## 1 Introduction
Ammonium (NH4+) and nitrate (NO3-) are two major forms of inorganic N available for plants in soil [14]. N metabolism in plants primarily includes NH4+/NO3- uptake, transport, and assimilation for amino acid synthesis (Figure 1). NH4+ and NO3- are absorbed in the roots by various transporters for ammonium (_AMTs_) and nitrate (_NRTs_) and form ion fluxes at the root surface. The genome of _Populus trichocarpa_ has been documented to contain 14 putative _AMTs_ and 79 putative _NRTs_[15]. A few studies have indicated that transcript levels of _AMTs_ and _NRTs_ (e.g., _AMT1.2_, _AMT1.6_, _NRT1.1_, _NRT2.4B_, _NRT2.4C_, and _NRT3.1C_) were responsive to the environmental N supply. A non-invasive micro-test technique (NMT), a powerful tool in the investigation of in situ ion fluxes, has been widely used to explore the electrophysiological processes of nitrogen acquisition at the root surface [16]. Spatial and temporal dynamics of net ion fluxes and influences of environmental factors such as pH in the media on net ion fluxes have been reported for roots of maize, barley, rice, poplar, and species of eucalyptus and coniferous [3; 17; 18; 19]. Additionally, many studies assume that \({}^{15}\)N at natural abundance levels acts as a tracer, and significant discrimination has been observed for plants [20]. Discrimination is positive in most biological systems; therefore, the product has a lower \(\delta^{15}\)N value than the substrate [21]. After uptake, NO3- is reduced to NH4+ via nitrate reductase (NR) and nitrite reductase (NiR). Then, NH4+ is assimilated to glutamine and glutamate by glutamine synthetase and glutamate synthase (GS/GOGAT) or the glutamate dehydrogenase (GDH) pathways (Figure 1). Glutamine and glutamate are utilized as precursors to synthesize various amino acids and proteins. To date, however, few studies have investigated the NH4+/NO3- uptake and assimilation response to the external N supply to compare N metabolism in different poplar species under different environmental conditions. Little information is available about the C/N metabolism of various poplar species and how they balance growth and nutrient mobilization in the context of a variable N supply. _Populus simonii_ (Ps) is widely distributed on the Loess Plateau, whereas _Populus euramericana_ (Pe) ordinarily grows on the plains. Soils on the Loess Plateau in Northwest China are alkaline (pH 8-9) and low in available N [22; 23; 24]. However, Pe grows in the plains soil with sufficient available N. In our previous study, we showed that the slow-growing Ps had greater root growth and NH4+ assimilation under N-deficient conditions compared to plants with an unlimited N supply, indicating that the plants acclimated to a limiting N supply by allocating more carbon to the root system [23]. However, in another preliminary experiment, the fast-growing Pe was sensitive to low N supply and showed growth inhibition in both  
Figure 1: A schematic model of C/N metabolism in plants. Glyceraldehyde 3-phosphate (G3P) produced by CO2 fixation in Calvin cycle can be exported to the cytosol or used to synthesize starch in chloroplast. Sucrose is synthesized by the enzyme sucrose phosphate synthase (SPS) via the consumption of uridine diphosphate glucose (UDP-G) from the Calvin cycle and fructose-6-phosphate. Sucrose can be transported to the sink tissues and hydrolyzed to glucose and fructose by cell wall invertase (CWI) and vacuolar invertase (VI). The glucose is phosphorylated by hexokinase (HkKs) and further utilized for glycolysis and respiration. NH4+ and NO3 – are absorbed in the roots by various transporters for ammonium (_AMTs_) and nitrate (_NRT_s). After uptake, NO3 – is reduced to NH4+ via nitrate reductase (NR) and nitrite reductase (NiR). Then, NH4+ is assimilated to glutamine (Gln) and glutamate (Glu) by glutamine synthetase and glutamate synthase (GS/GOGAT) or the glutamate dehydrogenase (GDH) pathways. the belowground and aerial parts.