## 4 Discussion  
### The Response of GPP, R, and Ne to Nitrogen Addition  
In our study, nitrogen addition increased the carbon fixation rate (_GPP_), other simulated nitrogen deposition experiments at leaf and stand levels in N-limited forests also found similar results [40; 41; 42]. However, the effect of nitrogen deposition on _GPP_ or plant growth also varies with the amount of nitrogen deposition, showing a general non-linear relationship [43]. At the leaf level, studies have found that there is a \"threshold effect\" on the effect of nitrogen input on plant photosynthesis [19; 44; 45; 46; 47]. Excessive nitrogen addition will cause a nutritional imbalance in plants and inhibit photosynthesis [48; 49; 4]. However, we found that the accumulation of _GPP_ under HN treatment (69 kgN\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)) was the highest, followed by MN treatment (46 kgN\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)), which increased by 53.62 and 52.42% compared with the control, respectively. Similar results were gained by [PERSON] et al. [19] that the threshold of nitrogen addition was about 80 kgN\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\) for two-year-old _F. mandshurica_ seedlings in northeastern China. [PERSON] et al. [17] also found high nitrogen load (50 kgN\(\cdot\)ha\({}^{-1}\)) significantly increased the net photosynthetic rate of two eucalypt species. The reasons for these differences lay in many aspects. One of them is the difference in the soil's available N content. Some researchers found that when the soil available nitrogen content was not enough to meet the needs of plant growth, high nitrogen concentration can still promote the accumulation of biomass [42; 50]. In our study, the experimental soil was relatively poor in nutrients (concentrations of C and N: 1.19 \(\pm\) 0.30 and 0.12 \(\pm\) 0.03 g\(\cdot\)kg \({}^{-1}\)) and in a nitrogen-restricted state. As shown in Figure 6, the _GPP_ and _Ne_ under HN treatment were the highest at the beginning of the growing season (June), however, under MN were greater than that under HN in July and August (\(p>\) 0.05), which was probably because of the increasing of nitrogen input, the saplings under high nitrogen treatment tended to be saturated with nitrogen. In addition, the difference in measurement methods of the CO\({}_{2}\) gas exchange leads to different results. Some authors have already pointed out that the inadequacy of single-leaf-based sampling to represent variability encountered at the whole-canopy level (due to the differences in stomatal distribution) [21; 22]. We found the response of \(R\) to nitrogen addition was similar to that of _GPP_ (Figures 4-7). Probably due to the relationship between leaf respiration and photosynthesis, which can be explained in the following two aspects. On the one hand, sugars produced by photosynthesis are the main substrates for respiration [51]. On the other hand, respiration provides the carbon backbone for photosynthesis and energy for the synthesis of sugars. At the same time, the consumption of matter by respiration can also relieve the feedback inhibition of photosynthesis, thus further improving the efficiency of carbon assimilation [52]. Therefore, there is usually a positive correlation between respiration rate and photosynthetic rate (\(R^{2}=0.995\), \(p<0.01\)). Similarly, the \(R\) was the largest under HN treatment in our experiment, followed by MN treatment, 23.44 and 23.14% higher than that in control, respectively (Figure 7). However, some studies found \(R\) showed a decreasing trend under high nitrogen treatment [6; 53], which is probably due to the different response of photosynthetic rates to nitrogen addition levels, resulting in different photosynthetic substrates. In addition, leaf respiration varied with leaf age, canopy position, and tree species [54; 55; 56]. And beyond that, only leaf respiration was observed at the leaf level, while the respiration of trunk and branches also accounted for a large proportion of the total respiration, ignorance of them would result in measurement deviation.