Effects Of Different Levels Of Nitrogen Addition On The Diversity And Productivity Of Grassland Plant Community In Northern Agro-Pastoral Ecotone

In the past 30 years,due to the rapid development of industrialization and the excessive use of fertilizers,the atmospheric nitrogen deposition shows an increasing trend,especially in the grassland ecosystem located in agro-pastoral ecotone,which is more obviously affected by the nitrogen deposition.Increased nitrogen deposition affects plant community species composition,biodiversity,productivity,and ecosystem structure and function.In this study,the semi-arid grassland ecosystem in northern agro-pastoral ecotone was studied.In April 2017,different level of nitrogen addition tests were set up,and the addition level of 0 g N·m-2(N0),1 g N·m-2(N1),2 g N·m-2(N2),4 g N·m-2(N4),8 g N·m-2(N8),16 g N·m-2(N16),24 g N·m-2(N24)and 32 g N· m-2(N32)were added.During the 2017-2018 growth season,the coverage,abundance and height of the plant community were investigated at the beginning of each month(June-August),and the biomass was measured in the early August.The net underground productivity of the year was determined by endogenous growth method front mid-april to mid-October.The results show that:1.Short-term nitrogen addition increased the plant community abundance,height and coverage.In 2017,the plant community characteristics were the highest under N16 treatment and the lowest under N32 treatment.In 2018,the abundance and coverage of plant community under N16 treatment were higher than those under other treatments,and the height of plant community was higher with N24 treatment.Nitrogen addition treatment had a significant effect on plant community height(P < 0.05),and the interannual grassland plant community abundance,height and coverage were significantly increased(P < 0.001).2.Short-term nitrogen addition reduced the diversity of grassland species.In June and July of 2017,the richness index of the control area was higher than that of nitrogen addition treatment.In August,N8 treatment had the highest richness index,followed by N1 and N16 treatment.In July and August,the diversity index of N16 treatment was the largest,and the richness and diversity index of N32 treatment were the lowest for three consecutive months.In June 2018,the richness and diversity index of N8 treatment were the highest,while the richness and diversity index of N0 treatment were the highest in July and August.Nitrogen addition treatment had no significant influence on the evenness of grassland.For two consecutive years,the number of species in the control area was higher than that treated with nitrogen addition.The species number and diversity of interannual grassland decreased significantly(P < 0.001).3.In the two consecutive years of experiments,the aboveground biomass of plants treated with N16 was the highest(286.0 g · m-2,569.8 g · m-2),and the aboveground biomass of the control area was the lowest(131.8 g · m-2,338.3 g · m-2).Nitrogen addition at different levels had a significant impact on underground biomass of grassland(P < 0.05).In 2017,the addition of 2 g of N m-2 was the highest(1957.48 g · m-2),and the difference was significant in the 0-10 cm soil layer(P < 0.05).N16 treatment was the highest,followed by N2 treatment.2018 underground biomass highest treated with N16(2206.9 g · m-2),and in the 0 to 30 cm soil layer,N16 treatment of underground biomass were significantly higher than other treatment(P < 0.05),the control for two consecutive years of plant root cap ratio is significantly higher than adding nitrogen treatment(P < 0.05);In 2017,nitrogen addition had a significant influence on the net underground primary productivity of grassland(P < 0.05),among which N24 treatment was the highest(681.1 g · m-2).The difference of net underground primary productivity of grassland in 2018 was not significant,but it was higher under N16 treatment(977.1 g · m-2).Nitrogen addition treatment had significant effects on above-ground biomass and underground biomass of grassland(P < 0.05),and inter-annual above-ground biomass and net underground primary productivity of grassland increased significantly(P < 0.001).In conclusion,16 g N · m-2 nitrogen addition level is the saturated nitrogen addition level in this region.Beyond this addition level,productivity and species diversity begin to decline.