relative wet year.(4)At the peak of the growing season,N addition had a significant effect on the height of perennial grasses,but no response to its height in other seasons.N addition had no significant effect on the coverage of perennial grasses during the whole growing season.The addition of P and NP had no significant effect on the height and coverage of perennial grasses during the growing season.(5)The addition of N and P in each growing season had no significant effect on the height and coverage of perennial forbs..(6)In the whole growing season,the addition of N and P alone had no significant effect on the height and coverage of annual and biennial herbs.At the peak of the growing season,NP addition had a significant effect on the height and coverage of annual and biennial herbs.To sum up,the response of aboveground productivity of temperate grassland plant communities to long-term N addition is relatively weak.Low level N addition also had no inhibitory effect on the plant community diversity in the sample site,while high level N addition and NP addition reduced the plant community diversity.Nitrogen fertilizer increased the biomass of perennial grasses and phosphorus fertilizer increased the biomass of annual and biennial herbs.During the growing season,the effects of nitrogen and phosphorus addition on the height and coverage of different functional groups in the plant community were significantly different.