## 1 Introduction  
Earth system models (ESMs) have been used to predict the climate's response to increased CO\({}_{2}\) emissions (or concentrations), but uncertainty in land carbon (C) feedbacks results in a wide spread of uncertainty in model results [1]. Part of this uncertainty lies in a general lack of knowledge of the physical processes responsible for the land feedbacks on the C cycle, which makes estimating the land C sink difficult. Adding to this uncertainty is the response of ecosystems in the face of multiple stressors, the impacts of which may be non-additive and will certainly be highly variable across ecosystems. One prime example is the current increase in nitrogen (N) deposition and the predicted increase in drought [2]. Most field studies isolate one environmental component (e.g., drought, elevated CO\({}_{2}\), N deposition) to study plant behavior. This results in an incomplete knowledge of an ecosystem's reaction to multiple stressors, which limits our forecasting capability. Vegetation responses have a strong influence on C storage. As plants adjust their partitioning strategy for C and other nutrients to optimize uptake while limiting costs to ensure survival, the quantity and quality of C stored and the way nutrients are recycled will change. Anthropogenic production of reactive N from fossil fuel combustion and fertilizer synthesis has led to an increased availability of reactive N in ecosystems, and this has become a global problem [3]. The current N deposition rate in some systems is over 10 kg ha\({}^{-1}\) year\({}^{-1}\)[4], which is well over the deposition rate from natural sources at 0.5 kg ha\({}^{-1}\) year\({}^{-1}\)[5]. Nitrogen deposits from industrial and agriculture activities have led to significant N loading in soils, particularly in regions of Europe and the eastern United States [6; 7]. These N additions are within the critical load of N for sensitive ecosystems [8; 9]. This N loading may be further exacerbated by increases in N availability from climate influences (i.e., warming and elevated CO\({}_{2}\)) on internal N cycling, which can increase decomposition, mineralization, and biological nitrogen fixation (BNF) [10; 11; 12]. The consequences of N loading are well known and include changes in biodiversity [7; 13; 14], composition [15; 16], productivity [17], leaching [18; 19], and possibly nitrification [17]. Since N is strongly tied to C, understanding the impact of increased N availability in ecosystems in the context of climate change is crucial if we are to understand whether increased N availability will strengthen or weaken the land's C sink capacity. Understanding the vegetative response to N deposition is increasingly difficult due a wide variability of responses between and within species and across climate and soil regimes, as well as contrasting behavior when faced with other elements of climate change (e.g., elevated CO\({}_{2}\) or drought). For example, increased N availability generally results in an increase in aboveground woody biomass and a decrease in root biomass, but experiments with drought indicate an increase in root biomass at the expense of aboveground tissue [20; 21; 22; 23]. This has led to the conclusion that N deposition may increase ecosystem vulnerability to drought [24], but some studies find that N deposition may enhance the recovery of some species post drought [25]. Therefore, a large uncertainty exists for vegetation response to increased N deposition under drought, especially because many of these antagonistic responses exhibit nonlinear behavior. Predictions of climate change indicate an increase in the frequency and severity of drought [2] in many regions that are already water stressed. For example, in the Amazon, the dry season is expected to lengthen and intensify, and the area affected by seasonal drought is expected to expand by up to 0.75 million km\({}^{2}\) by the end of the century [26].