## 3 Results
### Seasonal and Interannual Variations of Observed C Fluxes and ET before and after Fertilization  
The seasonal variations in the monthly EC-measured C fluxes and ET in the three stands before and after fertilization are presented in Figure 2. Monthly GPP values during the 4 post-fertilization years were similar to the means during the 9 non-fertilized years (Figure 2a). The monthly values of \(R\) during the 4 post-fertilization years were close to the 9-year pre-fertilization means for Jan-May but lower for the later months except for June and July in 2009 (Figure 2b). Therefore, the monthly NEP values during the 4 post-fertilization years were greater than the 9-year pre-fertilization average values for March-December, especially in June, August and September (Figure 2c). Similarly, for HDF88, monthly NEP values during the 4 post-fertilization years at HDF88 were generally greater than the pre-fertilization average values for 2002 to 2006 (Figure 2g). In the case of HDF00, compared with the 2001-2006 means, the largest increases in monthly values of NEP during 2007-2010 occurred in June, July and August (Figure 2k). The monthly GPP values during 2007-2010 at HDF88 and HDF00 were higher than the mean values for the pre-fertilization periods (Figure 2e,i). The monthly \(R\) values for HDF00 were also generally higher than the monthly means in the pre-fertilization period (Figure 2j). Furthermore, at DF49, the maximum NEP occurred in April and May (Figure 2c), while maximum NEP at HDF88 and HDF00 occurred in June and July (Figure 2g,k). Figure 1: Comparisons of mean monthly values of environmental variables observed at DF49 for the pre-fertilization (1998–2006) and post-fertilization years (2007–2010). The error bars are \(\pm\) 1 standard deviation (SD) for all the pre-fertilization data. (**a**) air temperature above the canopy (\(T_{a}\)); (**b**) 5-cm depth soil temperature (\(T_{s}\)); (**c**) 0–30-cm soil water content (\(\theta\)); (**d**) water vapor pressure deficit (\(D\)); (**e**) total downwelling photosynthetically active radiation (\(Q\)) and (**f**) net radiation (\(R_{n}\)). Most of the monthly ET values during 2007-2010 at DF49 and HDF88 were close to the monthly means in the pre-fertilization period (Figure 2d,h). However, at HDF00, monthly ET values for May-July during 2009 to 2010 were significantly higher than the means for the pre-fertilization period (Figure 2l). Moreover, for DF49, the monthly values of ET during January to April in 2010 were higher than the previous 9-year average values (Figure 2d). To better examine the interannual variations of EC-measured C fluxes, ET, WUE and LUE, the annual measured values are compared in Figure 3. The results indicate that EC-measured NEP during the post-fertilization four years increased in all three stands (Figure 3c). Annual NEP at DF49 remained  
Figure 2: Comparisons of mean monthly values of eddy-covariance tower measured gross primary productivity (GPP), ecosystem respiration (\(R\)), net ecosystem productivity (NEP) and evapotranspiration (ET) in all three stands for the pre-fertilization and post-fertilization years (2007–2010). The means of measured carbon component fluxes and ET at DF49, HDF88 and HDF00 are shown for 1998 to 2006, 2002 to 2006 and 2001 to 2006, respectively. (**a–d**) for DF49; (**e–h**) for HDF88; and (**i–l**) for HDF00. The error bars are \(\pm\) 1 SD for all the pre-fertilization data. stable over post-fertilization years with values about 200 g C m\({}^{-2}\) year\({}^{-1}\) higher than in the pre-fertilization period, despite the post-fertilization years being cooler and wetter than the pre-fertilization years (Figure 1a,c). However, at HDF88, annual NEP consistently increased with increasing stand age (Figure 3c). The interannual variations in GPP and \(R\) at DF49 since 1998 show that \(R\) decreased while GPP showed little change during 2007-2010 (Figure 3a,b). However, for the other two stands, annual \(R\) increased in the first post-fertilization year and decreased in the next three years (Figure 3b).