## Results
### Root and Leaf Activities of Enzymes Involved in C Assimilation  
In Ps, low N supply had no effect on the root and foliar SPS activities (Figure 3A,C). However, in Pe, the SPS activities in both roots and leaves were stimulated by high N supply (Figure 3A,C). The root SUS activity was increased with increasing N supply in Ps but remained unaltered in Pe (Figure 3B). The foliar sucrose synthase (SUS) activity was also unaffected by N availability in both Ps and Pe (Figure 3D). The HxK activity in the roots was inhibited in Ps by high N supply, but the  
Figure 2: Contents of sucrose (**A,D**), fructose (**B,E**) and glucose (**C,F**) of _P. simonii_ (Ps) and _Populus eumaricana_ (Pe) under 0.01, 1, and 10 mM NH\({}_{4}\)NO\({}_{3}\). Bars labelled with different letters indicate significant difference between the treatments. _p_-Values of the ANOVAs of species, N treatment, and their interaction are indicated. * \(p\) < 0.05; ** \(p\) < 0.01; ** \(p\) < 0.001; ns, not significant. opposite pattern was observed in Pe (Figure 3E). However, the HxK activity in the leaves was increased by high N supply in both species (Figure 3F). Generally, with medium and high N supply, Ps showed lower SPS activity than Pe (Figure 3A). However, with low N supply, the foliar SPS activity was higher in Ps than in Pe (Figure 3B). The SUS activity in roots was lower in Ps than in Pe at all three N levels, and the SUS activity in leaves was similar in the two poplar species (Figure 3D). At all three N levels, the foliar HxK activity in Pe was higher than in Ps (Figure 3F). The root CWI activity was not affected by the N supply in Ps, but was increased by high N supply in Pe (Figure S3). In Ps, the root VI activity decreased with high N supply, but in Pe, it was not affected by N supply (Figure S3). Low N supply induced a higher foliar CWI activity in Ps (Figure S3). The foliar VI activity was also unaffected by the N supply (Figure S3). The CWI and VI activities were generally lower in Ps than in Pe, especially with medium and high N supply (Figure S3). However, under low N supply, the foliar CWI activity was higher in Ps than Pe (Figure S3). Net Fluxes of NH4\({}^{+}\), NO3\({}^{-}\), and H\({}^{+}\); Accumulation of NH4\({}^{+}\), NO3\({}^{-}\), and NO2\({}^{-}\); and \(\delta^{15}\)N Content  
Generally, in both Ps and Pe, the net NH4\({}^{+}\) and NO3\({}^{-}\) fluxes were significantly decreased by N deficiency (Figure 4A,B and Figure S4). In contrast, both species showed increases in the net H\({}^{+}\) flux in  
Figure 3: Sucrose phosphate synthase (**A,D**), sucrose synthase (**B,E**), and hexokinase (**C,F**) activities of _P. simonii_ (Ps) and _Populus euramericana_ (Pe) under 0.01, 1, and 10 mM NH4 NO3. Bars labelled with different letters indicate significant difference between the treatments. _p_-Values of the ANOVAs of species, N treatment, and their interaction are indicated. * \(p\) < 0.05; **_p_ < 0.01; *** \(p\) < 0.001; ns, not significant. response to a low N supply (Figure 4C). With low and medium N supply, Ps and Pe exhibited a similar net NH4\({}^{+}\) flux (Figure 4A). However, with high N supply, the net NH4\({}^{+}\) flux was greater in Pe than in Ps (Figure 4A). With low N supply, the net NO3\({}^{-}\) flux was similar between the two species, but with high N supply, it was higher in Pe than in Ps (Figure 4B). With both low and high N supply, net H\({}^{+}\) flux was higher in Ps than in Pe (Figure 4C). In both species, the root NH4\({}^{+}\) concentration decreased with decreasing N supply levels (Figure 5A). Further, in both poplar species, the N supply had no effect on the NO3\({}^{-}\) concentration in roots and leaves (Figure 5B,E). The NO2\({}^{-}\) concentration in the roots decreased in Ps but increased in Pe in response to high N supply (Figure 5C). However, the foliar NH4\({}^{+}\) concentration increased in Pe in response to high N supply (Figure 5D). Low N supply increased the foliar NO2\({}^{-}\) concentration in both Ps and Pe (Figure 5F).