Effects Of Long-term Nitrogen And Phosphorus Addition On Plant Community Diversity And Biomass Stability In An Alpine Grassland

Global changes have caused a substantial increase in the availability of nitrogen and phosphorus in ecosystems.Atmospheric nitrogen and phosphorus deposition,as important exogenous nutrient input pathways,has a profound impact on the nutrient cycling in grassland ecosystem.The growth and productivity of plants in terrestrial ecosystems are generally limited by the availability of nitrogen and phosphorus nutrients,and natural grasslands are affected as well.Therefore,elucidate the effects of nitrogen and phosphorus on community structure,community diversity and productivity of grassland plants through long-term nitrogen and phosphorus addition experiments,we can understand and predict the changes of grassland ecosystem under the aggravation of nitrogen and phosphorus deposition in the future,which is an important basis for sustainable utilization of grassland and restoration of degraded grassland.This study adopted the design concept of random block,and set up four treatments including controls（CK）,nitrogen addition(N,10 g N m^-2 a^-1),phosphorus addition(P,5 g P m^-2 a^-1),and simultaneous addition of nitrogen and phosphorus（NP）in the alpine grassland near Haibei station in Qinghai province.The composition and structure of plant community and the above-ground biomass of each species were monitored continuously in the long-term（7-10 years of continuous nutrient addition,2017-2020）experiments,and we analyzed the impact of nitrogen and phosphorus additions on community diversity,aboveground biomass and its stability at spatial and temporal scales.The main results are as follows:1)The community composition and structure were changed by the long-term addition of nitrogen and phosphorus.N,P and NP treatments had a significant negative effect on communityαdiversity.The decrease ofαdiversity in N and P treatments was mainly result from the loss of rare species,while the decrease ofαdiversity in NP treatments was result from the loss of dominant,common and rare species.N and NP treatments significantly reduced communityγdiversity.2)The effects of long-term nitrogen and phosphorus addition on the spatial and temporal scaleβdiversity of plant communities were different among different types of indicators.For the spatial diversity of plant community,β_Jaccard was significantly decreased in P treatment,where convergence succession may have occurred on the spatial scale,whileβ_Bray andβ_Jaccard were not significantly changed in N and NP treatments.In NP treatment,β_Add was significantly decreased,whileβ_Mult was significantly increased.For the temporal diversity of plant community,β_Bray was significantly increased in N,P and NP treatments.The plant community composition had a tendency of directional change under N treatment,which did not occur under P and NP treatments.3)Aboveground biomass significantly increased in long-term N and NP treatments,and the increase of aboveground biomass in NP treatment was higher than that in N treatment.The increase of aboveground biomass in N treatment was mainly contributed by grasses,and the increase of aboveground biomass in NP treatment was mainly contributed by rare species/forbs.There was a significant negative correlation between aboveground biomass and species richness under nitrogen and phosphorus addition conditions.4)There were no significant changes in the temporal stability of plant community biomass,population stability,different functional groups,common species and rare species biomass in the long-term nitrogen and phosphorus addition（during the investigation）.Although NP treatment significantly reduced species asynchrony,dominant species stability and species diversity,it did not change the temporal stability of plant community biomass.There was no significant relationship between the temporal stability of plant community biomass atγscale andαscale.In addition,the effects of nitrogen and phosphorus addition on the stability of plant community biomass were not significant at spatial scale.These results indicate that the aboveground biomass of the plant community increased by nitrogen and phosphorus additions during the 7-10 years of continuous nutrient supplementation,while at the cost of the loss of community diversity and the changes in community composition and structure.The stability of plant community biomass did not change significantly in the late period of long-term nutrient addition,but the loss of species diversity was more pronounced at the later stage of long-term nutrient addition.The stability of plant community biomass still had the risk of decreasing,more attention should be paid to the effect of nitrogen and phosphorus addition on plant community in the future.