## 1 Introduction  
In recent years, the atmospheric nitrogen deposition rate has risen dramatically due to rapid agricultural, industrial, and urban development. The monitoring study found that the nitrogen deposition of European forests was 25-60 kg N\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)[1], and that of Los Angeles forests was 35-50 kg N\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)[2], the annual bulk nitrogen deposition in China has increased from 13.2 kg N\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\) in the 1980s to 21.1 kg N\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\) in the 2000s [3]. Forests are the most important part of terrestrial ecosystems and the direct carriers of nitrogen deposition. Increasing enhanced atmospheric nitrogen deposition increases the availability of nitrogen in forest ecosystems and affects the carbon cycle by influencing the photosynthesis and respiration of plant's canopy leaves [4, 5, 6, 7]. For now, there are some routine methods to predict changes in forest carbon cycles, including eddy covariance techniques and biomass survey methods [8; 9]. However, the eddy covariance technology needs large homogenous areas to be applied and is not suitable for control experiments with multi-plot grades. In addition, low friction velocity (laminar conditions) often makes consistent measurements difficult during night-time [10; 11; 12]. [PERSON] et al. [13] found the eddy covariance approach, compared with the biometric and chamber methods, overestimated net ecosystem production by 25%, and underestimated ecosystem respiration by 10% and gross primary production by 3%. The differences between eddy covariance and the other methods were greater at the sites with complex topography and dense canopy than at the sites with flat topography and open canopy. Some other studies on biomass carbon sink and net ecosystem production estimation based on forest inventory data at the regional scale have developed considerably [14; 15], however, which is limited in terms of replication (due to labor-intensive) and time resolution (often performed over annual or longer time intervals) [16]. In addition, this method is difficult to explain the influence of different factors on forest carbon source and sink changes in mechanism, especially in the context of global climate change. Therefore, substantial researches have recently been conducted on the effects of nitrogen deposition on the forest canopy carbon cycle at the leaf level [17; 18; 19]. Nevertheless, single-leaf measurements can be problematic for several reasons. First, the forest ecosystems usually have high heterogeneity due to the complexity of vegetation. Not only do environmental factors such as sunlight, temperature and humidity, wind speed, and wind direction in the forest have spatial gradients, but they also have the characteristics of time gradients. Changes in these environmental factors will directly affect the gas exchange characteristics of plants [20]. Additionally, carbon assimilation generally varies with different parts and different azimuth blades (e.g., due to differences in stomatal distribution) [21; 22]. Furthermore, the leaf-chamber measurements were discontinuous (once every few days and even to once a month), which ignores the carbon assimilation rate dependent upon the age and developmental stage of the plant [23]. Such limitations can be avoided by using a novel automatic-chamber system, which allows long term and continuous monitoring of changes in net photosynthetic and respiration of the canopies under different meteorological conditions at a whole plant scale. This way not only quantitatively evaluates the impact of environmental changes on the physiological and biochemical processes of a single plant, but is also the key to linking single leaf scale and ecosystem scale. Most importantly, it can provide a reference to accurate estimation of gas exchange between different scales.