## 7 Conclusions  
We have provided a review of the impacts of N and drought on ecosystems and a list of future model recommendations that serves to address the missing processes needed in LSMs to capture those interactions. While this list is extensive, it is by no means exhaustive. Our suggestions target two co-occurring stressors, but additional stresses will also be present that should be considered such as warming, elevated CO\({}_{2}\), and herbivory. For example, herbivory could reduce the N benefits of elevated productivity from consumption [145] or be a source of N deposition. Effects from elevated CO\({}_{2}\) can vary; CO\({}_{2}\) fertilization can help mitigate extreme heat and drought [146], or fertilization effects on productivity may be eliminated during drought and nutrient limitation [147]. These additional impacts on ecosystems are important, considering that CO\({}_{2}\) fertilization currently plays a larger role than N deposition in increasing productivity [148]. Another example is the interactions that exist between N availability, drought, and air temperature. It is thought that forest productivity should increase with temperature at the global scale, but droughts often associated with increases in temperature have lowered the NPP potential in some regions [149]. For instance, tropical forests can increase Net Ecosystem Exchange (NEE) with modest increases in air temperature, and drought effects could be ameliorated with increases in atmospheric CO\({}_{2}\)[150]. The interactions of temperature with drought and N can also affect the surface properties of forests beyond evapotranspiration. Drought sensitive forests (deciduous forests) tend to have a higher surface albedo than drought resistant ones (e.g., conifer forests), affecting the regional energy balance [151]. Further, increases in temperature can increase decomposition and N mineralization rates in the absence of drought, but N mineralization will not respond to temperature if moisture is the limiting factor [152]. These biophysical properties are important in understanding the global net effect of drought and N deposition on biosphere climate interactions [153] and are not the focus of this work. It is important to note that, in most elevated temperature ecosystem experiments, drought is a secondary effect caused by elevated temperature treatments [154; 155], making it difficult to distinguish primary from integrated responses of forests to drought and temperature on biogeochemical and biophysical processes. This study only focused on two stressors to ecosystems, N and drought. Although some of the identified processes targeted by this manuscript may help models to capture impacts from other climate-related effects, additional ecosystem processes will need to be addressed in the future. As the community integrates additional processes in models to capture vegetation responses to N inputs, we can revisit the resulting C uptake to evaluate the sink capacity of the terrestrial surface. However, special care should be taken when implementing additional parameters and processes into models, particularly so that models are not over-parameterized and so that the resulting C cycle response is not constrained by incomplete observational data. In the near term, model sensitivity studies can be used to determine the most sensitive parameters and processes that drive changes in C and nutrient cycles. More research on the structural uncertainty of models can provide insight on those processes that have strong feedbacks or introduce instability. Finally, when rigorous model testing through benchmarking is complete, we can focus on additional questions related to the fate of C in the land. Improving estimates of the global terrestrial C sink is a priority for ESM development. In order to improve the predictions and reduce uncertainty, model development should focus on the processes that will be affected by multiple co-occurring stressors such as N deposition and drought.