## 3 Results
### Effects of Nitrogen Addition on GPP, R, and Ne
#### 3.3.2 Seasonal Dynamics and Monthly Accumulation of Net Crown CO\({}_{2}\) Exchange  
As shown in Figure 5a, the daily accumulation of \(GPP\) increased with the nitrogen addition levels, with the average values being 1.16, 1.30, 1.76, and 1.79 \(\mathrm{G}\mathrm{C}\mathrm{m}^{-2}\mathrm{d}^{-1}\) in the control, low, medium, and high nitrogen treatments, respectively. There is a significant difference between LN and MN (\(p<0.05\)), while there is no significant difference between CK and LN, and MN and HN treatments (\(p>0.05\)). We also observed that daily sums of \(GPP\) have obvious seasonal variation from June to September. With seasonal vegetation senescence, the values of \(GPP\) decreased gradually, which were below the average values at the end of August. At the end of the growing season (25 September), the value of \(GPP\) declined sharply under control, low, and medium nitrogen treatments, which were an order  
Figure 4: Monthly averaged diurnal variations in photosynthetic rate (**a**), respiration rate (**b**), and net photosynthetic rate (**c**) of _Fraxinus mandshurica_ saplings after nitrogen addition and the control during the growing season (from 1 June to 30 September). CK, LN, MN and HN represent control (no nitrogen addition), low nitrogen addition (23 \(\mathrm{kgN}\cdot\mathrm{h}\mathrm{m}^{-2}\cdot\mathrm{a}^{-1}\)), medium nitrogen addition (46 \(\mathrm{kgN}\cdot\mathrm{h}\mathrm{m}^{-2}\cdot\mathrm{a}^{-1}\)) and high nitrogen addition (69 \(\mathrm{kgN}\cdot\mathrm{h}\mathrm{m}^{-2}\cdot\mathrm{a}^{-1}\)), respectively. of magnitude lower than the average values and ranged from 0.06 to 0.10 g Cm\({}^{-2}\) d\({}^{-1}\). The same was true of HN, but which occurred 3 days later (28 September). Similarly, the daytime respiration (\(R_{d}\)) and the night-time respiration (\(R_{n}\)) increased with nitrogen addition levels, which also have obvious seasonal variation (Figure 5b,c). Affected by \(T_{air}\), the peak occurred in early August or late July. The peak values of \(R_{d}\) in the control, low, medium, and high nitrogen treatments were 0.15, 0.18, 0.24, and 0.22 g C m\({}^{-2}\)d\({}^{-1}\), respectively (Figure 5b). Moreover, all nitrogen addition treatments showed a smaller magnitude changes in \(R_{n}\) than in \(R_{d}\), which ranged from 0.04 to 0.08 g C m\({}^{-2}\)d\({}^{-1}\), 0.05 to 0.10 g C m\({}^{-2}\) d\({}^{-1}\), 0.06 to 0.12 g C m\({}^{-2}\) d\({}^{-1}\), 0.06 to 0.12 g C m\({}^{-2}\) d\({}^{-1}\) in control and low, medium and high nitrogen addition treatments, respectively (the inhibitory effect of light on dark respiration was not considered in this paper). Owing to the small range of \(R\), the response of \(Ne\) to nitrogen addition was similar to \(GPP\), with MN, HN higher than LN (\(p<0.05\)) (Figure 5d). At the end of the growing season (25 September), the value of \(GPP\) declined sharply to less than \(R\), leading to the change of \(Ne\) from a negative value to a positive value, indicating the crown shifted from carbon sink to carbon source. However, the change of \(Ne\) under HN occurred 3 days later (28 September). Nitrogen addition increased crown carbon sink from June to September, with MN and HN higher than LN (\(p<0.05\)) (Figure 6). The monthly accumulation negative \(Ne\) value ranged from \(-11.25\) to \(-42.42\) g C m\({}^{-2}\)month\({}^{-1}\) in controls, whereas which ranged from \(-13.78\) to \(-44.61\), \(-21.82\) to \(-59.95\) and \(-23.65\) to \(-65.47\) g C m\({}^{-2}\)month\({}^{-1}\) for the same time in low, medium, and high nitrogen addition treatments. In addition, the monthly cumulative amount of \(GPP\) and \(Ne\) dropped gradually under all treatments from June to September.