## 4 Interactions between N and Drought  
The interactions between N and drought are difficult to determine because (1) the effects can depend on the timing of N deposition relative to drought; (2) most experiments are done with young trees or herbaceous plants and not with mature vegetation; (3) many studies impose only weak drought conditions that might not result in drought-N feedbacks [74]; and (4) the impacts vary with ecosystem and plant traits. However, in general, the effects from drought and N deposition are interdependent [24] and not always additive [75; 76]. A list of studies that have examined N deposition-drought interactions is included in Table S1.  
Drought seems to negate the increase in productivity observed from increased N deposition [77; 78], although the effects of N addition may alleviate some of the impacts of drought on growth [78]. This can be partially attributed to the countering effects of N and drought on photosynthesis; in particular, N deposition tends to increase photosynthetic capacity, while drought limits photosynthesis. Other interactions between these two types of stress also occur. For example, N deposition tends to increase water use efficiency (WUE), but, during a severe drought, plants may lose the ability to utilize N, leading to a loss of biomass [77]. Some ecosystems experience a decline in above- and belowground biomass [24; 79] and a loss of root functionality [79]. In some cases, changes in allocation between above- and belowground biomass result in higher root-to-shoot ratios [76].  
The timing of a drought event is also an important factor considering N level effects. Plants are more susceptible to drought when N availability increases before drought because it leads to higher productivity, thereby increasing evaporative demands [24]. This can even result in a loss of N allocation control and adaptive strategies to mitigate drought effects [24]. Finally, increased N availability can lower concentrations of compounds that are important for stress tolerance, which could also increase mortality under more severe drought conditions [80].  
Plant age also plays a role in determining the response of N deposition combined with drought. [PERSON] [81] found that the reduction in root biomass due to combined N deposition and drought was more severe for young seedlings than older trees. Trees tend to allocate more biomass to roots as they age, so younger trees are more susceptible to drought, and drought following N deposition further increases sensitivity to drought [76].  
However, N deposition during drought may not always be detrimental. For example, drought can lock nutrients in an immobilized state by reducing the soil water available for decomposition [28], but N deposition can increase available nutrients. Increased N deposition can also aid the recovery after drought of some species [25]. For trees that preferentially shift allocations to stems under N deposition and drought [82], water stress may be alleviated by the extra water storage in the tree. In desert systems, N deposition relieved some of the negative effects of water stress through increases in root weight, leaf number, leaf area, biomass, and decreased root-to-shoot ratios [80; 83].