## 2 Observations of N Impacts  
Nitrogen is a critical component in ecosystems; it drives productivity through photosynthesis processes [39]. Increased N deposition results in increased production because N is generally the most limiting nutrient for growth [37; 40]. [PERSON] et al. [41] estimate that an additional 175 PgC has been stored since the pre-industrial period due to increased N deposition. This increase results mostly from changes in photosynthesis (i.e., increase in foliar N) or shifts in C allocation to increase light availability (Figure 1 and Table S1). Several mechanisms will allow plants to respond to changes in N such as the ability to increase photosynthetic capacity, plasticity to alter C partitioning of resources, and the presence or absence of other limiting factors in the system (e.g., water, P, mycorrhizal associations, etc.). An overview of some of the consequences of N deposition in ecosystems is included in Figure 1.  
Figure 1: Effects of N deposition on ecosystems, based on the concepts of [PERSON] et al. [7]. Up arrows represent an increase, and down arrows represent a decrease. Three main pathways exist: changes in foliar or leaf N, changes to biomass partitioning, and increases in biomass N. Changes in leaf or plant N generally lead to a positive feedback by increasing N in the litter pool. Changes in the partitioning of biomass can lead to changes in competition for resources that will ultimately affect species distribution. In general, the impacts of N deposition occur over short timescales (¡5 years), while changes to species composition and disturbance occur over longer timescales [7]. Nitrogen deposition studies frequently find an increase in foliar N that results in decreased C:N ratios of leaves [42; 43; 44]. This relationship between N deposition and foliar N content has even been used to estimate critical loads of N in Europe [45]. [PERSON] et al. [42] suggested that the increase in foliar N is responsible for the increase in aboveground net primary productivity (ANPP) that is found under elevated N availability, considering the relationship N plays in photosynthetic capacity and C assimilation [39; 46]. However, reductions in N use efficiency and C allocation shifts away from mycorrhizalae may also result in increases in ANPP [47]. Increases in productivity can also lead to increases in litter production that can ultimately lead to increased N in soils. Gains in productivity can be the result of changes in patterns of C partitioning within the plant, and they often seem to be correlated with increases in above ground biomass [42; 48; 49]. Most increases in aboveground biomass from increased N deposition are allocated to stems [42; 50; 51], particularly for small-diameter trees [52]. This response results in faster biomass accumulation, which produces taller, skinnier trees [52]. Although this is more likely to affect young trees or seedlings exposed to elevated N, Du and Fang [53] also found weak growth in a mature forest. This may increase the mortality of young trees as a result of light limitation or the respiratory costs of early rapid growth [52]. However, the increase in growth is not consistent or linear for all species. For example, [PERSON] et al. [50] found that tropical forests had the least response to N deposition compared to temperate and boreal forests, and in some cases a negative growth relationship exists at high N inputs [49]. Herbaceous plants also experience biomass increases from N deposition [48; 54], and under low levels of N deposition they can increase aboveground biomass more than trees [51]. The increase in aboveground biomass is expected to come at the expense of belowground inputs [48]. However, it is unclear how roots respond to increased N availability; this response can include increases in productivity, even though decreases in biomass occur [55]. This occurs when increases in root turnover with N availability result in root biomass decreases over long time scales, even though C allocated to the roots likely increases.