The Response Of Productivity And Species Diversity To Warming And Nitrogen Addition In Stipa Breviflora Desert Steppe

Long-term climate warming and atmospheric nitrogen deposition aroused chronic resource alterations in ecosystem,meanwhile ecosystem sequentially responsed to such chronic alterations.However,in grassland ecosystems,especially the desert grassland ecosystems,evidence for the sequential response mechanism of plant community to long-term warming and nitrogen addition is still lacking.A study was carried out in Stipa breviflora desert steppe in Inner Mongolia.In this study,a split-plot design with warming as the primary factor and nitrogen addition as the secondary factor was used.There were four treatments,including the control(CK),nitrogen addition(N),warming(W)and warming plus nitrogen addition(WN).Each treatment had six replicates.During the growing season in 2015 and 2016,we investigated aboveground and belowground net primary productivity,root standing crop and species diversity of plant community under different treatments.At the same time,dynamic variations of soil temperature and soil moisture were monitored.The major findings are as follows:1.Warming,nitrogen addition and their interactions had no effects on aboveground net primary productivity in plant community;while the aboveground net primary productivity of perennial grasses was significantly reduced in the nitrogen addition treatment(P = 0.0409)during the two growing season both in 2015 and 2016.2.During the growing season in 2016,warming,nitrogen addition and their interactions had no effects on belowground net primary productivity of plant community,but warming significantly reduced belowground net primary productivity in 10-20 cm soil layer(P =0.0129).The allocation ratio of belowground net primary productivity in each layer was not affected by warming,nitrogen addition and their interactions.3.Warming reduced BNPP/ANPP(P = 0.0319),whereas no effects of nitrogen addition and its interaction with warming were detected in 2016.Both warming and nitrogen addition altered the relationship of trade-off between above-and below-ground net primary productivity in 2016.4.In 2016,warming significantly decreased root standing crop in 0-10 cm and 10-20 cm soil layers and in plant community(P<0.05),however,nitrogen addition and their interactions had no effects on root standing crop in each layer and plant community.In addition,none of warming,nitrogen addition and their interactions influenced the allocation ratio of root standing crop in each layer.5.During the two growing seasons,warming markedly reduced species richness,Shannon-Wiener index and Margalef index of plant community(P<0.01),whereas the effects of warming on Pielou index varied with year(P=0.0028).Warming stimulated Pielou index in drought year(2015),but reduced Pielou index in wet year(2016).Nitrogen addition siginificantly reduced species richness of shrubs and semi-shrubs(P = 0.0007),while it had no effect on species richness of plant community.Warming significantly interacted with nitrogen addition in terms of their effects on species richness of perennial forbs and shrubs and semi-shrubs(P<0.05).These results suggest that long-term warming and nitrogen addition would change plant community structure and composition in Stipa breviflora desert steppe.In addition,warming and nitrogen also would vary the allocation strategy of plant net primary productivity between above-and below-ground.