Effects of elevated ozone on the uptake and allocation of macronutrients in poplar saplings above- and belowground

Ground-level ozone (O₃) is a secondary air pollutant and affects the roots and soil processes of trees. Therefore, O₃ can affect the uptake and allocation of nutrients in trees, which merits further clarification. A fumigation experiment with five O₃ levels was conducted in 15 open top chambers for two poplar clones, and the concentrations of six macronutrients (N, P, K, S, Ca, Mg) in different organs and leaf positions were determined. Under all O₃ levels, the concentration of mobile nutrients (N and P) was higher in upper leaves than in lower leaves, while the non-mobile nutrients (Ca and S) concentration was the opposite. Relative to charcoal filtered ambient air (CF), high O₃ treatment (NF60) significantly increased the concentration of mobile nutrients K and Mg in upper leaves by 38 % and 33 %, in lower leaves by 142 % and 65 %, respectively, which suggested the effect of O₃ on their concentrations was greater at the lower leaf position than at the upper leaf position. Elevated O₃ significantly increased the macronutrient concentrations in most organs. The effects of O₃ on nutrient concentrations were attributed using graphical vector analysis, suggested that the increase of nutrient concentration in the shoots was attributed to excessive nutrient stocks, while their increase in root was attributed to the "concentration" effect. Compared to CF, NF60 also reduced the root-to-shoot ratio of N, P, S, K, Ca and Mg stocks by 34 %, 39 %, 37 %, 64 %, 46 % and 42 %, respectively, indicating the allocation of increased nutrients to shoots in response to O₃ stress. Changes in the allocation pattern of nutrients in different leaf positions and organs of poplar were primarily in response to O₃ stress since these nutrients play important roles in some physiological processes. These results will help improve the plantation nutrient utilization by optimizing fertilizer management regimes under O₃ pollution.