## 4 Discussion
### Mechanisms behind the Treatment Effect on GPP and \(\mathrm{F_{760}}\) at Leaf and Canopy Scale  
The effect of canopy structure on \(\mathrm{F_{760}}\) manifests itself mainly through variation in APAR and \(\mathrm{Fesc_{fw}}\) (Figures 6 and 2i,l, respectively). With the path analysis, we conclude that %graminoids positively influences APAR that leads to an increase of \(\mathrm{F_{760\text{leaf,fw}}}\) indirectly. Moreover, %graminoids negatively influence \(\mathrm{Fesc_{fw}}\). The changes of canopy structure mediated by changes in plant community at plot level are the most important factors controlling the pathway between \(\mathrm{F_{760\text{leaf,fw}}}\) and \(\mathrm{F_{760}}\), and ultimately GPP and \(\mathrm{F_{760}}\). By analyzing the relationships between different components measured in the manipulative experiment presented here, we were able to disentangle the pathways, mostly unknown [14; 20], through which N% influence the different components of the LUE equations. Our results show that the largest effect of N% on fluorescence emission is not direct, but rather mediated by APAR and Ts (Figure 6), which in turn affect \(\mathrm{F_{760\text{leaf,fw}}}\). There are two indirect ways in which N% affects \(\mathrm{F_{760\text{leaf,fw}}}\): (i) Higher N% in the green fraction of the vegetation is associated to an increase of photosynthetic pigments and in particular Cab in leaves [64] and in the canopy [22], which ultimately has a positive effect on APAR [15; 66]. Increase in APAR causes higher fluorescence emission at leaf and canopy level (Figure 6) [67]. There are contrasting results in the literature regarding the effect of N% on fluorescence and all the studies conducted at the leaf level [14; 15; 26]. Our study at canopy level supports the findings in [15] that at varying levels of N available APAR modulates \(\mathrm{F_{760\text{leaf,fw}}}\) and \(\mathrm{F_{760}}\), and its relationship with GPP. (ii) N% influences positively \(\mathrm{F_{760\text{leaf,fw}}}\) through Ts. N% has a negative effect on Ts and \(\mathrm{F_{760\text{leaf,fw}}}\) exhibits a negative relationship with Ts. The first hypothesized mechanism is related with the observed increased in Albedo\({}_{400-900}\) (Figure 3e,f) associated with the higher N%. The effect of N% on albedo, despite being quite debated in the literature [68; 69], has been demonstrated both at canopy scale [70; 71] and at leaf level [72] and has to do with the increase in near infra-red (NIR) reflectance that is larger than the decrease of the reflectance in the visible region due to higher Cab and light absorption. Therefore, the increase of Albedo\({}_{400-900}\) with increasing N% results in less available energy in the canopy, which eventually leads to a decrease of Ts if other conditions such as soil moisture and VPD are similar [69; 72]. The second has to do with the modulation of transpiration due to the fertilization (Figure 3g,h), which cools down the canopy, as the leaf surfaces lose heat when water evaporates through the stomata. Our estimate of LEISO show an increase in N and NP treatments during the peak of the growing season, but it is not significant (Figure S3a,b) and lower than the changes in Albedo\({}_{400-900}\) for N, NP and P, in particular in 2015 (Figure 3c,d). Given the strong response of GPP in the N and NP treatments in 2015 (Figure 2b), the mild change in LEISO (Figure S3a,b) suggests an increase of water use efficiency, which is backed by \(\xi^{13}\)C measurements, which show a significant increase in the N and NP treatment of Campaign 6 (Figure S2) (where less negative values correspond to higher WUE [73]). Therefore, we can conclude that, although transpiration might be involved in the regulation of Ts at the peak of the season, biophysical variables such as Albedo\({}_{400-900}\) are much more affected by N% and contribute to reduce Ts. Given that a large amount of N is invested in Rubisco protein [23], N can impact directly the carboxylation rates.