## 1 Introduction
The southwest and central United States are forecast to experience increases in drought severity that exceed the severest mega drought events at millennial scales [27]. Areas that do not experience a change in total precipitation may still experience changes in the timing and intensity of rainfall events. Plants under drought stress can experience hydraulic failure, C starvation, and increased vulnerability to disease, pests, and fire. Drought during peak growth periods will likely be more detrimental; juvenile and shallow-rooted plants are most susceptible to drought, whereas plants that are able to modify biogeochemical feedbacks will increase their chance of survival [28]. In general, drought results in decreased stomatal conductance and decreased net primary productivity [27]. In the past, drought events over the Amazon have reduced C storage through mortality and slowed growth [29]; increased events in the future could result in a weaker C sink of boreal, temperate, and tropical biomes across the globe [30]. This would contrast with the increased productivity that can result from high N deposition. Most ecosystems have experienced an extreme drought in the past, but extreme drought in combination with other climate impacts such as N deposition can put unprecedented amounts of stress on ecosystems. Given that the co-occurrence of drought and increased N availability are highly probable, these climate effects must be considered together to predict ecosystem behavior. To answer questions about the future of the terrestrial C sink, we need to understand the behavior of ecosystems under long-term chronic N deposition and drought, as well as the interactions between these phenomena. ESMs are important tools for exploring the relationships between climate and vegetation responses, but current models might not be able to capture these processes. The ESM community has made great strides in improving the C cycle in land surface models (LSMs) due to improvements to biogeochemical and hydrological cycles. Some of these advances constrain land C sequestration by imposing N limitations on C fertilization [31], water table, and inundation dynamics [32] and even hydraulic redistribution [33]. However, these models still cannot capture mortality in trees from drought, changes in biomass partitioning due to nutrient limitations, or even acclimation [34]. For example, [PERSON] et al. [35] found that 14 LSMs overestimate seasonal drought, due partly to how models handle soil hydrology and plant moisture stress. [PERSON] et al. [36] also found that LSMs exaggerate drought effects because most models assume the same drought sensitivity for all vegetation types. Adding the N cycle to LSMs results in productivity decreases that range from 7% to 64% [37], but the feedbacks between C and N are weak once a steady state is reached [38]. This suggests that LSMs still lack key processes. In order to simulate N deposition-drought interactions, we hypothesize that additional model developments are necessary to mimic ecosystem stress responses, particularly if LSMs are to represent the C cycle accurately. This review seeks to determine whether the current ESM framework of the C cycle is sufficient to capture the vegetation response to the combined effects of drought and growing N loading in ecosystems. To that end, this paper will review the observed impacts on C uptake and allocation due to increased N availability, drought, and their combined effects. Next will be a review of how current ESMs represent the coupled carbon-nitrogen model for C uptake and partitioning. We will present recent developments in several state-of-the-art LSMs, including the limitations of the representations for capturing interactions from N deposition and drought stress. Finally, we will discuss which additional processes should be the focus of future model developments to reduce uncertainty and improve predictive power.