Response Of Community Function And Structure To Nitrogen Addition In The Loess Plateau Region

With the increasing influence of climate change,the ecological effects of increasing nitrogen deposition is becoming a research hotspot to scholars.However,the research on the loess plateau region with limited nitrogen which has lower nitrogen deposition rate is still less,and the loess plateau region may be more sensitive to nitrogen enrichment.In addition one of these studies so far focused on the response of grassland ecosystem to nitrogen addition just with a single management(fencing or grazing).So,in this study we added nitrogen in a series levels to simulate nitrogen deposition,explored the effects of nitrogen addition on soil nutrient content,community aboveground biomass,leaf functional traits,community composition,species diversity,and the relationship between species diversity and aboveground biomass in long-term(30 years)fenced and grazed grassland on the loess plateau region.We also analyzed the differences and similarities in responses of grassland and tried to explain the underlying ecological mechanisms of these responses under the two typical grassland managements.The main results are as follows:(1)Nitrogen addition significantly enhanced soil nitric nitrogen,ammonium nitrogen in fenced grassland and nitric nitrogen in grazed grassland.While grazing obviously decreased soil nitrogen,ammonium nitrogen,total nitrogen and available phosphorus compared to fenced grassland.(2)Nitrogen addition elevated the community aboveground biomass significantly both in fenced and grazed grassland,and the maximum emerged respectively at the N5 and N10 levels.Nitrogen addition had no significant effect on functional group biomass in fenced grassland,but it improved the proportion of grass species group biomass and reduced the proportion of forbs species group biomass in the total biomass markedly.Compared to the fenced grassland,grazing and fertilization improved the proportion of edible grass.(3)With the increase of nitrogen addition levels,dominant species in fenced grassland with smaller SLA(specific leaf area)acquired more light by absorbing available nitrogen in the soil to increase Nmass(leaf nitrogen content),changed the species pattern,community composition and species diversity.The grazed grassland kept the species diversity through conserving internal resource in plant by keeping SLA unchanged and even decreased.(4)Nitrogen addition reduced species diversity of grassland ecosystem in loess plateau region,but species diversity was bigger in grazed grassland overall.According to these results,we draw the following conclusions:(1)Species in fenced and grazed grassland took different ways in acquiring resources and survival strategies to response the changes in the habitat and keep species diversity.(2)Nitrogen addition reduced species diversity of grassland ecosystem in loess plateau region,the main reasons are accumulation of litter biomass and transformation of competition from underground nutrients to light on the surface which resulted in changing of community species.(3)Compared to the fenced and fertilized grassland,grazing counteracted the negative effects of nitrogen addition to species diversity,slowed down the rate of descent to some extent.