## Results
### Description of Fertilization Effects on Fluxes, Optical Data, and Vegetation Characteristics
Group differences in **(a,b,i,l)** were analyzed with ANOVA with the Welch correction and individual differences among groups were evaluated with the Games–Howell post hoc test. _“**_” refers to a significant difference from the control treatment with \(p\) value \(<0.05\) and _“**_” refers to a significant difference from the control treatment with \(p\) value \(<0.01\). All treatments show a significant increase in Albedo\({}_{400-900}\) during Campaigns 5 (\(F_{3,18}\)= 29.3, \(p<0.01\)) and 6 (\(F_{3,26}\)= 13.6, \(p<0.01\)) in 2015, but no significant treatment-induced changes in Albedo\({}_{400-900}\) are observed in 2014. Ts shows significant differences in Campaign 6 for the N, NP and P treatments (\(F_{3,26}\)= 13.5, \(p<0.01\)). LE\({}_{\text{ISO}}\) follows the phenological cycle with lower values in 2015 (Figure S3a). There are differences in LE\({}_{\text{ISO}}\) among treatments (such as the increase during Campaign 2 of 2014 for N and NP), but these appeared not significant according to the ANOVA. LE\({}_{\text{ISO}}\) estimates are consistent also with independent simulations with SCOPE (Figure S3c). Instead, significant differences in %graminoids among treatments occur mainly in 2015 in Campaigns 5 (\(F_{3,18}\)= 9.4, \(p<0.01\)) and 6 (\(F_{3,26}\)= 13.3, \(p<0.01\)) with lower %graminoids in N and NP treatments. %Forbs also present significant differences in 2015 by increasing in the N treatment (in comparison with the C treatment) (\(F_{3,18}\)= 8.8, \(p<0.01\)) and in Campaign 6 in the N and NP treatment (\(F_{3,26}\)= 11.5, \(p<0.01\)) (Figure S14d). %Legumes is marginal and does not change significantly among treatments (Figure S14e,f). Temporal Variability of GPP-F\({}_{760}\) and GPP-F\({}_{760\text{leaf,fw}}\) Relationship among Treatments  
The results of the ANCOVA show that, in 2014, the intercept of the C treatment is significantly different from the other treatments for both F\({}_{760}\) (as shown in previous studies [8; 37] and F\({}_{760\text{leaf,fw}}\) (\(p<0.05\) and \(p<0.05\), respectively) (Figure 4 and Table S1). Figure 4: Scatterplot of observed fluorescence at 760 nm from top of canopy (F\({}_{760}\)) vs. Gross Primary Production (GPP) for 2014 (**a**) and for 2015 (**c**); and directional fluorescence emitted by all leaves at 760 nm calculated from forward SCOPE runs (F\({}_{760\text{leaf,fw}}\)) vs. GPP for 2014 (**b**) and for 2015 (**d**). Data are divided for the four treatments; control (C), nitrogen addition (N), nitrogen and phosphorus addition (NP) and phosphorus addition (P). P values of the interaction treatment with independent variable (in comparison with the control treatment, C) from an analysis of covariance (ANCOVA) are reported in the bottom-right of each panel. Colored lines represent the regression from the ordinary least square regression. In 2015, the intercept is different for the C treatment for both F\({}_{760}\) and F\({}_{760\text{leaf,fw}}\) (\(\rho<0.01\) for both) and for the NP treatment with \(p<0.05\) for both F\({}_{760}\) and F\({}_{760\text{leaf,fw}}\). In 2015, for the N treatment, there is no significant interaction between F\({}_{760}\) and treatment (Figure 4c), but there is a significant interaction between F\({}_{760\text{leaf,fw}}\) and the N treatment (\(p<0.05\)) (Figure 4d), with significant differences of the GPP-F\({}_{760\text{leaf,fw}}\) relationship. There is no significant effect of the year on the GPP-F\({}_{760}\) relationship. For each treatment, \(p=0.706\), \(p=0.323\), \(p=0.927\) and \(p=0.992\) for N, P and NP and C, respectively. Instead, when substituting F\({}_{760}\) with F\({}_{760\text{leaf,fw}}\), the effect of the year is not significant in the treatments C and P (\(p=0.659\) and \(p\)=0.742), but is significant for the NP treatment with \(p<0.05\), and barely not significant for the N treatment with \(p=0.057\).