## 5 Outlook  
High N input through animal manure, mineral fertilizer, and N deposition are common in croplands and can produce nutrient leaching and eutrophication of surface waters [83; 84; 85]. In Switzerland, N deposition is the source that provides the least amount of N inputs in agricultural soils [19]. However, critical loads (threshold below which adverse environmental effects do not occur according to present knowledge) of N deposition have been exceeded in most of the forest and semi-natural ecosystems [59]. Despite N deposition decreasing since the mid-1990s, N deposition remains still too high in many areas [60]. In particular, the highest N exceedances occur in the lowlands where intensive management practices lead to high ammonia emissions because of high manure inputs [59; 86]. In our study, the predictive performance of N deposition decreased in those areas with high intensity of use. N can stimulate growth differently according to species-specific physiology and other co-limiting nutrients, such as phosphorous and potassium [23; 87]. However, in those cultivated areas, growth is regulated by specific management practices. Detailed information about management practices (e.g., fertilization and irrigation), soil components, and crop types could shed light on the impact of N deposition on C fixation response in managed areas. Therefore, we recommend further research in that field. Determining nutrient balance helps characterize N dynamics and the influence of nutrient sources in managed agroecosystems [88; 89].  
On the other hand, N can be a driver, not only of biomass but also of ecosystem respiration in grasslands and croplands, which can turn C sinks into sources [10; 90; 91]. In addition, controlling factors of GPP response can have opposite effects. For example, low quantities of N deposition promote C storage while high temperatures produce C losses [24; 92]. Taking into account these interactions is of utter importance because of ever more frequent extreme climatic episodes worldwide. Therefore, quantifying the impact of N deposition on the C budget together with other factors, such as land management practices and extreme climatic events, is crucial to understanding ecosystem processes. The emission-based model adopted in this study to quantify N deposition provided good estimates for large areas and produced similar results when compared with alternative methods [60]. Therefore, this approach is considered a reliable source of information for studies carried out at a national scale. In addition, we suggest the development of further lines of research, taking into account the climate inter-annual variability in the study areas, which could provide answers to the low correlation found between precipitation and GPP. Model simulations have shown the importance of coupling N and C cycles to account for vegetation dynamics and to improve global patterns of C and N fluxes [93; 94]. There are still limitations on modelling sources of fixed N and land management changes, as well as coupling terrestrial C and N cycles [95; 96]. Nevertheless, we expect our study to contribute to the inclusion of N deposition datasets in Earth system models to better account for N dynamics. This could improve predictions of C uptake in different ecosystems [97; 98; 99].