Effects of elevated ozone and nitrogen addition on leaf nitrogen metabolism in poplar

Aims Ozone(O_(3))pollution and nitrogen(N)deposition/fertilization often simultaneously affect plant growth.However,research of their interactive effects on leaf N metabolism is still scarce.We investigated their interactive effects,aiming to better understand plant N metabolism processes and biogeochemical cycles under high 03 pollution and N deposition/fertilization.Methods Poplar saplings were exposed to two O_(3)levels(NF,non-filtered ambient air;NF60,NF+60 ppb O_(3))and four N treatments(NO,no N added;N50,NO+50 kg N ha^(-1)yr^(-1);N100,NO+100 kg N ha^(-1)yr^(-1);N200,NO+200 kg N ha^(-1)yr^(-1)in open-top chambers for 95 days.The indicators related to leaf N metabolism were analyzed,including the activities of N-metabolizing enzymes and the contents of total N,NO_(3)^(-)-N,NH_(4)^(+)-N,total amino acid(TAA)and total soluble protein(TSP)in the leaves.Important Findings NF60 stimulated the activities of nitrate reductase(NR)by 47.2%at August relative to NF,and stimulated glutamine synthetase(GS)by 57.3%when averaged across all N treatments and sampling times.In contrast,O_(3)did not significantly affect TSP and even reduced TAA content in August.Relative to NO,N200 significantly increased light-saturated rate of CO_(2)assimilation(Asat)by 24%,and increased total N content by 70.3%and 43.3%in August and September,respectively,while it reduced photosynthetic N-use efficiency by 26.1%in August.These results suggest that the increase in Asat and total N content are uncoupled,and that the surplus N is not used to optimize the capacity for carbon assimilation under high N treatment.Simultaneously,high N treatment significantly promoted leaf N metabolism by increasing NO_(3)^(-)-N contents,NH_(4)^(+)-N contents,TAA contents and the activities of NR and GS.There was no significant interaction between O_(3)and N for all variables.