Figure S1. Different processes control foliar Δ15N and Δ13C. Temperature has a strong effect on Δ15N, whereas stomatal conductance has a strong effect on Δ13C. The Δ15N model described in this article was parameterized to describe a tropical forest canopy by specifying soil source 15N = 0, tree molar C:N stoichiometry of 200:1, and atmospheric [NH3] = 5 ppb. The Δ13C model of Farquhar et al. (1982) was parameterized using isotope effects for boundary layer conductance (ab = 2.9), stomatal conductance (a = 4.4), mesophyll conductance (am = 1:8), Rubisco (b = 29), photorespiration (f = 11.6), and dark respiration (e = −6).

Figure S2. Foliar Δ15N and Δ13C vary with canopy height in a tropical forest. After filtering the Ehleringer et al. (2010) dataset to exclude samples <1 m in height (i.e. predominantly C4 grasses), foliar Δ13C was significantly positively correlated with Δ15N (Δ15N = β * Δ13C + α: β = 0.71, α = −22.7, t = 666.8, d.f. = 685, P < 0.001).

Figure S3. Influence of assumptions about control of the NH+4(aq) pool on the overall isotope effect associated with leaf-atmosphere NH3 exchange. The contours illustrate the enrichment in 15N (‰) of glutamate when (a) the NH+4(aq) pool is fixed at 200 μM versus (b) allowed to vary linearly with flux through the photorespiratory NH3 cycle. In both cases, results are plotted for a leaf importing 100 nmol NH-3(aq) m−2 s−1 while photosynthesizing at 1500 μmol PAR m−2 s−1 and 394 ppm CO2(g).

Appendix S1. Parameterization.

Appendix S2. Equation set.

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