## 3 Results
### Effects of Nitrogen Addition on GPP, R, and Ne  
#### 3.3.1 Diurnal Variation of Crown CO\({}_{2}\) Exchange  
The measurements showed that nitrogen addition increased the crown photosynthetic rate, with MN and HN, were larger than that of LN in every month (Figure 4a). At the beginning of nitrogen addition (June), the peak value of photosynthetic rate under HN treatment was highest, however, which under MN treatment it was greater than that of HN and LN in July and August. At the end of the growing season, the peak value of photosynthetic rate in HN was slightly higher than that in MN, but there was no significantly different (\(p>0.05\)), which was 2.14 and 2.12 \(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), respectively. However, the influence of different nitrogen addition levels on the diurnal variation in crown photosynthetic rate showed a similar pattern. In general, the crown photosynthetic rate increased with the increasing of _PAR_, reaching the peak at around 7:00-9:00, and decreasing in the afternoon. However, the photosynthesis weakened as photoinhibition occurred at noon under different nitrogen treatments in some months. In the afternoon, a small peak appeared at about 14:00-15:00 under some nitrogen treatments with the temperature and _VPD_ dropped. Among the different nitrogen treatments, CK treatment showed double peak phenomenon in each month, while LN and MN only in August and September, and HN in June and August showed that. We also found the peak values of crown photosynthetic rate ranged from \(-3.44\) to \(-5.52\)\(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), \(-3.15\) to \(-5.04\)\(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), \(-2.78\) to \(-4.56\)\(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), \(-1.66\) to \(-3.25\)\(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) in June, July, August, and September, respectively, which showed a variable and downward trend during the growing season. The response of crown respiration rate to nitrogen addition was similar to that of photosynthetic rate, with MN and HN higher than that of LN (\(p<0.05\)), the peak of respiration rate under MN treatment was greater than that under HN and other treatments during the growing season (Figure 4b). The peak of respiration rate ranged from 0.18 to 0.27 \(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), 0.21 to 0.34 \(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), 0.19 to 0.29 \(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), 0.17 to 0.25 \(\mu\)mol CO\({}_{2}\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\) in June, July, August, and September, respectively, which were an order of magnitude lower than that of crown photosynthetic rate. The daytime crown respiration rate (5:00-19:00) firstly increased and decreased later, and the maximum value appeared at around 14:00-15:00. However, the night-time crown respiration rate (19:00-5:00) linearly declined at around 0:00-5:00 and 21:00-23:00, with the highest values appearing in the HN treatment. Diurnal variation of respiration rate varied from month to month, but the daytime crown respiration rate was always greater than the night-time crown respiration rate. Figure 3: Variations in air temperature difference (\(\Delta T\)) (**a**) and vapor pressure deficit difference (\(\Delta VPD\)) (**b**) between inside and outside of the chamber were measured at the end of the 6 min chamber closing period with incident light (_PAR_). Non-linear and linear regressions (black lines) are also shown. Owing to the small change of respiration rate, the response of net photosynthetic rate to nitrogen addition was similar to the photosynthetic rate with MN, HN was higher than that of LN (\(p<0.05\)) (Figure 4c), and the diurnal variation exhibited double peak phenomenon under different nitrogen treatments in some months. In addition, the patterns of daytime uptake and night-time release were evident.