## 1 Introduction  
Due to the dual pressure of environmental problems and energy issues, forestation has become critical to prevent soil erosion and desertification, regulate climate, mitigate CO\({}_{2}\) emissions, and generate bioenergy resources. China is the country possessing the largest artificial forest in the world [1]. Considering the timber production and environmental adaptability, plantations in North China mainly rely on poplars. The genus _Populus_ consists of 30-40 species that have been widely used for afforestation in temperate and boreal regions [2]. However, fertile soils in plains areas are often intensively used for agriculture to meet food demands. Poplar plantations have often been established on marginal lands and, therefore, suffer from N deficiency [3]. In this context, it is particularly important to better understand the responses of _Populus_ to different N supply levels and to select poplar species that tolerate N deficiency and/or N fertilizer. High biomass and a short coppricing rotation make poplar an important tree species for the paper industry, biofuels, and ecological environmental protection [4]. Many studies have indicated that a sufficient N supply induces greater photosynthesis, greater biomass production, and accelerated C/N metabolism in plants [5; 6; 7]. For instance, the responses of the belowground biomass and lignin to high N supply are more pronounced in _Populus tremula \(\times\) Populus tremuloides_ than in _Populus trichocarpa_[5]. N-fertilization increased the photosynthetic rate and nitrogen use efficiency of _Populus alba \(\times\) Populus glandulosa_ more than _Populus popularis_[8]. These results indicate that the genus _Populus_ contains dozens of species that show varied C/N metabolism and molecular regulation in response to varying N availability. However, it remains to be elucidated how poplars that differ in their tolerance to low N supply balance growth and C/N physiology, and the selection of poplar species adapted to particular afforestation site conditions is of particular interest. C metabolism in plants primarily includes carbon fixation, carbohydrate anabolism and carbohydrate utilization. Related to these processes, sucrose plays pivotal roles in plant growth, metabolism regulation, and stress resistance [9]. Sucrose influences and reflects photosynthesis, C export, and nutrient mobilization (Figure 1). Sucrose is the primary product of photosynthesis, the form in which carbohydrates are transported from the photosynthetic tissues (i.e., the source) to non-photosynthetic tissues (i.e., the sink) and the initial building block of most natural organic matter [10]. In the photosynthetic tissues, 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. Then, sucrose-6-phosphate phosphatase (SPP) hydrolyzes sucrose-6-phosphate into sucrose (Figure 1) [11]. Sucrose can be exported to the apoplast and transported into the sieve element via the plasmodesmata by sucrose transporter (SUT) proteins [12]. Subsequently, turgor pressure provides the driving force for the long-distance transport of sucrose from the source to the sink tissues. Sucrose can be hydrolyzed in the sink tissues to glucose and fructose by cell wall invertase (CWI) and vacuolar invertase (VI). The glucose is phosphorylated by hexokinase (HxKs) and further utilized for glycolysis and respiration. However, comparisons of C metabolism and photosynthetic partitioning in poplars that show large differences in growth remain scarce. Plants can use a wide range of N forms, ranging from simple inorganic N compounds (ammonium and nitrate) to organic N, such as proteins. Many studies have provided evidence for plant uptake of organic N in the field but direct evidence that organic N contributes significantly to plant N nutrition is still lacking [13]. Thus, research on plant N nutrition has had a strong focus on inorganic N forms [13].